write essay about greenhouse effect

Essay on Greenhouse Effect for Students and Children

500 words essay on greenhouse effect.

The past month, July of 2019, has been the hottest month in the records of human history. This means on a global scale, the average climate and temperatures are now seen a steady rise year-on-year. The culprits of this climate change phenomenon are mainly pollution , overpopulation and general disregard for the environment by the human race. However, we can specifically point to two phenomenons that contribute to the rising temperatures – global warming and the greenhouse effect. Let us see more about them in this essay on the greenhouse effect.

The earth’s surface is surrounded by an envelope of the air we call the atmosphere. Gasses in this atmosphere trap the infrared radiation of the sun which generates heat on the surface of the earth. In an ideal scenario, this effect causes the temperature on the earth to be around 15c. And without such a phenomenon life could not sustain on earth.

However, due to rapid industrialization and rising pollution, the emission of greenhouse gases has increased multifold over the last few centuries. This, in turn, causes more radiation to be trapped in the earth’s atmosphere. And as a consequence, the temperature on the surface of the planet steadily rises. This is what we refer to when we talk about the man-made greenhouse effect.

Causes of Greenhouse Effect

As we saw earlier in this essay on the greenhouse effect, the phenomenon itself is naturally occurring and an important one to sustain life on our planet. However, there is an anthropogenic part of this effect. This is caused due to the activities of man.

The most prominent among this is the burning of fossil fuels . Our industries, vehicles, factories, etc are overly reliant on fossil fuels for their energy and power. This has caused an immense increase in emissions of harmful greenhouse gasses such as carbon dioxide, carbon monoxide, sulfides, etc. This has multiplied the greenhouse effect and we have seen a steady rise in surface temperatures.

Other harmful activities such as deforestation, excessive urbanization, harmful agricultural practices, etc. have also led to the release of excess carbon dioxide and made the greenhouse effect more prominent. Another harmful element that causes harm to the environment is CFC (chlorofluorocarbon).

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Some Effects of Greenhouse Effect

Even after overwhelming proof, there are still people who deny the existence of climate change and its devastating pitfalls. However, there are so many effects and pieces of evidence of climate change it is now undeniable. The surface temperature of the planet has risen by 1c since the 19th century. This change is largely due to the increased emissions of carbon dioxide. The most harm has been seen in the past 35 years in particular.

The oceans and the seas have absorbed a lot of this increased heat. The surfaces of these oceans have seen a rise in temperatures of 0.4c. The ice sheets and glaciers are also rapidly shrinking. The rate at which the ice caps melt in Antartica has tripled in the last decade itself. These alarming statistics and facts are proof of the major disaster we face in the form of climate change.

600 Words Essay on Greenhouse Effect

A Greenhouse , as the term suggests, is a structure made of glass which is designed to trap heat inside. Thus, even on cold chilling winter days, there is warmth inside it. Similarly, Earth also traps energy from the Sun and prevents it from escaping back. The greenhouse gases or the molecules present in the atmosphere of the Earth trap the heat of the Sun. This is what we know as the Greenhouse effect.

Greenhouse Gases

These gases or molecules are naturally present in the atmosphere of the Earth. However, they are also released due to human activities. These gases play a vital role in trapping the heat of the Sun and thereby gradually warming the temperature of Earth. The Earth is habitable for humans due to the equilibrium of the energy it receives and the energy that it reflects back to space.

Global Warming and the Greenhouse Effect

The trapping and emission of radiation by the greenhouse gases present in the atmosphere is known as the Greenhouse effect. Without this process, Earth will either be very cold or very hot, which will make life impossible on Earth.

The greenhouse effect is a natural phenomenon. Due to wrong human activities such as clearing forests, burning fossil fuels, releasing industrial gas in the atmosphere, etc., the emission of greenhouse gases is increasing.

Thus, this has, in turn, resulted in global warming . We can see the effects due to these like extreme droughts, floods, hurricanes, landslides, rise in sea levels, etc. Global warming is adversely affecting our biodiversity, ecosystem and the life of the people. Also, the Himalayan glaciers are melting due to this.

There are broadly two causes of the greenhouse effect:

I. Natural Causes

• Some components that are present on the Earth naturally produce greenhouse gases. For example, carbon dioxide is present in the oceans, decaying of plants due to forest fires and the manure of some animals produces methane , and nitrogen oxide is present in water and soil.
• Water Vapour raises the temperature by absorbing energy when there is a rise in the humidity.
• Humans and animals breathe oxygen and release carbon dioxide in the atmosphere.

II. Man-made Causes

• Burning of fossil fuels such as oil and coal emits carbon dioxide in the atmosphere which causes an excessive greenhouse effect. Also, while digging a coal mine or an oil well, methane is released from the Earth, which pollutes it.
• Trees with the help of the process of photosynthesis absorb the carbon dioxide and release oxygen. Due to deforestation the carbon dioxide level is continuously increasing. This is also a major cause of the increase in the greenhouse effect.
• In order to get maximum yield, the farmers use artificial nitrogen in their fields. This releases nitrogen oxide in the atmosphere.
• Industries release harmful gases in the atmosphere like methane, carbon dioxide , and fluorine gas. These also enhance global warming.

All the countries of the world are facing the ill effects of global warming. The Government and non-governmental organizations need to take appropriate and concrete measures to control the emission of toxic greenhouse gases. They need to promote the greater use of renewable energy and forestation. Also, it is the duty of every individual to protect the environment and not use such means that harm the atmosphere. It is the need of the hour to protect our environment else that day is not far away when life on Earth will also become difficult.

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What is the greenhouse effect?

The greenhouse effect is the process through which heat is trapped near Earth's surface by substances known as 'greenhouse gases.' Imagine these gases as a cozy blanket enveloping our planet, helping to maintain a warmer temperature than it would have otherwise. Greenhouse gases consist of carbon dioxide, methane, ozone, nitrous oxide, chlorofluorocarbons, and water vapor. Water vapor, which reacts to temperature changes, is referred to as a 'feedback', because it amplifies the effect of forces that initially caused the warming.

Scientists have determined that carbon dioxide plays a crucial role in maintaining the stability of Earth's atmosphere. If carbon dioxide were removed, the terrestrial greenhouse effect would collapse, and Earth's surface temperature would drop significantly, by approximately 33°C (59°F).

Greenhouse gases are part of Earth's atmosphere. This is why Earth is often called the 'Goldilocks' planet – its conditions are just right, not too hot or too cold, allowing life to thrive. Part of what makes Earth so amenable is its natural greenhouse effect, which maintains an average temperature of 15 ° C (59 ° F) . However, in the last century, human activities, primarily from burning fossil fuels that have led to the release of carbon dioxide and other greenhouse gases into the atmosphere, have disrupted Earth's energy balance. This has led to an increase in carbon dioxide in the atmosphere and ocean. The level of carbon dioxide in Earth’s atmosphere has been rising consistently for decades and traps extra heat near Earth's surface, causing temperatures to rise.

• The Greenhouse Effect (UCAR)
• NASA's Climate Kids: Meet the Greenhouse Gases! (downloadable and printable cards)
• NASA's Climate Kids: What Is the Greenhouse Effect?

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Understanding Global Change

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Greenhouse effect

Life as we know it would be impossible if not for the greenhouse effect, the process through which heat is absorbed and re-radiated in that atmosphere. The intensity of a planet’s greenhouse effect is determined by the relative abundance of greenhouse gases in its atmosphere. Without greenhouse gases, most of Earth’s heat would be lost to outer space, and our planet would quickly turn into a giant ball of ice. Increase the amount of greenhouse gases to the levels found on the planet Venus, and the Earth would be as hot as a pizza oven! Fortunately, the strength of Earth’s greenhouse effect keeps our planet within a temperature range that supports life

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What is the greenhouse effect, earth system models about the greenhouse effect, how human activities influence the greenhouse effect, explore the earth system, investigate, links to learn more.

For the classroom:

• Teaching Resources

Global Change Infographic

The greenhouse effect occurs in the atmosphere, and is an essential part of How the Earth System Works. Click the image on the left to open the Understanding Global Change Infographic . Locate the greenhouse effect icon and identify other topics that cause changes to, or are affected by, the greenhouse effect.

Adapted from the Environmental Protection Agency greenhouse effect file

Greenhouse gases such as methane, carbon dioxide, nitrous oxide, and water vapor  significantly affect the amount of energy in the Earth system, even though they make up a tiny percentage of Earth’s atmosphere.  Solar radiation that passes through the atmosphere and reaches Earth’s surface is either reflected or absorbed . Reflected sunlight doesn’t add any heat to the Earth system because this energy bounces back into space.

However, absorbed sunlight increases the temperature of Earth’s surface, and the warmed surface re-radiates as long-wave radiation (also known as infrared radiation). Infrared radiation is invisible to the eye, but we feel it as heat.

If there were not any greenhouse gases in the atmosphere, all that heat would pass directly back into space. With greenhouse gases present, however, most of the long-wave radiation coming from Earth’s surface is absorbed and then re-radiated in all directions many times before passing back into space. Heat that is re-radiated downward, toward the Earth, is absorbed by the surface and re-radiated again.

Clouds also influence the greenhouse effect. A thick, low cloud cover can enhance the reflectivity of the atmosphere, reducing the amount of solar radiation reaching Earth’s surface, but clouds high in the atmosphere can intensify the greenhouse effect by re-radiating heat from the Earth’s surface.

Altogether, this cycle of absorption and re-radiation by greenhouse gases impedes the loss of heat from our atmosphere to space, creating the greenhouse effect. Increases in the amount of greenhouses gases will mean that more heat is trapped, increasing the amount of energy in the Earth system (Earth’s energy budget), and raising Earth’s temperature. This increase in Earth’s average temperature is also known as global warming.

This Earth system model is one way to represent the essential processes and interactions related to the greenhouse effect. Hover over the icons for brief explanations; click on the icons to learn more about each topic. Download the Earth system models on this page. There are a few ways that the relationships among these topics can be represented and explained using the Understanding Global Change icons ( download examples ).  

The greenhouse effect, which influences Earth’s average temperature, affects many of the processes that shape global climate and ecosystems.  This model shows some of the other parts of the Earth system that the greenhouse effect influences, including the water cycle and water temperature .

Humans directly affect the greenhouse effect through activities that result in greenhouse gas emissions. The Earth system model below includes some of the ways that human activities increase the amount of greenhouse gases in the atmosphere. Releasing greenhouse gases intensifies the greenhouse effect, and increases Earth’s average air temperatures (also known as global warming). Hover over or click on the icons to learn more about these human causes of change and how they influence the greenhouse effect.

Click the scene icons and bolded terms on this page to learn more about these process and phenomena.

Learn more in these real-world examples, and challenge yourself to  construct a model  that explains the Earth system relationships.

• Ancient fossils and modern climate change
• How Global Warming Works
• NASA:  Global Climate Change:  A Blanket Around the Earth
• UCAR Center for Science Education: The Greenhouse Effect
• IPCC:  What is the Greenhouse Effect?
• Indicators of Change (NCA.2014)
• Human influence on the greenhouse effect
• The Carbon Cycle and Earth’s Climate

What Is the Greenhouse Effect?

Watch this video to learn about the greenhouse effect! Click here to download this video (1920x1080, 105 MB, video/mp4). Click here to download this video about the greenhouse effect in Spanish (1920x1080, 154 MB, video/mp4).

How does the greenhouse effect work?

As you might expect from the name, the greenhouse effect works … like a greenhouse! A greenhouse is a building with glass walls and a glass roof. Greenhouses are used to grow plants, such as tomatoes and tropical flowers.

A greenhouse stays warm inside, even during the winter. In the daytime, sunlight shines into the greenhouse and warms the plants and air inside. At nighttime, it's colder outside, but the greenhouse stays pretty warm inside. That's because the glass walls of the greenhouse trap the Sun's heat.

A greenhouse captures heat from the Sun during the day. Its glass walls trap the Sun's heat, which keeps plants inside the greenhouse warm — even on cold nights. Credit: NASA/JPL-Caltech

The greenhouse effect works much the same way on Earth. Gases in the atmosphere, such as carbon dioxide , trap heat similar to the glass roof of a greenhouse. These heat-trapping gases are called greenhouse gases .

During the day, the Sun shines through the atmosphere. Earth's surface warms up in the sunlight. At night, Earth's surface cools, releasing heat back into the air. But some of the heat is trapped by the greenhouse gases in the atmosphere. That's what keeps our Earth a warm and cozy 58 degrees Fahrenheit (14 degrees Celsius), on average.

Earth's atmosphere traps some of the Sun's heat, preventing it from escaping back into space at night. Credit: NASA/JPL-Caltech

How are humans impacting the greenhouse effect?

Human activities are changing Earth's natural greenhouse effect. Burning fossil fuels like coal and oil puts more carbon dioxide into our atmosphere.

NASA has observed increases in the amount of carbon dioxide and some other greenhouse gases in our atmosphere. Too much of these greenhouse gases can cause Earth's atmosphere to trap more and more heat. This causes Earth to warm up.

What reduces the greenhouse effect on Earth?

Just like a glass greenhouse, Earth's greenhouse is also full of plants! Plants can help to balance the greenhouse effect on Earth. All plants — from giant trees to tiny phytoplankton in the ocean — take in carbon dioxide and give off oxygen.

The ocean also absorbs a lot of excess carbon dioxide in the air. Unfortunately, the increased carbon dioxide in the ocean changes the water, making it more acidic. This is called ocean acidification .

More acidic water can be harmful to many ocean creatures, such as certain shellfish and coral. Warming oceans — from too many greenhouse gases in the atmosphere — can also be harmful to these organisms. Warmer waters are a main cause of coral bleaching .

This photograph shows a bleached brain coral. A main cause of coral bleaching is warming oceans. Ocean acidification also stresses coral reef communities. Credit: NOAA

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The greenhouse effect

Discovering Geology — Climate change

‘Greenhouse gases’ are crucial to keeping our planet at a suitable temperature for life. Without the natural greenhouse effect, the heat emitted by the Earth would simply pass outwards from the Earth’s surface into space and the Earth would have an average temperature of about -20°C.

Greenhouse gases

The greenhouse effect: some of the infrared radiation from the Sun passes through the atmosphere, but most is absorbed and re-emitted in all directions by greenhouse gas molecules and clouds. The effect of this is to warm the Earth’s surface and the lower atmosphere.  BGS © UKRI.

A greenhouse gas is called that because it absorbs infrared radiation from the Sun in the form of heat, which is circulated in the atmosphere and eventually lost to space. Greenhouse gases also increase the rate at which the atmosphere can absorb short-wave radiation from the Sun, but this has a much weaker effect on global temperatures.

The CO 2 released from the burning of fossil fuels is accumulating as an insulating blanket around the Earth, trapping more of the Sun’s heat in our atmosphere. Actions carried out by humans are called anthropogenic actions; the anthropogenic release of CO 2 contributes to the current  enhanced greenhouse effect [1] .

Which gases cause the greenhouse effect?

The contribution that a greenhouse gas makes to the greenhouse effect depends on how much heat it absorbs, how much it re-radiates and how much of it is in the atmosphere.

In descending order, the gases that contribute most to the Earth’s greenhouse effect are:

• water vapour (H 2 O)
• carbon dioxide (CO 2 )
• nitrous oxide(N 2 O)
• methane (CH 4 )
• ozone (O 3 )

In terms of the amount of heat these gases can absorb and re-radiate (known as their global warming potential or GWP), CH 4 is 23 times more effective and N 2 O is 296 times more effective than CO 2 . However, there is much more CO 2 in the Earth’s atmosphere than there is CH 4 or N 2 O.

Not all the greenhouse gas that we emit to the atmosphere remains there indefinitely. For example, the amount of CO 2  in the atmosphere and the amount of CO 2  dissolved in surface waters of the oceans stay in equilibrium, because the air and water mix well at the sea surface. When we add more CO 2  to the atmosphere, a proportion of it dissolves into the oceans.

Anthropogenic greenhouse gases

Since the start of the Industrial Revolution in the mid-18th century, human activities have greatly increased the concentrations of greenhouse gases in the atmosphere. Consequently, measured atmospheric concentrations of CO 2 are many times higher than pre-industrial levels.

Overview of global anthropogenic greenhouse gas emissions in 2017; figures here are expressed in CO2-equivalents. Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2015 (EPA, 2017).

Main sources of anthropogenic greenhouse gases

Carbon dioxide levels are substantially higher now than at any time in the last 750 000 years. The burning of fossil fuels has elevated CO 2 levels from an atmospheric concentration of approximately 280 parts per million (ppm) in pre-industrial times to over 400 ppm in 2018. This is a 40 per cent increase since the start of the Industrial Revolution.

CO 2 concentrations are increasing at a rate of about 2–3 ppm/year and are expected to exceed 900 ppm by the end of the 21st century.

If this continues, together with rising emissions of CH 4   and other greenhouse gases, by 2100 the global average surface temperature could have increased by up to 4.8°C compared to pre-industrial levels. Consequently, some scientists suggest goals to limit concentrations to keep temperature change below +2°C.  This would include substantial cuts in anthropogenic greenhouse gas emissions by the middle of the 21st century through large-scale changes in energy systems and land use.

In 2010, the burning of coal, natural gas and oil for electricity and heat was the largest single source of global greenhouse gas emissions (25 per cent). By comparison, in 2010, 14 per cent of global greenhouse gas emissions came from fossil fuels burned for road, rail, air and marine transportation.

Agriculture, deforestation and other changes in land use account for one quarter of net anthropogenic greenhouse gas emissions. According to a United Nations report, livestock is responsible for about 14.5 per cent of this. The main sources of emissions are:

• feed production and processing (45 per cent)
• outputs of greenhouse gases during digestion by cows (39 per cent)
• manure decomposition (10 per cent

The rest is attributable to the processing and transportation of animal products.

Higher concentrations of atmospheric CH 4 are also caused by changes in land and wetland use, pipeline losses and landfill emissions. The use of fertilisers can also lead to higher N 2 O concentrations.

Agriculture is estimated to be the main driver for around 80 per cent of deforestation worldwide. Source: Pixabay .

Cement manufacture contributes CO 2 to the atmosphere when calcium carbonate is heated, producing lime and CO 2 .

Estimates vary, but it is widely accepted that the cement industry produces between five and eight per cent of global anthropogenic CO 2 emissions, of which 50 per cent is produced from the chemical process itself and 40 per cent from burning fuel to power that process. The amount of CO 2 emitted by the cement industry is more than 900 kg of CO 2 for every 1000 kg of cement produced.

Aerosols are small particles suspended in the atmosphere that can be produced when we burn fossil fuels. Other anthropogenic sources of aerosols include pollution from cars and factories, chlorofluorocarbons (CFCs) used in refrigeration systems and CFCs and halons used in fire suppression systems and manufacturing processes. Aerosols can also be produced naturally from a number of natural processes e.g. forest fires, volcanoes and isoprene emitted from plants.

We know that greenhouse gases provide a warming effect to Earth’s surface, but aerosol pollution in the atmosphere can counteract this warming effect. For example, sulphate aerosols from fossil fuel combustion exert a cooling influence by reducing the amount of sunlight that reaches the Earth.

Aerosols also have a detrimental impact on human health and affect other parts of the climate system, such as rainfall.

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1. Enhanced Greenhouse effect

'Greenhouse gases' are actually crucial to keeping our planet at a habitable temperature, without them the Earth would be about minus 17 degrees! Anthropogenic or human release of carbon dioxide is what is contributing to an additional or enhanced greenhouse effect.

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The Science of Climate Change Explained: Facts, Evidence and Proof

Definitive answers to the big questions.

Credit... Photo Illustration by Andrea D'Aquino

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By Julia Rosen

Ms. Rosen is a journalist with a Ph.D. in geology. Her research involved studying ice cores from Greenland and Antarctica to understand past climate changes.

• Published April 19, 2021 Updated Nov. 6, 2021

The science of climate change is more solid and widely agreed upon than you might think. But the scope of the topic, as well as rampant disinformation, can make it hard to separate fact from fiction. Here, we’ve done our best to present you with not only the most accurate scientific information, but also an explanation of how we know it.

How do we know climate change is really happening?

How much agreement is there among scientists about climate change, do we really only have 150 years of climate data how is that enough to tell us about centuries of change, how do we know climate change is caused by humans, since greenhouse gases occur naturally, how do we know they’re causing earth’s temperature to rise, why should we be worried that the planet has warmed 2°f since the 1800s, is climate change a part of the planet’s natural warming and cooling cycles, how do we know global warming is not because of the sun or volcanoes, how can winters and certain places be getting colder if the planet is warming, wildfires and bad weather have always happened. how do we know there’s a connection to climate change, how bad are the effects of climate change going to be, what will it cost to do something about climate change, versus doing nothing.

Climate change is often cast as a prediction made by complicated computer models. But the scientific basis for climate change is much broader, and models are actually only one part of it (and, for what it’s worth, they’re surprisingly accurate ).

For more than a century , scientists have understood the basic physics behind why greenhouse gases like carbon dioxide cause warming. These gases make up just a small fraction of the atmosphere but exert outsized control on Earth’s climate by trapping some of the planet’s heat before it escapes into space. This greenhouse effect is important: It’s why a planet so far from the sun has liquid water and life!

However, during the Industrial Revolution, people started burning coal and other fossil fuels to power factories, smelters and steam engines, which added more greenhouse gases to the atmosphere. Ever since, human activities have been heating the planet.

We know this is true thanks to an overwhelming body of evidence that begins with temperature measurements taken at weather stations and on ships starting in the mid-1800s. Later, scientists began tracking surface temperatures with satellites and looking for clues about climate change in geologic records. Together, these data all tell the same story: Earth is getting hotter.

Average global temperatures have increased by 2.2 degrees Fahrenheit, or 1.2 degrees Celsius, since 1880, with the greatest changes happening in the late 20th century. Land areas have warmed more than the sea surface and the Arctic has warmed the most — by more than 4 degrees Fahrenheit just since the 1960s. Temperature extremes have also shifted. In the United States, daily record highs now outnumber record lows two-to-one.

Where it was cooler or warmer in 2020 compared with the middle of the 20th century

This warming is unprecedented in recent geologic history. A famous illustration, first published in 1998 and often called the hockey-stick graph, shows how temperatures remained fairly flat for centuries (the shaft of the stick) before turning sharply upward (the blade). It’s based on data from tree rings, ice cores and other natural indicators. And the basic picture , which has withstood decades of scrutiny from climate scientists and contrarians alike, shows that Earth is hotter today than it’s been in at least 1,000 years, and probably much longer.

In fact, surface temperatures actually mask the true scale of climate change, because the ocean has absorbed 90 percent of the heat trapped by greenhouse gases . Measurements collected over the last six decades by oceanographic expeditions and networks of floating instruments show that every layer of the ocean is warming up. According to one study , the ocean has absorbed as much heat between 1997 and 2015 as it did in the previous 130 years.

We also know that climate change is happening because we see the effects everywhere. Ice sheets and glaciers are shrinking while sea levels are rising. Arctic sea ice is disappearing. In the spring, snow melts sooner and plants flower earlier. Animals are moving to higher elevations and latitudes to find cooler conditions. And droughts, floods and wildfires have all gotten more extreme. Models predicted many of these changes, but observations show they are now coming to pass.

Back to top .

There’s no denying that scientists love a good, old-fashioned argument. But when it comes to climate change, there is virtually no debate: Numerous studies have found that more than 90 percent of scientists who study Earth’s climate agree that the planet is warming and that humans are the primary cause. Most major scientific bodies, from NASA to the World Meteorological Organization , endorse this view. That’s an astounding level of consensus given the contrarian, competitive nature of the scientific enterprise, where questions like what killed the dinosaurs remain bitterly contested .

Scientific agreement about climate change started to emerge in the late 1980s, when the influence of human-caused warming began to rise above natural climate variability. By 1991, two-thirds of earth and atmospheric scientists surveyed for an early consensus study said that they accepted the idea of anthropogenic global warming. And by 1995, the Intergovernmental Panel on Climate Change, a famously conservative body that periodically takes stock of the state of scientific knowledge, concluded that “the balance of evidence suggests that there is a discernible human influence on global climate.” Currently, more than 97 percent of publishing climate scientists agree on the existence and cause of climate change (as does nearly 60 percent of the general population of the United States).

So where did we get the idea that there’s still debate about climate change? A lot of it came from coordinated messaging campaigns by companies and politicians that opposed climate action. Many pushed the narrative that scientists still hadn’t made up their minds about climate change, even though that was misleading. Frank Luntz, a Republican consultant, explained the rationale in an infamous 2002 memo to conservative lawmakers: “Should the public come to believe that the scientific issues are settled, their views about global warming will change accordingly,” he wrote. Questioning consensus remains a common talking point today, and the 97 percent figure has become something of a lightning rod .

To bolster the falsehood of lingering scientific doubt, some people have pointed to things like the Global Warming Petition Project, which urged the United States government to reject the Kyoto Protocol of 1997, an early international climate agreement. The petition proclaimed that climate change wasn’t happening, and even if it were, it wouldn’t be bad for humanity. Since 1998, more than 30,000 people with science degrees have signed it. However, nearly 90 percent of them studied something other than Earth, atmospheric or environmental science, and the signatories included just 39 climatologists. Most were engineers, doctors, and others whose training had little to do with the physics of the climate system.

A few well-known researchers remain opposed to the scientific consensus. Some, like Willie Soon, a researcher affiliated with the Harvard-Smithsonian Center for Astrophysics, have ties to the fossil fuel industry . Others do not, but their assertions have not held up under the weight of evidence. At least one prominent skeptic, the physicist Richard Muller, changed his mind after reassessing historical temperature data as part of the Berkeley Earth project. His team’s findings essentially confirmed the results he had set out to investigate, and he came away firmly convinced that human activities were warming the planet. “Call me a converted skeptic,” he wrote in an Op-Ed for the Times in 2012.

Mr. Luntz, the Republican pollster, has also reversed his position on climate change and now advises politicians on how to motivate climate action.

A final note on uncertainty: Denialists often use it as evidence that climate science isn’t settled. However, in science, uncertainty doesn’t imply a lack of knowledge. Rather, it’s a measure of how well something is known. In the case of climate change, scientists have found a range of possible future changes in temperature, precipitation and other important variables — which will depend largely on how quickly we reduce emissions. But uncertainty does not undermine their confidence that climate change is real and that people are causing it.

Earth’s climate is inherently variable. Some years are hot and others are cold, some decades bring more hurricanes than others, some ancient droughts spanned the better part of centuries. Glacial cycles operate over many millenniums. So how can scientists look at data collected over a relatively short period of time and conclude that humans are warming the planet? The answer is that the instrumental temperature data that we have tells us a lot, but it’s not all we have to go on.

Historical records stretch back to the 1880s (and often before), when people began to regularly measure temperatures at weather stations and on ships as they traversed the world’s oceans. These data show a clear warming trend during the 20th century.

Global average temperature compared with the middle of the 20th century

+0.75°C

–0.25°

Some have questioned whether these records could be skewed, for instance, by the fact that a disproportionate number of weather stations are near cities, which tend to be hotter than surrounding areas as a result of the so-called urban heat island effect. However, researchers regularly correct for these potential biases when reconstructing global temperatures. In addition, warming is corroborated by independent data like satellite observations, which cover the whole planet, and other ways of measuring temperature changes.

Much has also been made of the small dips and pauses that punctuate the rising temperature trend of the last 150 years. But these are just the result of natural climate variability or other human activities that temporarily counteract greenhouse warming. For instance, in the mid-1900s, internal climate dynamics and light-blocking pollution from coal-fired power plants halted global warming for a few decades. (Eventually, rising greenhouse gases and pollution-control laws caused the planet to start heating up again.) Likewise, the so-called warming hiatus of the 2000s was partly a result of natural climate variability that allowed more heat to enter the ocean rather than warm the atmosphere. The years since have been the hottest on record .

Still, could the entire 20th century just be one big natural climate wiggle? To address that question, we can look at other kinds of data that give a longer perspective. Researchers have used geologic records like tree rings, ice cores, corals and sediments that preserve information about prehistoric climates to extend the climate record. The resulting picture of global temperature change is basically flat for centuries, then turns sharply upward over the last 150 years. It has been a target of climate denialists for decades. However, study after study has confirmed the results , which show that the planet hasn’t been this hot in at least 1,000 years, and probably longer.

Scientists have studied past climate changes to understand the factors that can cause the planet to warm or cool. The big ones are changes in solar energy, ocean circulation, volcanic activity and the amount of greenhouse gases in the atmosphere. And they have each played a role at times.

For example, 300 years ago, a combination of reduced solar output and increased volcanic activity cooled parts of the planet enough that Londoners regularly ice skated on the Thames . About 12,000 years ago, major changes in Atlantic circulation plunged the Northern Hemisphere into a frigid state. And 56 million years ago, a giant burst of greenhouse gases, from volcanic activity or vast deposits of methane (or both), abruptly warmed the planet by at least 9 degrees Fahrenheit, scrambling the climate, choking the oceans and triggering mass extinctions.

In trying to determine the cause of current climate changes, scientists have looked at all of these factors . The first three have varied a bit over the last few centuries and they have quite likely had modest effects on climate , particularly before 1950. But they cannot account for the planet’s rapidly rising temperature, especially in the second half of the 20th century, when solar output actually declined and volcanic eruptions exerted a cooling effect.

That warming is best explained by rising greenhouse gas concentrations . Greenhouse gases have a powerful effect on climate (see the next question for why). And since the Industrial Revolution, humans have been adding more of them to the atmosphere, primarily by extracting and burning fossil fuels like coal, oil and gas, which releases carbon dioxide.

Bubbles of ancient air trapped in ice show that, before about 1750, the concentration of carbon dioxide in the atmosphere was roughly 280 parts per million. It began to rise slowly and crossed the 300 p.p.m. threshold around 1900. CO2 levels then accelerated as cars and electricity became big parts of modern life, recently topping 420 p.p.m . The concentration of methane, the second most important greenhouse gas, has more than doubled. We’re now emitting carbon much faster than it was released 56 million years ago .

30 billion metric tons

Carbon dioxide emitted worldwide 1850-2017

Rest of world

Other developed

European Union

Developed economies

Other countries

United States

E.U. and U.K.

These rapid increases in greenhouse gases have caused the climate to warm abruptly. In fact, climate models suggest that greenhouse warming can explain virtually all of the temperature change since 1950. According to the most recent report by the Intergovernmental Panel on Climate Change, which assesses published scientific literature, natural drivers and internal climate variability can only explain a small fraction of late-20th century warming.

Another study put it this way: The odds of current warming occurring without anthropogenic greenhouse gas emissions are less than 1 in 100,000 .

But greenhouse gases aren’t the only climate-altering compounds people put into the air. Burning fossil fuels also produces particulate pollution that reflects sunlight and cools the planet. Scientists estimate that this pollution has masked up to half of the greenhouse warming we would have otherwise experienced.

Greenhouse gases like water vapor and carbon dioxide serve an important role in the climate. Without them, Earth would be far too cold to maintain liquid water and humans would not exist!

Here’s how it works: the planet’s temperature is basically a function of the energy the Earth absorbs from the sun (which heats it up) and the energy Earth emits to space as infrared radiation (which cools it down). Because of their molecular structure, greenhouse gases temporarily absorb some of that outgoing infrared radiation and then re-emit it in all directions, sending some of that energy back toward the surface and heating the planet . Scientists have understood this process since the 1850s .

Greenhouse gas concentrations have varied naturally in the past. Over millions of years, atmospheric CO2 levels have changed depending on how much of the gas volcanoes belched into the air and how much got removed through geologic processes. On time scales of hundreds to thousands of years, concentrations have changed as carbon has cycled between the ocean, soil and air.

Today, however, we are the ones causing CO2 levels to increase at an unprecedented pace by taking ancient carbon from geologic deposits of fossil fuels and putting it into the atmosphere when we burn them. Since 1750, carbon dioxide concentrations have increased by almost 50 percent. Methane and nitrous oxide, other important anthropogenic greenhouse gases that are released mainly by agricultural activities, have also spiked over the last 250 years.

We know based on the physics described above that this should cause the climate to warm. We also see certain telltale “fingerprints” of greenhouse warming. For example, nights are warming even faster than days because greenhouse gases don’t go away when the sun sets. And upper layers of the atmosphere have actually cooled, because more energy is being trapped by greenhouse gases in the lower atmosphere.

We also know that we are the cause of rising greenhouse gas concentrations — and not just because we can measure the CO2 coming out of tailpipes and smokestacks. We can see it in the chemical signature of the carbon in CO2.

Carbon comes in three different masses: 12, 13 and 14. Things made of organic matter (including fossil fuels) tend to have relatively less carbon-13. Volcanoes tend to produce CO2 with relatively more carbon-13. And over the last century, the carbon in atmospheric CO2 has gotten lighter, pointing to an organic source.

We can tell it’s old organic matter by looking for carbon-14, which is radioactive and decays over time. Fossil fuels are too ancient to have any carbon-14 left in them, so if they were behind rising CO2 levels, you would expect the amount of carbon-14 in the atmosphere to drop, which is exactly what the data show .

It’s important to note that water vapor is the most abundant greenhouse gas in the atmosphere. However, it does not cause warming; instead it responds to it . That’s because warmer air holds more moisture, which creates a snowball effect in which human-caused warming allows the atmosphere to hold more water vapor and further amplifies climate change. This so-called feedback cycle has doubled the warming caused by anthropogenic greenhouse gas emissions.

A common source of confusion when it comes to climate change is the difference between weather and climate. Weather is the constantly changing set of meteorological conditions that we experience when we step outside, whereas climate is the long-term average of those conditions, usually calculated over a 30-year period. Or, as some say: Weather is your mood and climate is your personality.

So while 2 degrees Fahrenheit doesn’t represent a big change in the weather, it’s a huge change in climate. As we’ve already seen, it’s enough to melt ice and raise sea levels, to shift rainfall patterns around the world and to reorganize ecosystems, sending animals scurrying toward cooler habitats and killing trees by the millions.

It’s also important to remember that two degrees represents the global average, and many parts of the world have already warmed by more than that. For example, land areas have warmed about twice as much as the sea surface. And the Arctic has warmed by about 5 degrees. That’s because the loss of snow and ice at high latitudes allows the ground to absorb more energy, causing additional heating on top of greenhouse warming.

Relatively small long-term changes in climate averages also shift extremes in significant ways. For instance, heat waves have always happened, but they have shattered records in recent years. In June of 2020, a town in Siberia registered temperatures of 100 degrees . And in Australia, meteorologists have added a new color to their weather maps to show areas where temperatures exceed 125 degrees. Rising sea levels have also increased the risk of flooding because of storm surges and high tides. These are the foreshocks of climate change.

And we are in for more changes in the future — up to 9 degrees Fahrenheit of average global warming by the end of the century, in the worst-case scenario . For reference, the difference in global average temperatures between now and the peak of the last ice age, when ice sheets covered large parts of North America and Europe, is about 11 degrees Fahrenheit.

Under the Paris Climate Agreement, which President Biden recently rejoined, countries have agreed to try to limit total warming to between 1.5 and 2 degrees Celsius, or 2.7 and 3.6 degrees Fahrenheit, since preindustrial times. And even this narrow range has huge implications . According to scientific studies, the difference between 2.7 and 3.6 degrees Fahrenheit will very likely mean the difference between coral reefs hanging on or going extinct, and between summer sea ice persisting in the Arctic or disappearing completely. It will also determine how many millions of people suffer from water scarcity and crop failures, and how many are driven from their homes by rising seas. In other words, one degree Fahrenheit makes a world of difference.

Earth’s climate has always changed. Hundreds of millions of years ago, the entire planet froze . Fifty million years ago, alligators lived in what we now call the Arctic . And for the last 2.6 million years, the planet has cycled between ice ages when the planet was up to 11 degrees cooler and ice sheets covered much of North America and Europe, and milder interglacial periods like the one we’re in now.

Climate denialists often point to these natural climate changes as a way to cast doubt on the idea that humans are causing climate to change today. However, that argument rests on a logical fallacy. It’s like “seeing a murdered body and concluding that people have died of natural causes in the past, so the murder victim must also have died of natural causes,” a team of social scientists wrote in The Debunking Handbook , which explains the misinformation strategies behind many climate myths.

Indeed, we know that different mechanisms caused the climate to change in the past. Glacial cycles, for example, were triggered by periodic variations in Earth’s orbit , which take place over tens of thousands of years and change how solar energy gets distributed around the globe and across the seasons.

These orbital variations don’t affect the planet’s temperature much on their own. But they set off a cascade of other changes in the climate system; for instance, growing or melting vast Northern Hemisphere ice sheets and altering ocean circulation. These changes, in turn, affect climate by altering the amount of snow and ice, which reflect sunlight, and by changing greenhouse gas concentrations. This is actually part of how we know that greenhouse gases have the ability to significantly affect Earth’s temperature.

For at least the last 800,000 years , atmospheric CO2 concentrations oscillated between about 180 parts per million during ice ages and about 280 p.p.m. during warmer periods, as carbon moved between oceans, forests, soils and the atmosphere. These changes occurred in lock step with global temperatures, and are a major reason the entire planet warmed and cooled during glacial cycles, not just the frozen poles.

Today, however, CO2 levels have soared to 420 p.p.m. — the highest they’ve been in at least three million years . The concentration of CO2 is also increasing about 100 times faster than it did at the end of the last ice age. This suggests something else is going on, and we know what it is: Since the Industrial Revolution, humans have been burning fossil fuels and releasing greenhouse gases that are heating the planet now (see Question 5 for more details on how we know this, and Questions 4 and 8 for how we know that other natural forces aren’t to blame).

Over the next century or two, societies and ecosystems will experience the consequences of this climate change. But our emissions will have even more lasting geologic impacts: According to some studies, greenhouse gas levels may have already warmed the planet enough to delay the onset of the next glacial cycle for at least an additional 50,000 years.

The sun is the ultimate source of energy in Earth’s climate system, so it’s a natural candidate for causing climate change. And solar activity has certainly changed over time. We know from satellite measurements and other astronomical observations that the sun’s output changes on 11-year cycles. Geologic records and sunspot numbers, which astronomers have tracked for centuries, also show long-term variations in the sun’s activity, including some exceptionally quiet periods in the late 1600s and early 1800s.

We know that, from 1900 until the 1950s, solar irradiance increased. And studies suggest that this had a modest effect on early 20th century climate, explaining up to 10 percent of the warming that’s occurred since the late 1800s. However, in the second half of the century, when the most warming occurred, solar activity actually declined . This disparity is one of the main reasons we know that the sun is not the driving force behind climate change.

Another reason we know that solar activity hasn’t caused recent warming is that, if it had, all the layers of the atmosphere should be heating up. Instead, data show that the upper atmosphere has actually cooled in recent decades — a hallmark of greenhouse warming .

So how about volcanoes? Eruptions cool the planet by injecting ash and aerosol particles into the atmosphere that reflect sunlight. We’ve observed this effect in the years following large eruptions. There are also some notable historical examples, like when Iceland’s Laki volcano erupted in 1783, causing widespread crop failures in Europe and beyond, and the “ year without a summer ,” which followed the 1815 eruption of Mount Tambora in Indonesia.

Since volcanoes mainly act as climate coolers, they can’t really explain recent warming. However, scientists say that they may also have contributed slightly to rising temperatures in the early 20th century. That’s because there were several large eruptions in the late 1800s that cooled the planet, followed by a few decades with no major volcanic events when warming caught up. During the second half of the 20th century, though, several big eruptions occurred as the planet was heating up fast. If anything, they temporarily masked some amount of human-caused warming.

The second way volcanoes can impact climate is by emitting carbon dioxide. This is important on time scales of millions of years — it’s what keeps the planet habitable (see Question 5 for more on the greenhouse effect). But by comparison to modern anthropogenic emissions, even big eruptions like Krakatoa and Mount St. Helens are just a drop in the bucket. After all, they last only a few hours or days, while we burn fossil fuels 24-7. Studies suggest that, today, volcanoes account for 1 to 2 percent of total CO2 emissions.

When a big snowstorm hits the United States, climate denialists can try to cite it as proof that climate change isn’t happening. In 2015, Senator James Inhofe, an Oklahoma Republican, famously lobbed a snowball in the Senate as he denounced climate science. But these events don’t actually disprove climate change.

While there have been some memorable storms in recent years, winters are actually warming across the world. In the United States, average temperatures in December, January and February have increased by about 2.5 degrees this century.

On the flip side, record cold days are becoming less common than record warm days. In the United States, record highs now outnumber record lows two-to-one . And ever-smaller areas of the country experience extremely cold winter temperatures . (The same trends are happening globally.)

So what’s with the blizzards? Weather always varies, so it’s no surprise that we still have severe winter storms even as average temperatures rise. However, some studies suggest that climate change may be to blame. One possibility is that rapid Arctic warming has affected atmospheric circulation, including the fast-flowing, high-altitude air that usually swirls over the North Pole (a.k.a. the Polar Vortex ). Some studies suggest that these changes are bringing more frigid temperatures to lower latitudes and causing weather systems to stall , allowing storms to produce more snowfall. This may explain what we’ve experienced in the U.S. over the past few decades, as well as a wintertime cooling trend in Siberia , although exactly how the Arctic affects global weather remains a topic of ongoing scientific debate .

Climate change may also explain the apparent paradox behind some of the other places on Earth that haven’t warmed much. For instance, a splotch of water in the North Atlantic has cooled in recent years, and scientists say they suspect that may be because ocean circulation is slowing as a result of freshwater streaming off a melting Greenland . If this circulation grinds almost to a halt, as it’s done in the geologic past, it would alter weather patterns around the world.

Not all cold weather stems from some counterintuitive consequence of climate change. But it’s a good reminder that Earth’s climate system is complex and chaotic, so the effects of human-caused changes will play out differently in different places. That’s why “global warming” is a bit of an oversimplification. Instead, some scientists have suggested that the phenomenon of human-caused climate change would more aptly be called “ global weirding .”

Extreme weather and natural disasters are part of life on Earth — just ask the dinosaurs. But there is good evidence that climate change has increased the frequency and severity of certain phenomena like heat waves, droughts and floods. Recent research has also allowed scientists to identify the influence of climate change on specific events.

Let’s start with heat waves . Studies show that stretches of abnormally high temperatures now happen about five times more often than they would without climate change, and they last longer, too. Climate models project that, by the 2040s, heat waves will be about 12 times more frequent. And that’s concerning since extreme heat often causes increased hospitalizations and deaths, particularly among older people and those with underlying health conditions. In the summer of 2003, for example, a heat wave caused an estimated 70,000 excess deaths across Europe. (Human-caused warming amplified the death toll .)

Climate change has also exacerbated droughts , primarily by increasing evaporation. Droughts occur naturally because of random climate variability and factors like whether El Niño or La Niña conditions prevail in the tropical Pacific. But some researchers have found evidence that greenhouse warming has been affecting droughts since even before the Dust Bowl . And it continues to do so today. According to one analysis , the drought that afflicted the American Southwest from 2000 to 2018 was almost 50 percent more severe because of climate change. It was the worst drought the region had experienced in more than 1,000 years.

Rising temperatures have also increased the intensity of heavy precipitation events and the flooding that often follows. For example, studies have found that, because warmer air holds more moisture, Hurricane Harvey, which struck Houston in 2017, dropped between 15 and 40 percent more rainfall than it would have without climate change.

It’s still unclear whether climate change is changing the overall frequency of hurricanes, but it is making them stronger . And warming appears to favor certain kinds of weather patterns, like the “ Midwest Water Hose ” events that caused devastating flooding across the Midwest in 2019 .

It’s important to remember that in most natural disasters, there are multiple factors at play. For instance, the 2019 Midwest floods occurred after a recent cold snap had frozen the ground solid, preventing the soil from absorbing rainwater and increasing runoff into the Missouri and Mississippi Rivers. These waterways have also been reshaped by levees and other forms of river engineering, some of which failed in the floods.

Wildfires are another phenomenon with multiple causes. In many places, fire risk has increased because humans have aggressively fought natural fires and prevented Indigenous peoples from carrying out traditional burning practices. This has allowed fuel to accumulate that makes current fires worse .

However, climate change still plays a major role by heating and drying forests, turning them into tinderboxes. Studies show that warming is the driving factor behind the recent increases in wildfires; one analysis found that climate change is responsible for doubling the area burned across the American West between 1984 and 2015. And researchers say that warming will only make fires bigger and more dangerous in the future.

It depends on how aggressively we act to address climate change. If we continue with business as usual, by the end of the century, it will be too hot to go outside during heat waves in the Middle East and South Asia . Droughts will grip Central America, the Mediterranean and southern Africa. And many island nations and low-lying areas, from Texas to Bangladesh, will be overtaken by rising seas. Conversely, climate change could bring welcome warming and extended growing seasons to the upper Midwest , Canada, the Nordic countries and Russia . Farther north, however, the loss of snow, ice and permafrost will upend the traditions of Indigenous peoples and threaten infrastructure.

It’s complicated, but the underlying message is simple: unchecked climate change will likely exacerbate existing inequalities . At a national level, poorer countries will be hit hardest, even though they have historically emitted only a fraction of the greenhouse gases that cause warming. That’s because many less developed countries tend to be in tropical regions where additional warming will make the climate increasingly intolerable for humans and crops. These nations also often have greater vulnerabilities, like large coastal populations and people living in improvised housing that is easily damaged in storms. And they have fewer resources to adapt, which will require expensive measures like redesigning cities, engineering coastlines and changing how people grow food.

Already, between 1961 and 2000, climate change appears to have harmed the economies of the poorest countries while boosting the fortunes of the wealthiest nations that have done the most to cause the problem, making the global wealth gap 25 percent bigger than it would otherwise have been. Similarly, the Global Climate Risk Index found that lower income countries — like Myanmar, Haiti and Nepal — rank high on the list of nations most affected by extreme weather between 1999 and 2018. Climate change has also contributed to increased human migration, which is expected to increase significantly .

Even within wealthy countries, the poor and marginalized will suffer the most. People with more resources have greater buffers, like air-conditioners to keep their houses cool during dangerous heat waves, and the means to pay the resulting energy bills. They also have an easier time evacuating their homes before disasters, and recovering afterward. Lower income people have fewer of these advantages, and they are also more likely to live in hotter neighborhoods and work outdoors, where they face the brunt of climate change.

These inequalities will play out on an individual, community, and regional level. A 2017 analysis of the U.S. found that, under business as usual, the poorest one-third of counties, which are concentrated in the South, will experience damages totaling as much as 20 percent of gross domestic product, while others, mostly in the northern part of the country, will see modest economic gains. Solomon Hsiang, an economist at University of California, Berkeley, and the lead author of the study, has said that climate change “may result in the largest transfer of wealth from the poor to the rich in the country’s history.”

Even the climate “winners” will not be immune from all climate impacts, though. Desirable locations will face an influx of migrants. And as the coronavirus pandemic has demonstrated, disasters in one place quickly ripple across our globalized economy. For instance, scientists expect climate change to increase the odds of multiple crop failures occurring at the same time in different places, throwing the world into a food crisis .

On top of that, warmer weather is aiding the spread of infectious diseases and the vectors that transmit them, like ticks and mosquitoes . Research has also identified troubling correlations between rising temperatures and increased interpersonal violence , and climate change is widely recognized as a “threat multiplier” that increases the odds of larger conflicts within and between countries. In other words, climate change will bring many changes that no amount of money can stop. What could help is taking action to limit warming.

One of the most common arguments against taking aggressive action to combat climate change is that doing so will kill jobs and cripple the economy. But this implies that there’s an alternative in which we pay nothing for climate change. And unfortunately, there isn’t. In reality, not tackling climate change will cost a lot , and cause enormous human suffering and ecological damage, while transitioning to a greener economy would benefit many people and ecosystems around the world.

Let’s start with how much it will cost to address climate change. To keep warming well below 2 degrees Celsius, the goal of the Paris Climate Agreement, society will have to reach net zero greenhouse gas emissions by the middle of this century. That will require significant investments in things like renewable energy, electric cars and charging infrastructure, not to mention efforts to adapt to hotter temperatures, rising sea-levels and other unavoidable effects of current climate changes. And we’ll have to make changes fast.

Estimates of the cost vary widely. One recent study found that keeping warming to 2 degrees Celsius would require a total investment of between $4 trillion and $60 trillion, with a median estimate of $16 trillion, while keeping warming to 1.5 degrees Celsius could cost between $10 trillion and $100 trillion, with a median estimate of $30 trillion. (For reference, the entire world economy was about $88 trillion in 2019.) Other studies have found that reaching net zero will require annual investments ranging from less than 1.5 percent of global gross domestic product to as much as 4 percent . That’s a lot, but within the range of historical energy investments in countries like the U.S.

Now, let’s consider the costs of unchecked climate change, which will fall hardest on the most vulnerable. These include damage to property and infrastructure from sea-level rise and extreme weather, death and sickness linked to natural disasters, pollution and infectious disease, reduced agricultural yields and lost labor productivity because of rising temperatures, decreased water availability and increased energy costs, and species extinction and habitat destruction. Dr. Hsiang, the U.C. Berkeley economist, describes it as “death by a thousand cuts.”

As a result, climate damages are hard to quantify. Moody’s Analytics estimates that even 2 degrees Celsius of warming will cost the world $69 trillion by 2100, and economists expect the toll to keep rising with the temperature. In a recent survey , economists estimated the cost would equal 5 percent of global G.D.P. at 3 degrees Celsius of warming (our trajectory under current policies) and 10 percent for 5 degrees Celsius. Other research indicates that, if current warming trends continue, global G.D.P. per capita will decrease between 7 percent and 23 percent by the end of the century — an economic blow equivalent to multiple coronavirus pandemics every year. And some fear these are vast underestimates .

Already, studies suggest that climate change has slashed incomes in the poorest countries by as much as 30 percent and reduced global agricultural productivity by 21 percent since 1961. Extreme weather events have also racked up a large bill. In 2020, in the United States alone, climate-related disasters like hurricanes, droughts, and wildfires caused nearly $100 billion in damages to businesses, property and infrastructure, compared to an average of $18 billion per year in the 1980s.

Given the steep price of inaction, many economists say that addressing climate change is a better deal . It’s like that old saying: an ounce of prevention is worth a pound of cure. In this case, limiting warming will greatly reduce future damage and inequality caused by climate change. It will also produce so-called co-benefits, like saving one million lives every year by reducing air pollution, and millions more from eating healthier, climate-friendly diets. Some studies even find that meeting the Paris Agreement goals could create jobs and increase global G.D.P . And, of course, reining in climate change will spare many species and ecosystems upon which humans depend — and which many people believe to have their own innate value.

The challenge is that we need to reduce emissions now to avoid damages later, which requires big investments over the next few decades. And the longer we delay, the more we will pay to meet the Paris goals. One recent analysis found that reaching net-zero by 2050 would cost the U.S. almost twice as much if we waited until 2030 instead of acting now. But even if we miss the Paris target, the economics still make a strong case for climate action, because every additional degree of warming will cost us more — in dollars, and in lives.

Veronica Penney contributed reporting.

Illustration photographs by Esther Horvath, Max Whittaker, David Maurice Smith and Talia Herman for The New York Times; Esther Horvath/Alfred-Wegener-Institut

An earlier version of this article misidentified the authors of The Debunking Handbook. It was written by social scientists who study climate communication, not a team of climate scientists.

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3.2: The Greenhouse Effect

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• Page ID 36543

• Laci M. Gerhart-Barley
• College of Biological Sciences - UC Davis

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The process by which the atmosphere absorbs the sun’s energy and prevents it from being radiated back out to space has often been compared to that of a greenhouse, leading to the nickname the greenhouse effect . It is the same process that occurs when you leave your car sitting in the sun with the windows rolled up. The sun’s rays are powerful enough to pass through the glass of the windows (or through the Earth’s atmosphere) and be absorbed by the dashboard and seats (or the Earth’s surface); however, when these surfaces emit energy, it is not powerful enough to pass back out through the window glass (or atmosphere) and in so doing becomes trapped within the car (or atmosphere) causing it to warm. There are certain atmospheric gases, termed greenhouses gases (or GHGs) that behave like the car windows, increasing the amount of energy retained in the atmosphere, and increasing the amount of warming that occurs. In Figure 3.1.1 the greenhouse effect is represented by the curved arrow showing 95% of the energy emitted by the Earth’s surface that is reabsorbed, and the amount of GHGs in the atmosphere drive the size of this arrow.

The primary GHGs considered in this section are carbon dioxide (CO 2 ), methane (CH 3 ), and nitrous oxide (N 2 O). Scientists at the National Oceanic and Atmospheric Administration (NOAA) have been tracking concentrations of these and other GHGs in the atmosphere for decades (Fig 3.2.1) and have found that all three continue to steadily increase.

These increases are particularly pronounced when compared to past GHG concentrations. Scientists can measure past atmospheric composition in ice sheets in Greenland and Antarctica. As the ice sheets formed, small bubbles of air were trapped in the ice, and coring deep into the ice sheet (Fig 3.2.2a) allows scientists to reconstruct atmospheric composition from direct measurements as far back as 800,000 years (Fig 3.2.2b). From these measurements, we can see that atmospheric CO 2 levels (and temperature, as estimated from oxygen isotope composition of the ice itself) have fluctuated significantly throughout the past. The ice sheets document a clear pattern of periodic increases and decreases in CO 2 , which are coupled with increases and decreases in temperature. Periods of low CO 2 and low temperatures are glacial periods (also referred to as ‘ice ages’), and periods of high CO 2 and high temperature are inter-glacial periods. For the past 800,000 years, Earth has oscillated between glacial and inter-glacial periods roughly every 100,000 years. The maximum level that CO 2 concentrations reached in the last 800,000 years was approximately 300 parts per million (ppm). Global CO 2 levels are now over 400 ppm, a level that scientists estimate has not occurred on Earth since the Pliocene Epoch, approximately 3 million years ago.

This increase in CO 2 (as well as other GHGs) increases the amount of solar energy that is retained in the atmosphere as opposed to radiated back out to space, which increases the temperature of the Earth’s surface. Figure 3.2.3 shows the temperature anomaly for the decade 2014-2018 as compared to the average from 1951-1980. The National Aeronautics and Space Administration (NASA) reports that the global average temperature has increased approximately 1.4° Fahrenheit (0.8° C) since 1880, though that warming has not been evenly distributed across the Earth. As can be seen in Figure 3.2.3, the polar regions, particularly the Arctic, have warmed much more than other areas. This pattern is particularly concerning given the feedbacks that warming in polar regions may have on the melting of ice sheets and sea ice.

In 2018, the state of California released its Fourth Climate Change Assessment report, which outlines the impact that global climatic changes are having and will have on the state (Fig 3.2.4). Some regions, most notably southern California, have already experienced nearly 3°F increases in annual average temperatures since the beginning of the 20th century.

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Greenhouse Effect

By thomas schelling.

By Thomas Schelling,

What Is It?

The “greenhouse effect” is a complicated process by which the earth is becoming progressively warmer. The earth is bathed in sunlight, some of it reflected back into space and some absorbed. If the absorption is not matched by radiation back into space, the earth will get warmer until the intensity of that radiation matches the incoming sunlight. Some atmospheric gases absorb outward infrared radiation, warming the atmosphere. Carbon dioxide is one of these gases; so are methane, nitrous oxide, and the chlorofluorocarbons (CFCs). The concentrations of these gases are increasing, with the result that the earth is absorbing more sunlight and getting warmer.

This greenhouse phenomenon is truly the result of a “global common” (see The Tragedy of the Commons ). Because no one owns the atmosphere, no one has a sufficient incentive to take account of the change to the atmosphere caused by his or her emission of carbon. Also, carbon emitted has the same effect no matter where on earth it happens.

How Serious Is It?

The expected change in global average temperature for a doubling of CO 2 is 1.5 to 4.5 degrees centigrade. But translating a change in temperature into a change in climates is full of uncertainties. Meteorologists predict greater temperature change in the polar regions than near the equator. This change could cause changes in circulation of air and water. The results may be warmer temperatures in some places and colder in others, wetter climates in some places and drier in others.

Temperature is useful as an index of climate change. A band of about one degree covers variations in average temperatures since the last ice age. This means that climates will change more in the next one hundred years than in the last ten thousand. But to put this in perspective, remember that people have been migrating great distances for thousands of years, experiencing changes in climate greater than any being forecast.

The models of global warming project only gradual changes. Climates will “migrate” slowly. The climate of Kansas may become like Oklahoma’s, but not like that of Oregon or Massachusetts. But a caveat is in order: the models probably cannot project discontinuities because nothing goes into them that will produce drastic change. There may be phenomena that could produce drastic changes, but they are not known with enough confidence to introduce into the models.

Carbon dioxide has increased about 25 percent since the onset of the industrial revolution. The global average temperature rose almost half a degree during the first forty years of this century, was level for the next forty, and rose during the eighties. Yet whether or not we are witnessing the greenhouse effect is unknown because other decades-long influences such as changes in solar intensity and in the atmosphere’s particulate matter can obscure any smooth greenhouse trend. In other words, the increase in carbon dioxide will, by itself, cause the greenhouse effect, but other changes in the universe may offset it.

Even if we had confident estimates of climate change for different regions of the world, there would be uncertainties about the kind of world we will have fifty or a hundred years from now. Suppose the kind of climate change expected between now and, say, 2080 had already taken place, since 1900. Ask a seventy-five-year-old farm couple living on the same farm where they were born: would the change in the climate be among the most dramatic changes in either their farming or their lifestyle? The answer most likely would be no. Changes from horses to tractors and from kerosene to electricity would be much more important.

Climate change would have made a vastly greater difference to the way people lived and earned their living in 1900 than today. Today, little of our gross domestic product is produced outdoors, and therefore, little is susceptible to climate. Agriculture and forestry are less than 3 percent of total output, and little else is much affected. Even if agricultural productivity declined by a third over the next half-century, the per capita GNP we might have achieved by 2050 we would still achieve in 2051. Considering that agricultural productivity in most parts of the world continues to improve (and that many crops may benefit directly from enhanced photosynthesis due to increased carbon dioxide), it is not at all certain that the net impact on agriculture will be negative or much noticed in the developed world.

Its Effects on Developing Countries

Climate changes would have greater impact in underdeveloped countries. Agriculture provides the livelihoods of 30 percent or more of the population in much of the developing world. While there is no strong presumption that the climates prevailing in different regions fifty or a hundred years from now will be less conducive to food production, those people are vulnerable in a way that Americans and west Europeans are not. Nor can the impact on their health be dismissed. Parasitic and other vectorborne diseases affecting hundreds of millions of people are sensitive to climate.

Yet the trend in developing countries is to be less dependent on agriculture. If per capita income in such countries grows in the next forty years as rapidly as it has in the forty just past, vulnerability to climate change should diminish. This is pertinent to whether developing countries should make sacrifices to minimize the emission of gases that may change climate to their disadvantage. Their best defense against climate change will be their own continued development.

Population is an important factor. Carbon emissions in developing countries rise with population. For instance, if China holds population growth to near zero for the next couple of generations, it may do as much for the earth’s atmosphere as would a heroic anticarbon program coupled with 2 percent annual population growth. Furthermore, the most likely adverse impact of climate change would be on food production, and in the poorest parts of the world the adequacy of food depends on the number of mouths.

Why Should Developed Countries Do Anything?

Why might developed countries care enough about climate to do anything about it? The answer depends on how much people in developed countries care about people in developing countries and on how expensive it is to do something worthwhile. Abatement programs in a number of econometric models suggest that doing something worthwhile would cost about 2 percent of GNP in perpetuity. Two percent of the U.S. GNP is over $100 billion a year, and that is an annual cost that would continue forever.

One argument for doing something is that the developing countries are vulnerable, and we care about their well-being. But if the developed countries were prepared to invest, say, $200 billion a year in greenhouse gas abatement, explicitly for the benefit of developing countries fifty years or more from now, the developing countries would probably clamor, understandably, to receive the resources immediately in support of their continued development.

A second argument is that our natural environment may be severely damaged. This is the crux of the political debate over the greenhouse effect, but it is an issue that no one really understands. It is difficult to know how to value what is at risk, and difficult even to know just what is at risk. The benefits of slowing climate change by some particular amount are even more uncertain.

A third argument is that the conclusion I reported earlier—that climates will change slowly and not much—may be wrong. The models do not produce surprises. The possibility has to be considered that some atmospheric or oceanic circulatory systems may flip to alternative equilibria, producing regional changes that are sudden and extreme. A currently discussed possibility is in the way oceans behave. If the gulf stream flipped into a new pattern, the climatic consequences might be sudden and severe. (Paradoxically, global warming might severely cool western Europe.)

Is 2 percent of GNP forever, to postpone the doubling of carbon in the atmosphere, a big number or a small one? That depends on what the comparison is. A better question—assuming we were prepared to spend 2 percent of GNP to reduce the damage from climate change—is whether we might find better uses for the money.

I mentioned one such use—directly investing to improve the economies of the poorer countries. Another would be direct investment in preserving species or ecosystems or wilderness areas, if the alternative is to invest trillions in the reduction of carbon emissions.

What Solutions Are Proposed?

What can be done to reduce or offset carbon emissions? Reducing energy use and the carbon content of energy have received most of the attention. There are other possibilities. Trees store carbon. A new forest will absorb carbon until it reaches maturity; it then holds its carbon but does not absorb more. The area available for reforestation throughout the world suggests that reforestation can contribute, but not much.

Stopping or slowing deforestation is important for other reasons but is quantitatively more important than reforestation, partly because forest subsoils typically contain carbon greater than the amount in the trees themselves, and this carbon is subject to oxidation when the trees are removed.

Also, substances or objects can be put in orbit or in the stratosphere to reflect incoming sunlight. Some of these are as apparently innocuous as stimulating cloud formation and some as dramatic as huge mylar balloons in low earth orbit. If in decades to come the greenhouse impact confirms the more alarmist expectations, and if the costs of reducing emissions prove unmanageable, some of these “geoengineering” options will invite attention.

The main responses will be to adapt as the climate changes and to reduce carbon emissions. (CFCs are potent greenhouse gases and, if unchecked, might have rivaled carbon dioxide in decades to come. International actions to reduce or eliminate CFCs are making progress and are among the cheapest ways of reducing greenhouse emissions.)

It is improbable that the developing world, at least for the next several decades, will incur any significant sacrifice in the interest of reduced carbon, nor would it be advisable. Financing energy conservation, energy efficiency, and a switch from high-carbon to lower-carbon or noncarbon fuels in Asia and Africa would not only be a major economic enterprise, but also a complex effort in international diplomacy and politics. If successful, it would increase the costs to the developed world by at least another percent or two on top of the 2 percent I mentioned.

A universal carbon tax is a popular proposal among economists because it promises an efficient solution. A carbon tax set equally for all users worldwide would achieve a given reduction in the use of carbon at the lowest cost. If user A values his use of one ton of carbon at two thousand dollars more than its net-of-tax price, and if the tax is four hundred dollars per ton, he will continue to use the carbon because doing so is worthwhile. If user B values his use of one ton at only three hundred dollars more than the net-of-tax price, the tax will induce him to end his use. Thus the tax would eliminate the lowest-valued uses of carbon and would leave the highest-valued ones in place. A carbon tax would require no negotiation except over a tax rate and a formula for distributing the proceeds. But a tax rate that made a big dent in the greenhouse problem would have to be equivalent to around a dollar per gallon on motor fuel, and for the United States alone such a tax on coal, petroleum, and natural gas would currently yield close to half a trillion dollars per year in revenue, almost 10 percent of our GNP. It is doubtful that any greenhouse taxing agency would be allowed to collect that kind of revenue, or that a treaty requiring the United States to levy internal carbon taxation at that level would be ratified.

Tradable permits have been proposed as an alternative to the tax. The main possibilities are estimating “reasonable” emissions country by country and establishing commensurate quotas, or distributing tradable rights in accordance with some “equitable” criterion. Depending on how restrictive the emission rights might be, the latter amounts to distributing trillions of dollars (in present value terms), an unlikely prospect. If quotas are negotiated to correspond to countries’ currently “reasonable” emissions levels, they will surely be renegotiated every few years, and selling an emissions right will be perceived as evidence that a quota was initially too generous.

A helpful model for conceptualizing a greenhouse regime among the richer countries is the negotiations among the nations of Western Europe for distributing Marshall Plan aid after World War II. There was never a formula or explicit criterion, such as equalizing living standards, maximizing aggregate growth, or establishing a floor under levels of living. Baseline dollar-balance-of-payments deficits were a point of departure, but the negotiations took into account other factors such as investment needs and traditional consumption levels. The United States insisted that the recipients argue out and agree on shares. In the end they did not quite make it, the United States having to make the final allocation. But all the submission of data and open argument led, if not to consensus, to a reasonable appreciation of each nation’s needs. Distribution of Marshall Plan funds is the only model of multilateral negotiation involving resources commensurate with the cost of greenhouse abatement. (In the first year Marshall Plan funds were about 1.5 percent of U.S. GNP and—adjusting for overvalued currencies—probably 5 percent of recipient countries’ GNP.)

What the Marshall Plan model suggests is that the participants in a greenhouse regime would submit for each other’s scrutiny and cross-examination plans for reducing carbon emissions. The plans would be accompanied by estimates of emissions, but any commitments would be to the policies, not the emissions.

The alternative is commitments to specific levels of emissions. Because target dates would be a decade or two in the future, monitoring a country’s progress would be more ambiguous than monitoring the implementation of policies.

______________________________________________________________________________________________________________________________

Thomas C. Schelling is a professor of economics at the University of Maryland School of Public Affairs in College Park. For most of his professional life he was an economics professor at Harvard University. In 1991 he was president of the American Economic Association. He is an elected member of the National Academy of Sciences.

Ausubel, Jesse. “Does Climate Still Matter?” Nature 350, April 25, 1991, 649-52.

Cline, William R. The Greenhouse Effect: Global Economic Consequences. 1992.

Congressional Budget Office. Carbon Charges as a Response to Global Warming: The Effects of Taxing Fossil Fuels. 1990.

Dornbush, Rudiger, and James M. Poterba. Global Warming: Economic Policy Responses. 1991.

Nordhaus, William D. “The Cost of Slowing Climate Change: A Survey.” Energy Journal 12, no. 1 (1991): 37-66.

_______________________________________________________________________________________________________________________________

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Robert Bradley and Richard Fulmer, Those Old Oil Company Ads: Misleading, False, or Simply Reasonable? at Econlib, March 2, 2022.

Bryan Caplan, The Moral Case for Fossil Fuels: We Can Live With Warming , at EconLog, December 12, 2014.

Robert Murphy, The Economics of Climate Change , at Econlib, July 2009.

Pedro Schwartz, Climate Change: A Tragedy of the Commons? at Econlib, March 2020.

Pedro Schwartz, Climate Change: What is (Not) To Be Done , at Econlib, April 2020.

Judith Curry on Climate Change , EconTak podcast, December 23, 2013.

Martin Weitzman on Climate Change , EconTalk podcast, June 1, 2015.

RELATED CONTENT By Amy Willis

Data versus drama.

How Do We Reduce Greenhouse Gases?

To stop climate change , we need to stop the amount of greenhouse gases, like carbon dioxide, from increasing. For the past 150 years, burning fossil fuels and cutting down forests, which naturally pull carbon dioxide out of the air, has caused greenhouse gas levels to increase. There are two main ways to stop the amount of greenhouse gases from increasing: we can stop adding them to the air, and we can increase the Earth’s ability to pull them out of the air.

This is called climate mitigation . There is not one single way to mitigate climate change. Instead, we will have to piece together many different solutions to stop the climate from warming. Below are descriptions of the main methods that we can use.

Many of these solutions are already being implemented in places around the world. Some can be tackled by individuals, such as using less energy, riding a bike instead of driving, driving an electric car, and switching to renewable energy. Other actions to mitigate climate change involve communities, regions, or nations working together to make changes, such as switching power plants from burning coal or gas to renewable energy and growing public transit.

Use less electricity.

Taking steps to use less electricity, especially when it comes from burning coal or gas, can take a big bite out of greenhouse gas emissions. Worldwide, electricity use is responsible for a quarter of all emissions. 

Some steps that you can take to use less electricity are simple and save money, like replacing incandescent light bulbs with LED bulbs that use less electricity, adding insulation to your home, and setting the thermostat lower in the winter and higher in the summer, especially when no one is home. There are also new technologies that help keep buildings energy efficient, such as glass that reflects heat, low-flow water fixtures, smart thermostats, and new air conditioning technology with refrigerants that don’t cause warming. In urban and suburban environments, green or cool roofs can limit the amount of heat that gets into buildings during hot days and help decrease the urban heat island effect .

Green roof on the Walter Reed Community Center in Arlington, VA, US Credit: Arlington County on Flickr/CC BY-SA 2.0

Generate electricity without emissions.

Renewable energy sources include solar energy, geothermal energy, wind turbines, ocean wave and tidal energy, waste and biomass energy, and hydropower. Because they do not burn fossil fuels, these renewable energy sources do not release greenhouse gases into the atmosphere as they generate electricity. Nuclear energy also creates no greenhouse gas emissions, so it can be thought of as a solution to climate change. However, it does generate radioactive waste that needs long-term, secure storage.

Today, the amount of electricity that comes from renewable energy is growing. A few countries, such as Iceland and Costa Rica, now get nearly all of their electricity from renewable energy. In many other countries, the percentage of electricity from renewable sources is currently small (5 - 10%) but growing.

Wind turbines can be on land or in the ocean, where high winds are common. Credit: Nicholas Doherty on Unsplash

Shrink the footprint of food.

Today, about a fifth of global carbon emissions come from raising farm animals for meat. For example, as cattle digest food they burp, releasing methane, a powerful greenhouse gas, and their manure releases the greenhouse gases carbon dioxide and nitrous oxide. And forests, which take carbon dioxide out of the air, are often cut down so that cattle have space to graze.

Eating a diet that is mostly or entirely plant-based (such as vegetables, bread, rice, and beans) lowers emissions. According to the Drawdown Project , if half the population worldwide adopts a plant-rich diet by 2050, 65 gigatons of carbon dioxide would be kept out of the atmosphere over about 30 years. (For a sense of scale, 65 gigatons of carbon dioxide is nearly two-years-worth of recent emissions from fossil fuels and industry.) Reducing food waste can make an even larger impact, saving about 90 gigatons of carbon dioxide from the atmosphere over 30 years.

Eating a plant-rich diet lowers greenhouse gas emissions. Credit: Victoria Shes on Unsplash

Travel without making greenhouse gases.

Most of the ways we have to get from place to place currently rely on fossil fuels: gasoline for vehicles and jet fuel for planes. Burning fossil fuels for transportation adds up to 14% of global greenhouse gas emissions worldwide. We can reduce emissions by shifting to alternative technologies that either don’t need gasoline (like bicycles and electric cars) or don’t need as much (like hybrid cars). Using public transportation, carpooling, biking, and walking leads to fewer vehicles on the road and less greenhouse gases in the atmosphere. Cities and towns can make it easier for people to lower greenhouse gas emissions by adding bus routes, bike paths, and sidewalks.

Electric bicycles can be a way to get around without burning gasoline. Credit: Karlis Dambrans/CC BY 2.0

Reduce household waste.

Waste we put in landfills releases greenhouse gases. Almost half the gas released by landfill waste is methane, which is an especially potent greenhouse gas. Landfills are, in fact, the third largest source of methane emissions in the U.S., behind natural gas/petroleum use and animals raised for food production (and their manure). In the U.S., each member of a household produces an average of 2 kg (4.4 lbs) of trash per day. That's 726 kg (1660 lbs) of trash per person per year! Conscious choices, including avoiding unnecessary purchases, buying secondhand, eliminating reliance on single-use containers, switching to reusable bags, bottles, and beverage cups, reducing paper subscriptions and mail in favor of digital options, recycling, and composting, can all help reduce household waste.     

Reduce emissions from industry.

Manufacturing, mining for raw materials, and dealing with the waste all take energy. Most of the products that we buy — everything from phones and TVs to clothing and shoes — are created in factories, which produce up to about 20% of the greenhouse gases emitted worldwide.

There are ways to decrease emissions from manufacturing. Using materials that aren’t made from fossil fuels and don’t release greenhouse gases is a good start. For example, cement releases carbon dioxide as it hardens, but there are alternative products that don’t create greenhouse gases. Similarly, bioplastics made from plants are an alternative to plastics that come from fossil fuels. Companies can also use renewable energy sources to power factories and ship the products that they create in fuel-saving cargo ships.

Take carbon dioxide out of the air.

Along with reducing the amount of carbon dioxide that we add to the air, we can also take action to increase the amount of carbon dioxide we take out of the air. The places where carbon dioxide is pulled out of the air are called carbon sinks. For example, planting trees, bamboo, and other plants increases the number of carbon sinks. Conserving forests, grasslands, peatlands, and wetlands, where carbon is held in plants and soils, protects existing carbon sinks. Farming methods such as planting cover crops and crop rotation keep soils healthy so that they are effective carbon sinks. There are also carbon dioxide removal technologies, which may be able to pull large amounts of greenhouse gases out of the atmosphere.

As the trees and other plants in a forest use sunlight to create the food they need, they are also pulling carbon dioxide out of the air. Credit: B NW on Unsplash

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Greenhouse effect.

Global warming is a major concern for modern day society. As global temperatures rise, the glacial caps melt and species of plants and animals adapted to a specific climate are put at risk of extinction. A major contributing factor to this global warming is a process called the greenhouse effect. As human activity increases, so does the global CO2 levels. CO2 is a heavy gas, meaning that it will pass visible light, while reflecting infrared light. This causes the CO2 in the atmosphere to build and retain heat. The rapid increase in CO2 levels is one of the major causes of the increase in the global temperature.

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Greenhouse Effect

Table of Contents

What is the Greenhouse Effect?

Greenhouse gases, causes of greenhouse effect, effects of greenhouse effect, runaway greenhouse effect, greenhouse effect definition.

“Greenhouse effect is the process by which radiations from the sun are absorbed by the greenhouse gases and not reflected back into space. This insulates the surface of the earth and prevents it from freezing.”

A greenhouse is a house made of glass that can be used to grow plants. The sun’s radiations warm the plants and the air inside the greenhouse. The heat trapped inside can’t escape out and warms the greenhouse which is essential for the growth of the plants. Same is the case in the earth’s atmosphere.

During the day the sun heats up the earth’s atmosphere. At night, when the earth cools down the heat is radiated back into the atmosphere. During this process, the heat is absorbed by the greenhouse gases in the earth’s atmosphere. This is what makes the surface of the earth warmer, that makes the survival of living beings on earth possible.

However, due to the increased levels of greenhouse gases, the temperature of the earth has increased considerably. This has led to several drastic effects.

Let us have a look at the greenhouse gases and understand the causes and consequences of greenhouse effects with the help of a diagram.

Also Read:  Global Warming

“Greenhouse gases are the gases that absorb the infrared radiations and create a greenhouse effect. For eg., carbondioxide and chlorofluorocarbons.” Greenhouse Effect Diagram

The Diagram shows Greenhouse Gases such as carbon dioxide are the primary cause for the Greenhouse Effect

The major contributors to the greenhouse gases are factories, automobiles, deforestation , etc. The increased number of factories and automobiles increases the amount of these gases in the atmosphere. The greenhouse gases never let the radiations escape from the earth and increase the surface temperature of the earth. This then leads to global warming.

Also Read:  Our Environment

The major causes of the greenhouse effect are:

Burning of Fossil Fuels

Fossil fuels are an important part of our lives. They are widely used in transportation and to produce electricity. Burning of fossil fuels releases carbon dioxide. With the increase in population, the utilization of fossil fuels has increased. This has led to an increase in the release of greenhouse gases in the atmosphere.

Deforestation

Plants and trees take in carbon dioxide and release oxygen. Due to the cutting of trees, there is a considerable increase in the greenhouse gases which increases the earth’s temperature.

Nitrous oxide used in fertilizers is one of the contributors to the greenhouse effect in the atmosphere.

Industrial Waste and Landfills

The industries and factories produce harmful gases which are released in the atmosphere.

Landfills also release carbon dioxide and methane that adds to the greenhouse gases.

The main effects of increased greenhouse gases are:

Global Warming

It is the phenomenon of a gradual increase in the average temperature of the Earth’s atmosphere. The main cause for this environmental issue is the increased volumes of greenhouse gases such as carbon dioxide and methane released by the burning of fossil fuels, emissions from the vehicles, industries and other human activities.

Depletion of  Ozone Layer

Ozone Layer protects the earth from harmful ultraviolet rays from the sun. It is found in the upper regions of the stratosphere. The depletion of the ozone layer results in the entry of the harmful UV rays to the earth’s surface that might lead to skin cancer and can also change the climate drastically.

The major cause of this phenomenon is the accumulation of natural greenhouse gases including chlorofluorocarbons, carbon dioxide, methane, etc.

Smog and Air Pollution

Smog is formed by the combination of smoke and fog. It can be caused both by natural means and man-made activities.

In general, smog is generally formed by the accumulation of more greenhouse gases including nitrogen and sulfur oxides. The major contributors to the formation of smog are automobile and industrial emissions, agricultural fires, natural forest fires and the reaction of these chemicals among themselves.

Acidification of Water Bodies

Increase in the total amount of greenhouse gases in the air has turned most of the world’s water bodies acidic. The greenhouse gases mix with the rainwater and fall as acid rain. This leads to the acidification of water bodies.

Also, the rainwater carries the contaminants along with it and falls into the river, streams and lakes thereby causing their acidification.

This phenomenon occurs when the planet absorbs more radiation than it can radiate back. Thus, the heat lost from the earth’s surface is less and the temperature of the planet keeps rising. Scientists believe that this phenomenon took place on the surface of Venus billions of years ago.

This phenomenon is believed to have occurred in the following manner:

• A runaway greenhouse effect arises when the temperature of a planet rises to a level of the boiling point of water. As a result, all the water from the oceans converts into water vapour, which traps more heat coming from the sun and further increases the planet’s temperature. This eventually accelerates the greenhouse effect. This is also called the “positive feedback loop”.
• There is another scenario giving way to the runaway greenhouse effect. Suppose the temperature rise due to the above causes reaches such a high level that the chemical reactions begin to occur. These chemical reactions drive carbon dioxide from the rocks into the atmosphere. This would heat the surface of the planet which would further accelerate the transfer of carbon dioxide from the rocks to the atmosphere, giving rise to the runaway greenhouse effect.

In simple words, increasing the greenhouse effect gives rise to a runaway greenhouse effect which would increase the temperature of the earth to such an extent that no life will exist in the near future.

Also Read:  Environmental Issues

To learn more about what is the greenhouse effect, its definition, causes and effects, keep visiting BYJU’S website or download the BYJU’S app for further reference.

Frequently Asked Questions

What is global warming.

The gradual increase in temperature due to the greenhouse effect caused by pollutants, CFCs and carbon dioxide is called global warming. This phenomenon has disturbed the climatic pattern of the earth.

List gases which are responsible for the greenhouse effect.

The major greenhouse gases are: 1) Carbon dioxide 2) Methane 3) Water 4) Nitrous oxide 5) Ozone 6) Chlorofluorocarbons (CFCs)

What is the greenhouse effect?

What are the major causes of the greenhouse effect.

Burning of fossil fuels, deforestation, farming and livestock production all contribute to the greenhouse effect. Industries and factories also play a major role in the release of greenhouse gases.

What would have happened if the greenhouse gases were totally missing in the earth’s atmosphere?

Put your understanding of this concept to test by answering a few MCQs. Click ‘Start Quiz’ to begin!

Select the correct answer and click on the “Finish” button Check your score and answers at the end of the quiz

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The Greenhouse Effect [Text]

The Greenhouse Effect

If  not for greenhouse gases trapping heat in the atmosphere, the Earth would be a very cold place. Greenhouse gases keep the Earth warm through a process called the greenhouse effect.  See the video (2 minutes)  to learn more about it

What's in a Name? The “Greenhouse Effect”

A greenhouse is a building made of glass that allows sunlight to enter but traps heat inside, so the building stays warm even when it's cold outside. Because gases in the Earth's atmosphere also let in light but trap heat, many people call this phenomenon the “greenhouse effect.” The greenhouse effect works somewhat differently from an actual greenhouse, but the name stuck, so that's how we still refer to it today.

The action of a greenhouse is to allow solar radiation to come in (glass being transparent to sunlight), but prevent heat from escaping by  convective currents  (glass acting as a physical barrier to rising warm air). The 'greenhouse' warming in the atmosphere works by blocking the long-wave (terrestrial) radiation from escaping the earth-atmospheric system (greenhouse gases like carbon dioxide acting as a radiative barrier). 

Greenhouse Gases

Greenhouse gases trap heat in the atmosphere, which makes the Earth warmer. People are adding several types of greenhouse gases to the atmosphere, and each gas's effect on climate change depends on three main factors:

1.       How much?   People produce larger amounts of some greenhouse gases than others. Carbon dioxide is the greenhouse gas you hear people talk about the most. That's because we produce more carbon dioxide than any other greenhouse gas, and it's responsible for most of the warming.

2.       How long?   Some greenhouse gases stay in the atmosphere for only a short time, but others can stay in the atmosphere and affect the climate for thousands of years.

3.       How powerful?  Not all greenhouse gases are equal as some trap more heat than others. This is represented using Global Warming potential (GWP), a measure of how much heat a substance can trap in the atmosphere over a specific time period relative to carbon di-oxide. GWP can be used to compare the effects of different greenhouse gases. For example, methane has a GWP of 34 over a period of 100 years, which means 1 kg of methane will trap 34 times more heat than 1 kg of carbon dioxide.

Carbon dioxide is the most important greenhouse gas emitted by humans, but several other gases contribute to climate change, too.

Figure : The size of each piece of the pie represents the amount of warming that each gas is currently causing in the atmosphere as a result of emissions from people's activities. Source: Intergovernmental Panel on Climate Change, Fifth Assessment Report (2014)

More about the "Warm family " of green house gases 

Is  w ater vapour  a green house  gas?

Water can take form of water vapour, which is naturally present in atmosphere and has a strong effect on weather and climate. As planet gets warmer, more water evaporates from the earth’s surface and becomes vapour in the atmosphere. Water vapour is a green house gas, so more water in the atmosphere leads to even more warming. This is an example of positive looping where warming leads to even more warming.

Greenhouse gases come from all sorts of everyday activities, such as using electricity, heating our homes, and driving around town.

Figure : This pie chart shows the different activities that lead to greenhouse gas emissions. The largest pieces represent electricity production and transportation. Source: EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks (2016).

These greenhouse gases don't just stay in one place after they're added to the atmosphere. As air moves around the world, greenhouse gases become globally mixed, which means the concentration of a greenhouse gas like carbon dioxide is roughly the same no matter where you measure it. Even though some countries produce more greenhouse gases than others, emissions from every country contribute to the problem. That's one reason why climate change requires global action. The graph below shows how the world's total greenhouse gas emissions are continuing to increase every year.

Figure : This graph shows how the total amount of greenhouse gas emissions has been increasing around the world since 1990. Source: EPA's Climate Change Indicators (2016).

More about Carbon dioxide (CO 2 )

Carbon is an element that's found all over the world and in every living thing. Oxygen is another element that's in the air we breathe. When carbon and oxygen bond together, they form a colorless, odorless gas called carbon dioxide, which is a heat-trapping greenhouse gas. Whenever we burn fossil fuels such as coal, oil, and natural gas—whether it's to drive our cars, use electricity, or make products—we are producing carbon dioxide.

The atmosphere isn't the only part of the Earth that has carbon. The oceans store large amounts of carbon, and so do plants, soil, and deposits of coal, oil, and natural gas deep underground. Carbon naturally moves from one part of the Earth to another through the carbon cycle. But right now, by burning fossil fuels, people are adding carbon to the atmosphere (in the form of carbon dioxide) faster than natural processes can remove it. That's why the amount of carbon dioxide in the atmosphere is increasing, which is causing global climate change.

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• The GREENHOUSE EFFECT in EFFECT
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People treat the EARTH
 with little respect
 global warming comes to us
 live and direct
 the EARTH is becoming
 a burnt out wreck.
 You see the GREENHOUSE EFFECT
 is in EFFECT. Burning Tropical rain forest
 producing carbon dioxide
 increasing global warming
 sea levels start to rise
 weather patterns are changing
 there’s a lesson to be learnt
 the GREENHOUSE EFFECT is in EFFECT
 we could all get burnt. Greedy developers destroy land
 to build new factories
 killing the indigenous population
 with their economic policies
 animals and plant life become extinct
 along with cures for many diseases
 the chance of breathing fresh clean air
 for ever decreases. The GREE…
 The GREE…
 The GREENHOUSE EFFECT
 Humans treating the EARTH with little respect.
 The GREE…
 The GREE…
 The GREENHOUSE EFFECT
 The EARTH is becoming a nervous wreck. Trees get axed
 to feed the consumer
 atmospheric gases clash
 which equates to danger
 and life on EARTH gets
 stranger
 and now man has his regrets.
 Because
 The GREENHOUSE
 The GREENHOUSE
 The GREENHOUSE EFFECT is in EFFECT

Acknowledgments

Linked articles.

• In this issue Common sense, the least common sense? Carlos Alvarez-Dardet John R Ashton Journal of Epidemiology & Community Health 2005; 59 341-341 Published Online First: 14 Apr 2005.

Read the full text or download the PDF:

The Greenhouse Effect and our Planet

The greenhouse effect happens when certain gases, which are known as greenhouse gases, accumulate in Earth’s atmosphere. Greenhouse gases include carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), ozone (O 3 ), and fluorinated gases.

Biology, Ecology, Earth Science, Geography, Human Geography

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Greenhouse gases include gases such as carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), ozone (O 3 ), and fluorinated gases. These greenhouse gases allow the sun's light to shine onto Earth's surface. Then the gases, such as ozone, trap the heat that reflects back from the surface inside Earth's atmosphere . The gases act like the glass walls of a greenhouse. In other words, they are warming.

The greenhouse effect happens when these gases gather in Earth's atmosphere. According to scientists, without the greenhouse effect, the average temperature of Earth would drop from 57 degrees Fahrenheit (14 degrees Celsius) to as low as negative 0.4 degrees F (minus 18 degrees C).

Do We Blame the Industrial Revolution ? Some greenhouse gases come from natural sources. For example, evaporation adds water vapor to the atmosphere. Animals and plants release carbon dioxide when they breathe. Methane is released naturally from decomposition, when soils and living things break down. Volcanoes —both on land and under the ocean —release greenhouse gases.

The Industrial Revolution happened in the late 1700s and early 1800s, when factories began producing more. Since then, people have been releasing larger quantities of greenhouse gases into the atmosphere. Greenhouse gas emissions increased 70 percent between 1970 and 2004. Emissions of carbon dioxide (CO 2 ), rose about 80 percent during that time.

The amount of CO 2 in the atmosphere far exceeds Earth's natural amount seen over the last 650,000 years.

Most of the CO 2 that people put into the atmosphere comes from burning fossil fuels . Cars, trucks, t rains and planes all burn fossil fuels. Many electric power plants do, as well. Another way humans release CO 2 into the atmosphere is by cutting down forests , because trees contain large amounts of carbon.

Human Activity + Greenhouse Gases = A Warming Earth People add methane to the atmosphere through livestock farming, landfills and fossil fuel production such as coal mining and natural gas processing. Nitrous oxide comes from agriculture and fossil fuel burning.

Fluorinated gases include chlorofluoro carbons (CFCs), hydro chlorofluoro carbons (HCFCs), and hydrofluorocarbons (HFCs). They are produced during the manufacturing of refrigeration and cooling products. Some come through aerosol cans , such as some hairsprays or spray paint.

As greenhouse gases increase, so does the temperature of Earth. The rise in Earth's average temperature contributed to by human activity is known as global warming .

The Greenhouse Effect and Climate Change Even slight increases in average global temperatures can have huge effects.

Perhaps the biggest effect is that glaciers and ice caps melt faster than usual. The meltwater d rains into the oceans , causing sea levels to rise.

Glaciers and ice caps cover about 10 percent of the world's land. They hold between 70 and 75 percent of the world's freshwater . If all of this ice melted, sea levels would rise about 70 meters (230 feet).

The Intergovernmental Panel on Climate Change says that the global sea level rose about 1.8 millimeters (0.07 inch) per year from 1961 to 1993. It rose about 3.1 millimeters (1/8 inch) per year since 1993.

This seems like only a tiny bit, but rising sea levels can cause flooding in cities along the coasts . This could force millions of people in low-lying areas out of their homes, such as in Bangladesh, the U.S. state of Florida, and the Netherlands.

Millions more people in countries such as Peru and India depend on water from melted glaciers . They use it for drinking, watering crops and hydroelectric power . Rapid loss of these glaciers would greatly hurt those countries.

Predictable Rain is Important to Many Greenhouse gas emissions also affect changes in precipitation , such as rain and snow .

In the 20th century, precipitation increased in eastern parts of North and South America, Northern Europe, and northern and Central Asia. However, it has decreased in parts of Africa, the Mediterranean, and southern Asia.

As climates change, so do the habitats for living things. Animals that are adapted to a certain climates might become threatened. Many humans depend on predictable rain patterns to grow specific crops . If the climate of an area changes, the people who live there may no longer be able to grow the crops they depend on for survival.

Scientists aren't the only Ones Who Can Help

• Drive less. Use  public transportation , carpool, walk, or ride a bike.
• Fly less. Airplanes produce huge amounts of greenhouse gas emissions.
• Reduce, reuse, and  recycle .
• Plant a tree. Trees absorb carbon dioxide, keeping it out of the atmosphere.
• Use less  electricity .
• Eat less meat. Cows are one of the biggest methane producers.
• Support alternative energy sources that don’t burn fossil fuels.

Artificial Gas

Chlorofluorocarbons (CFCs) are the only greenhouse gases not created by nature. They are created through refrigeration and aerosol cans.

CFCs, used mostly as refrigerants, are chemicals that were developed in the late 19th century and came into wide use in the mid-20th century.

Other greenhouse gases, such as carbon dioxide, are emitted by human activity, at an unnatural and unsustainable level, but the molecules do occur naturally in Earth's atmosphere.

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MIT Technology Review

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How I learned to stop worrying and love fake meat

Let’s stop inventing reasons to reject cultured meat and other protein alternatives that could dramatically cut climate emissions.

• James Temple archive page

Fixing our collective meat problem is one of the trickiest challenges in addressing climate change—and for some baffling reason, the world seems intent on making the task even harder.

The latest example occurred last week, when Florida governor Ron DeSantis signed a law banning the production, sale, and transportation of cultured meat across the Sunshine State. 

“Florida is fighting back against the global elite’s plan to force the world to eat meat grown in a petri dish or bugs to achieve their authoritarian goals,” DeSantis seethed in a statement.

Alternative meat and animal products—be they lab-grown or plant-based—offer a far more sustainable path to mass-producing protein than raising animals for milk or slaughter. Yet again and again, politicians, dietitians, and even the press continue to devise ways to portray these products as controversial, suspect, or substandard. No matter how good they taste or how much they might reduce greenhouse-gas emissions, there’s always some new obstacle standing in the way—in this case, Governor DeSantis, wearing a not-at-all-uncomfortable smile.  

The new law clearly has nothing to do with the creeping threat of authoritarianism (though for more on that, do check out his administration’s crusade to ban books about gay penguins). First and foremost it is an act of political pandering, a way to coddle Florida’s sizable cattle industry, which he goes on to mention in the statement.

Cultured meat is seen as a threat to the livestock industry because animals are only minimally involved in its production. Companies grow cells originally extracted from animals in a nutrient broth and then form them into nuggets, patties or fillets. The US Department of Agriculture has already given its blessing to two companies , Upside Foods and Good Meat, to begin selling cultured chicken products to consumers. Israel recently became the first nation to sign off on a beef version.

It’s still hard to say if cultured meat will get good enough and cheap enough anytime soon to meaningfully reduce our dependence on cattle, chicken, pigs, sheep, goats, and other animals for our protein and our dining pleasure. And it’s sure to take years before we can produce it in ways that generate significantly lower emissions than standard livestock practices today.

But there are high hopes it could become a cleaner and less cruel way of producing meat, since it wouldn’t require all the land, food, and energy needed to raise, feed, slaughter, and process animals today. One study found that cultured meat could reduce emissions per kilogram of meat 92% by 2030, even if cattle farming also achieves substantial improvements.

Those sorts of gains are essential if we hope to ease the rising dangers of climate change, because meat, dairy, and cheese production are huge contributors to greenhouse-gas emissions.

DeSantis and politicians in other states that may follow suit, including Alabama and Tennessee, are raising the specter of mandated bug-eating and global-elite string-pulling to turn cultured meat into a cultural issue, and kill the industry in its infancy. 

But, again, it’s always something. I’ve heard a host of other arguments across the political spectrum directed against various alternative protein products, which also include plant-based burgers, cheeses, and milks, or even cricket-derived powders and meal bars . Apparently these meat and dairy alternatives shouldn’t be highly processed, mass-produced, or genetically engineered, nor should they ever be as unhealthy as their animal-based counterparts. 

In effect, we are setting up tests that almost no products can pass, when really all we should ask of alternative proteins is that they be safe, taste good, and cut climate pollution.

The meat of the matter

Here’s the problem. 

Livestock production generates more than 7 billion tons of carbon dioxide, making up 14.5% of the world’s overall climate emissions, according to the United Nations Food and Agriculture Organization.

Beef, milk, and cheese production are, by far, the biggest problems, representing some 65% of the sector’s emissions. We burn down carbon-dense forests to provide cows with lots of grazing land; then they return the favor by burping up staggering amounts of methane, one of the most powerful greenhouse gases. Florida’s cattle population alone, for example, could generate about 180 million pounds of methane every year, as calculated from standard per-animal emissions . 

In an earlier paper , the World Resources Institute noted that in the average US diet, beef contributed 3% of the calories but almost half the climate pollution from food production. (If you want to take a single action that could meaningfully ease your climate footprint, read that sentence again.)

The added challenge is that the world’s population is both growing and becoming richer, which means more people can afford more meat. 

There are ways to address some of the emissions from livestock production without cultured meat or plant-based burgers, including developing supplements that reduce methane burps and encouraging consumers to simply reduce meat consumption. Even just switching from beef to chicken can make a huge difference .

Let’s clear up one matter, though. I can’t imagine a politician in my lifetime, in the US or most of the world, proposing a ban on meat and expecting to survive the next election. So no, dear reader. No one’s coming for your rib eye. If there’s any attack on personal freedoms and economic liberty here, DeSantis is the one waging it by not allowing Floridians to choose for themselves what they want to eat.

But there is a real problem in need of solving. And the grand hope of companies like Beyond Meat, Upside Foods, Miyoko’s Creamery, and dozens of others is that we can develop meat, milk, and cheese alternatives that are akin to EVs: that is to say, products that are good enough to solve the problem without demanding any sacrifice from consumers or requiring government mandates. (Though subsidies always help.)

The good news is the world is making some real progress in developing substitutes that increasingly taste like, look like, and have (with apologies for the snooty term) the “mouthfeel” of the traditional versions, whether they’ve been developed from animal cells or plants. If they catch on and scale up, it could make a real dent in emissions—with the bonus of reducing animal suffering, environmental damage, and the spillover of animal disease into the human population.

The bad news is we can’t seem to take the wins when we get them. 

The blue cheese blues

For lunch last Friday, I swung by the Butcher’s Son Vegan Delicatessen & Bakery in Berkeley, California, and ordered a vegan Buffalo chicken sandwich with a blue cheese on the side that was developed by Climax Foods , also based in Berkeley.

Late last month, it emerged that the product had, improbably, clinched the cheese category in the blind taste tests of the prestigious Good Food awards, as the Washington Post revealed .

Let’s pause here to note that this is a stunning victory for vegan cheeses, a clear sign that we can use plants to produce top-notch artisanal products, indistinguishable even to the refined palates of expert gourmands. If a product is every bit as tasty and satisfying as the original but can be produced without milking methane-burping animals, that’s a big climate win.

But sadly, that’s not where the story ended.

After word leaked out that the blue cheese was a finalist, if not the winner, the Good Food Foundation seems to have added a rule that didn’t exist when the competition began but which disqualified Climax Blue , the Post reported.

I have no special insights into what unfolded behind the scenes. But it reads at least a little as if the competition concocted an excuse to dethrone a vegan cheese that had bested its animal counterparts and left traditionalists aghast. 

That victory might have done wonders to help promote acceptance of the Climax product, if not the wider category. But now the story is the controversy. And that’s a shame. Because the cheese is actually pretty good. 

I’m no professional foodie, but I do have a lifetime of expertise born of stubbornly refusing to eat any salad dressing other than blue cheese. In my own taste test, I can report it looked and tasted like mild blue cheese, which is all it needs to do.

A beef about burgers

Banning a product or changing a cheese contest’s rules after determining the winner are both bad enough. But the reaction to alternative proteins that has left me most befuddled is the media narrative that formed around the latest generation of plant-based burgers soon after they started getting popular a few years ago. Story after story would note, in the tone of a bold truth-teller revealing something new each time: Did you know these newfangled plant-based burgers aren’t actually all that much healthier than the meat variety? 

To which I would scream at my monitor: THAT WAS NEVER THE POINT!

The world has long been perfectly capable of producing plant-based burgers that are better for you, but the problem is that they tend to taste like plants. The actual innovation with the more recent options like Beyond Burger or Impossible Burger is that they look and taste like the real thing but can be produced with a dramatically smaller climate footprint .

That’s a big enough win in itself. 

If I were a health reporter, maybe I’d focus on these issues too. And if health is your personal priority, you should shop for a different plant-based patty (or I might recommend a nice salad, preferably with blue cheese dressing).

But speaking as a climate reporter, expecting a product to ease global warming, taste like a juicy burger, and also be low in salt, fat, and calories is absurd. You may as well ask a startup to conduct sorcery.

More important, making a plant-based burger healthier for us may also come at the cost of having it taste like a burger. Which would make it that much harder to win over consumers beyond the niche of vegetarians and thus have any meaningful impact on emissions. WHICH IS THE POINT!

It’s incredibly difficult to convince consumers to switch brands and change behaviors, even for a product as basic as toothpaste or toilet paper. Food is trickier still, because it’s deeply entwined with local culture, family traditions, festivals and celebrations. Whether we find a novel food product to be yummy or yucky is subjective and highly subject to suggestion. 

And so I’m ending with a plea. Let’s grant ourselves the best shot possible at solving one of the hardest, most urgent problems before us. Treat bans and political posturing with the ridicule they deserve. Reject the argument that any single product must, or can, solve all the problems related to food, health, and the environment.

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Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base (1992)

Chapter: a questions and answers about greenhouse warming, appendix a questions and answers about greenhouse warming, the greenhouse effect: what is known, what can be predicted.

1. What is the "greenhouse effect"?

In simplest terms, "greenhouse gases" let sunlight through to the earth's surface while trapping "outbound" radiation. This alters the radiative balance of the earth (see Figure A.1) and results in a warming of the earth's surface. The major greenhouse gases are water vapor, carbon dioxide (CO 2 ), methane (CH 4 ), chlorofluorocarbons (CFCs) and hydrogenated chlorofluorocarbons (HCFCs), tropospheric ozone (O 3 ), and nitrous oxide (N 2 O). Without the naturally occurring greenhouse gases (principally water vapor and CO 2 ), the earth's average temperature would be nearly 35°C (63°F) colder, and the planet would be much less suitable for human life.

2. Why is it called the "greenhouse" effect?

The greenhouse gases in the atmosphere act in much the same way as the glass panels of a greenhouse, which allow sunlight through and trap heat inside.

3. Why have experts become worried about the greenhouse effect now?

Rising atmospheric concentrations of CO 2 , CH 4 , and CFCs suggest the possibility of additional warming of the global climate. The panel refers to warming due to increased atmospheric concentrations of greenhouse gases as "greenhouse warming." Measurements of atmospheric CO 2 show that the 1990 concentration of 353 parts per million by volume (ppmv) is about one-quarter larger than the concentration before the Industrial Revolution (prior

FIGURE A.1 Earth's radiation balance. The solar radiation is set at 100 percent; all other values are in relation to it. About 25 percent of incident solar radiation is reflected back into space by the atmosphere, about 25 percent is absorbed by gases in the atmosphere, and about 5 percent is reflected into space from the earth's surface, leaving 45 percent to be absorbed by the oceans, land, and biotic material (white arrows).

Evaporation and mechanical heat transfer inject energy into the atmosphere equal to about 29 percent of incident radiation (grey arrow). Radiative energy emissions from the earth's surface and from the atmosphere (straight black arrows) are determined by the temperatures of the earth's surface and the atmosphere, respectively. Upward energy radiation from the earth's surface is about 104 percent of incident solar radiation. Atmospheric gases absorb part (25 percent) of the solar radiation penetrating the top of the atmosphere and all of the mechanical heat transferred from the earth's surface and the outbound radiation from the earth's surface. The downward radiation from the atmosphere is about 88 percent and outgoing radiation about 70 percent of incident solar radiation.

Note that the amounts of outgoing and incoming radiation balance at the top of the atmosphere, at 100 percent of incoming solar radiation (which is balanced by 5 percent reflected from the surface, 25 percent reflected from the top of the atmosphere, and 70 percent outgoing radiation), and at the earth's surface, at 133 percent (45 percent absorbed solar radiation plus 88 percent downward radiation from the atmosphere balanced by 29 percent evaporation and mechanical heat transfer and 104 percent upward radiation). Energy transfers into and away from the atmosphere also balance, at the atmosphere line, at 208 percent of incident solar radiation (75 percent transmitted solar radiation plus 29 percent mechanical transfer from the surface plus 104 percent upward radiation balanced by 50 percent of incoming solar continuing to the earth's surface, 70 percent outgoing radiation, and 88 percent downward radiation). These different energy transfers are due to the heat-trapping effects of the greenhouse gases in the atmosphere, the reemission of energy absorbed by these gases, and the cycling of energy through the various components in the diagram. The accuracy of the numbers in the diagram is typically ±5.

This diagram pertains to a period during which the climate is steady (or unchanging); that is, there is no net change in heat transfers into earth's surface, no net change in heat transfers into the atmosphere, and no net radiation change into the atmosphere-earth system from beyond the atmosphere.

to 1750). Atmospheric CO 2 is increasing at about 0.5 percent per year. The concentration of CH 4 is about 1.72 ppmv, or slightly more than twice that before 1750. It is rising at a rate of 0.9 percent per year. CFCs do not occur naturally, and so they were not found in the atmosphere until production began a few decades ago. Continued increases in atmospheric concentrations of greenhouse gases would affect the earth's radiative balance and could cause a large amount of additional greenhouse warming. Increasing the capture of energy in this fashion is also called "radiative forcing." Other factors, such as variation in incoming solar radiation, could be involved.

4. Has there been greenhouse warming in the recent past?

Best estimates are that the average global temperature rose between 0.3° and 0.6°C over about the last 100 years. However, it is not possible to say with a high degree of confidence whether this is due to increased atmospheric concentrations of greenhouse gases or to other natural or human causes. The temperature record much before 1900 is not reliable for estimates of changes smaller than 1°C–1.8°F).

5. What about CO 2 and temperature in the prehistoric past?

According to best estimates based on analysis of air bubbles trapped in ice sheets, ocean and lake sediments, and other records from the geologic past, there have been three especially "warm" periods in the last 4 million years. The Holocene optimum occurred from 6,000 to 5,000 years ago. During that period, atmospheric concentrations of CO 2 were about 270 to 280 ppmv, and average air temperatures about 1°C (1.8°F) warmer than modern times. The Eemian interglacial period happened with its midpoint about 125,000 years ago. Atmospheric concentrations of CO 2 were 280 to 300 ppmv, and temperatures up to 2°C (3.6°F) warmer than now. The Pliocene climate optimum occurred between 4.3 and 3.3 million years ago. Atmospheric concentrations of CO 2 have been estimated for that period to be about 450 ppmv, with temperatures 3° to 4°C (5.4° to 7.2°F) warmer than modern times. The prehistoric temperature estimates are from evidence dependent

on conditions during growing seasons and probably are better proxies for summer than winter temperatures. The estimate for the Pliocene period is especially controversial.

6. What natural things affect climate in the long run?

On the geologic time scale, many things affect climate:

• Changes in solar output

• Changes in the earth's orbital path

• Changes in land and ocean distribution (tectonic plate movements and the associated changes in mountain geography, ocean circulation, and sea level)

• Changes in the reflectivity of the earth's surface

• Changes in atmospheric concentrations of trace gases (especially CO 2 and CH 4 )

• Changes of a catastrophic nature (such as meteor impacts or extended volcanic eruptions)

7. What is meant by ''atmospheric lifetime" and "sinks"?

These concepts can be illustrated by referring to what is called the "carbon cycle." When CO 2 is emitted into the atmosphere, it moves among four main sinks, or pools, of stored carbon: the atmosphere, the oceans, the soil, and the earth's biomass (plants and animals). The movement of CO 2 among these sinks is not well understood. About 45 percent of the total emissions of CO 2 from human activity since preindustrial times is missing in the current accounting of CO 2 in the atmosphere, oceans, soil, and biomass. Three possible sinks for this missing CO 2 have been suggested. First, more CO 2 may have been absorbed into the oceans than was thought. Second, the storage of CO 2 in terrestrial plant life may be greater than estimated. Third, more CO 2 may have been absorbed directly into soil than is thought. However, there is no direct evidence for any of these explanations accounting for all the missing CO 2 . CO 2 in the atmosphere is relatively "long-lived" in that it does not easily break down into its constituent parts. CH 4 , by contrast, decomposes in the atmosphere in about 10 years. The greenhouse gas with the longest atmospheric lifetime (except for CO 2 ), CFC-115, has an average atmospheric lifetime of about 400 years. The overall contribution of greenhouse gases to global warming depends on their atmospheric lifetime as well as their ability to trap radiation. Table A.1 shows the relevant characteristics of the principal greenhouse gases.

8. Do all greenhouse gases have the same effect?

Each gas has different radiative properties, atmospheric chemistry, typical atmospheric lifetime, and atmospheric concentration. For example, CFC-12 is roughly 15,800 times more efficient molecule for molecule at trapping heat than CO 2 . Because CFC-12 is a large, heavy molecule with many atoms and a

TABLE A.1 Key Greenhouse Gases Influenced by Human Activity

CO 2 molecule is small and light in comparison, there are fewer molecules of CFC-12 in each ton of CFC-12 emissions than CO 2 molecules in each ton of CO 2 emissions. Each ton of CFC-12 emissions is about 5,750 times more efficient at trapping heat than each ton of CO 2 . The comparatively greater amount of CO 2 in the atmosphere, however, means that it accounts for roughly half of the radiative forcing associated with the greenhouse effect.

9. Do greenhouse gases have different effects over time?

Yes. Figure A.2 shows projected changes in radiative forcing for different greenhouse gases between now and 2030. The potential increase for each

FIGURE A.2 Additional radiative forcing of principal greenhouse gases from 1990 to 2030 for different emission rates. The horizontal axis shows changes in greenhouse gas emissions ranging from completely eliminating emissions (-100 percent) to doubling current emissions (+100 percent). Emission changes are assumed to be linear from 1990 levels to the 2030 level selected. The vertical axis shows the change in radiative forcing in watts per square meter at the earth's surface in 2030. Each asterisk indicates the projected emissions of that gas assuming no additional regulatory policies, based on the Intergovernmental Panel on Climate Change estimates and the original restrictions agreed to under the Montreal Protocol, which limits emissions of CFCs. Chemical interactions among greenhouse gas species are not included.

For CO 2 emissions remaining at 1990 levels through 2030, the resulting change in radiative forcing can be determined in two steps: (1) Find the point on thecurvelabeled "CO 2 " that is vertically above 0 percent change on the bottom scale. (2) The radiative forcing on the surface-troposphere system can be read in watts per square meter by moving horizontally to the left-hand scale, or about 1 W/m 2 . These steps must be repeated for each gas. For example, the radiative forcing for continued 1990-level emissions of CH 4 through 2030 would be about 0.2W/m 2 .

SOURCE : Courtesy of Michael C. MacCracken.

gas is plotted for different emissions of each gas compared to 1990 emission levels. The figure shows the impact of different percentage changes in emissions (compared to 1990 emission rates) on the radiative forcing. Figure A.3 extends this to show the impact on equilibrium temperature for different sensitivities of the climatic system (in degrees Celsius).

10. What is meant by a "feedback" mechanism?

One example of a greenhouse warming feedback mechanism involves water vapor. As air warms, each cubic meter of air can hold more water vapor. Since water vapor is a greenhouse gas, this increased concentration of water vapor further enhances greenhouse warming. In turn, the warmer air can hold more water, and so on. This is an example of a positive feedback, providing a physical mechanism for "multiplying" the original impetus for change beyond its initial force.

Some mechanisms provide a negative feedback, which decreases the initial impetus. For example, increasing the amount of water vapor in the air may lead to forming more clouds. Low-level, white clouds reflect sunlight, thereby preventing sunlight from reaching the earth and warming the surface. Increasing the geographical coverage of low-level clouds would reduce greenhouse warming, whereas increasing the amount of high, convective clouds could enhance greenhouse warming. This is because high, convective clouds absorb energy from below at higher temperatures than they radiate energy into space from their tops, thereby effectively trapping energy. Satellite measurements indicate that clouds currently have a slightly negative effect on current planetary temperature. It is not known whether increased temperatures would lead to more low-level clouds or more high, convective clouds.

11. Can the temperature record be used to show whether or not greenhouse warming is occurring?

The estimated warming of between 0.3° and 0.6°C (0.5° and 1.1°F) over the last 100 years is roughly consistent with increased concentrations of greenhouse gases, but it is also within the bounds of "natural" variability for weather and climate. It cannot be proven to a high degree of confidence that this warming is the result of the increased atmospheric concentrations of greenhouse gases. There may be an underlying increase or decrease in average temperature from other, as yet undetected, causes.

12. What is the basis for predictions of global warming?

General circulation models (GCMs) are the principal tools for projecting climatic changes. GCMs project equilibrium temperature increases between 1.9° and 5.2°C (3.4° and 9.4°F) for greenhouse gas concentrations equivalent to a doubling of the preindustrial level of atmospheric CO 2 . The midpoint of this range corresponds to an average global climate warmer than

FIGURE A.3 Commitment to future warming. An incremental change in radiative forcing between 1990 and 2030 due to emissions of greenhouse gases implies a change in global average equilibrium temperature (see text). The scales on the right-hand side show two ranges of global average temperature responses. The first corresponds to a climate whose temperature response to an equivalent of doubling of the preindustrial level of CO 2 is 1°C; the second corresponds to a rise of 5°C for an equivalent doubling of CO 2 . These scales indicate the equilibrium commitment to future warming caused by emissions from 1990 through 2030. Assumptions are as in Figure A.2.

To determine equilibrium warming in 2030 due to continued emissions of CO 2 at the 1990 level, find the point on the curve labeled "CO 2 " that is vertically above 0 percent change on the bottom scale. The equilibrium warming on the right-hand scales is about 0.23°C (0.4°F) for a climate system with 1° sensitivity and about 1.2°C (2.2°F) for a system with 5° sensitivity. For CH 4 emissions continuing at 1990 levels through 2030, the equilibrium warming would be about 0.04°C (0.07°F) at 1°sensitivity and about 0.25°C (0.5°F) at 5° sensitivity. These steps must be repeated for each gas. Total warming associated with 1990-level emissions of the gases shown until 2030 would be about 0.41°C (0.7°F) at 1°sensitivity and about 2.2°C (4°F) at 5° sensitivity.

Scenarios of changes in committed future warming accompanying different greenhouse gas emission rates can be constructed by repeating this process for given emission rates and adding up the results.

any in the last 1 million years. The consequences of this amount of warming are unknown and may include extremely unpleasant surprises.

13. What is "equilibrium temperature"?

The oceans, covering roughly 70 percent of the earth's surface, absorb heat from the sun and redistribute it to the deep oceans slowly. It will be decades, perhaps centuries, before the oceans and the atmosphere fully redistribute the absorbed energy and the currently "committed" temperature rise is actually "realized." The temperature at which the system would ultimately come to rest given a particular level of greenhouse gas concentrations is called the "equilibrium temperature.'' Since atmospheric concentrations of greenhouse gases are constantly changing, the temperature measured at any time is the "transient" temperature, which lags behind the committed equilibrium warming. The lag depends in part on the sensitivity of the climate system and is believed to be between 10 and 100 years. This phenomenon makes it difficult to use temperature alone to "prove" that greenhouse warming is occurring.

14. How can we know when greenhouse warming is occurring?

The only tools we have for trying to produce credible scientific results are observations combined with theoretical calculation. Detecting additional greenhouse warming will require careful monitoring of temperature and other variables over years or even decades. Further development of numerical models will help characterize the climatic system, including the atmosphere, oceans, and land-based elements like forests and ice fields. However, only careful interpretation of actual measurements can reveal what has occurred and when.

15. How can credible estimates of future global warming be made?

Several approaches can be used. Scientific "first principles" can be used to estimate physical bounds on future trends. GCMs can be used to conduct "what if" experiments under differing conditions. Comparisons can be made with paleoclimatic data of previous interglacial periods. None of these methods is absolutely conclusive, but it is generally agreed that GCMs are the best available tools for predicting climatic changes. Substantial improvements in GCM capabilities are needed, however, for GCM forecasts to increase their credibility.

16. What influences future warming?

The amount of climatic warming depends on several things:

• The amount of sunlight reaching the earth

• Emission rates of greenhouse gases

• Chemical interactions of greenhouse gases in the atmosphere

• Atmospheric lifetimes of greenhouse gases until they decompose or transfer into sinks

• Effectiveness of positive or negative feedback mechanisms that enhance or reduce warming

• Human actions, which effect radiative forcing in both positive and negative directions

17. What are the major "unknowns" in predictions?

Major uncertainties include:

• Future emissions of greenhouse gases

• Role of the oceans and biosphere in uptake of heat and CO 2

• Amount of CO 2 and carbon in the atmosphere, oceans, biota, and soils

• Effectiveness of sinks for CO 2 and other greenhouse gases, especially CH 4

• Interactions between temperature change and cloud formation and the resulting feedbacks

• Effects of global warming on biological sources of greenhouse gases

• Interactions between changing climate and ice cover and the resulting feedbacks

• Amount and regional distribution of precipitation

• Other factors, like variation in solar radiation

18. How can the uncertainties best be handled?

Data can be arrayed to validate components of the models. Increasing the number of data sets can also help. In addition, the variation in GCM results can be compared to provide a sense of their "robustness." A major "intercomparison" of GCMs is being conducted, and has shown large differences in regional precipitation and reduction of snow and ice fields at high latitudes.

19. Are these changes associated with an equivalent doubling of the preindustrial level of atmospheric CO 2 that can be stated with confidence?

Because of the uncertainty in our understanding of various factors, projections reflect different levels of confidence.

20. What about storms and other extreme weather events?

The factors governing tropical storms are different from those governing mid-latitude storms and need to be considered separately.

One of the conditions for formation of typhoons or hurricanes today is a sea surface temperature of 26°C (79°F) or greater. With higher global average surface temperature, the area of sea with this temperature should be larger. Thus the number of hurricanes could increase. However, air pressure, humidity, and a number of other conditions also govern the creation and propagation of tropical cyclones. The critical temperature for their creation may increase as climate changes these other factors. There is no consistent indication whether tropical storms will increase in number or intensity as climate changes. Nor is there any evidence of change over the past several decades.

Mid-latitude storms are driven by equator-to-pole temperature contrast. In a warmer world, this contrast will probably weaken since surface temperatures in high latitudes are projected to increase more than at the equator (at least in the northern hemisphere). Higher in the atmosphere, however, the temperature contrast strengthens. Increased atmospheric water vapor could also supply extra energy to storm development. We do not currently know which of these factors would be more important and how mid-latitude storms would change in frequency, intensity, or location.

21. Can projections be improved?

Better computers alone will not solve the problems associated with positive and negative feedbacks. Better understanding of atmospheric physics and chemistry and better mathematical descriptions of relevant mechanisms in the models are also needed, as are data to validate models and their subcomponents. Significant improvements may require decades.

22. Is it possible to avoid the projected warming?

It is possible only at great expense or by incurring risks not now understood, unless the earth is itself self-correcting. Continued increases in atmospheric concentrations of greenhouse gases would probably result in additional global warming. Avoiding all future warming either would be very costly (if we significantly reduce atmospheric concentrations of greenhouse gases) or potentially very risk (if we use climate engineering). However, a comprehensive action program could slow or reduce the onset of greenhouse warming.

A Framework for Responding to Additional Greenhouse Warming

23. What kinds of responses to potential greenhouse warming are possible?

Human interventions in natural and economic activities can affect the net rate of change in the radiative forcing of the earth. It is useful to categorize the possible types of intervention into three types:

• Actions to eliminate or reduce emissions of greenhouse gases

• Actions to "offset" such emissions by removing such gases from the atmosphere, blocking solar radiation, or altering the earth's reflectivity or absorption of energy

• Actions to help human and ecologic systems adjust to new climatic conditions and events

In this study the panel analyzes the first two types of action together under the label of "mitigation," since they are aimed at avoiding or reducing greenhouse warming. The third type of action is here called "adaptation."

24. How can response options be evaluated?

The choice of response options to potential greenhouse warming can be guided by a standard cost-benefit approach, augmented to handle some important aspects of the issues involved. The anticipated impacts (both adverse and beneficial) can be arrayed to produce a "damage function" showing the anticipated costs (or benefits) associated with projected climatic changes. The mitigation and adaptation options can be arrayed similarly according to their respective costs and effectiveness to produce an "abatement cost function." Optimal policies involve balancing incremental costs and benefits, which is called cost-benefit balancing. A necessary condition for an optimal policy is that the level of policy chosen should be cost-effective (any step undertaken minimizes costs). Employing such guidelines requires estimating both the anticipated damages and the cost-effectiveness of alternative response options, and choosing a discount rate to use for assessing the current value of future expenditures or returns.

In practice, a full cost-benefit approach can only be approximated. It is impossible to determine in detail the impacts of climatic changes that will not occur for 40 or 100 years. Thus the damage function can be only roughly approximated. Estimation of the abatement cost function is considerably easier.

Responses to greenhouse warming should be regarded as investments in the future. Cost-effectiveness and cost-benefit balancing should guide the selection of options. In general, a mixed strategy employing some investment in many different alternatives will be most effective.

Impacts of Additional Greenhouse Warming

25. Can impacts of expected climatic changes be projected?

It currently is not possible to predict regional temperature, precipitation, and other effects of climate change with much confidence. And without quantitative projections of regional and local climatic changes, it is not possible to produce quantitative projections of the consequences of greenhouse warming.

Instead, the degree of "sensitivity" of affected human and natural systems to the projected changes can be estimated. The sensitivity of a particular system to the climate changes expected to accompany different amounts of additional greenhouse warming can be used to estimate the impacts of those changes.

A crucial aspect of the sensitivity of a system is the speed at which it can react. For example, investment decisions in many industries typically have a "life-cycle" of 10 years or less. Climatic changes associated with additional greenhouse warming are expected to emerge slowly enough that these industries may be expected to adjust as climate changes. Some industries, such as electric power production, have longer investment cycles, and might have more difficulty responding as quickly. Natural ecological systems would not be expected to anticipate climate change and probably would not be able to adapt as quickly as climatic conditions change.

The impacts of climate change are thus hard to assess because the response of human and natural systems to climate change must be included.

26. How can the impacts on affected systems be classified?

Likely impacts of climate change can be divided into four categories:

• Low sensitivity. The projected changes would likely have little effect on the system. An example is most industrial production not requiring large quantities of water. Temperature changes of the magnitude projected would not matter much for most industrial processes. These impacts do not give rise to much concern.

• High sensitivity, but adaptation possible at some cost. The system would likely adapt or otherwise cope with the projected changes without completely restructuring the system. An example is American agriculture. Although some crops would likely move into new locations, agricultural scientists and plant breeders would almost certainly develop new crops suitable for changed growing conditions. There would be costs, but food supply would not be interrupted. As a class, these impacts give rise to concern because the affected systems may have difficulty adapting.

• High sensitivity, and adaptation problematic. The system would be seriously affected, and adaptation would probably not be easy or effective. Natural communities of plants and animals would probably lose their current structure, and reformulate with different mixes of species. Some individual species, especially animals, would move to new locations. The natural landscape as we know it today would almost certainly be altered by a climate change at or above the midpoint of the range used in this study. These impacts are of considerable concern because the affected systems may not be able to adapt without assistance.

• Uncertain sensitivity, but cataclysmic consequences. The sensitivity of the system cannot be assessed with certainty, but the consequences would be extremely severe. An example is the possible shifting, slowing, or even stopping of major ocean currents like the Gulf Stream or the Japanese Current. These ocean currents strongly affect weather patterns, and changes in them could drastically alter weather in Europe or the West Coast of the United States. We have no credible way, however, of assessing the conditions that could lead to such shifts.

27. What are the likely impacts of climate change?

Human societies exhibit a wide range of adaptive mechanisms in the face of changing climatic events and conditions. Projected climatic changes, especially at the upper end of the range, may overwhelm human adaptive mechanisms in areas of marginal productivity and in countries where traditional coping mechanisms have been disrupted. In general, natural ecosystems would be much more sorely stressed, probably beyond their capacities for adjustment. For example, even temperature changes at the lower end of the range would result in shifts of local climates at rates faster than the movement of long-lived trees with large seeds.

A comprehensive catalog of beneficial and harmful impacts is not available. Nor is an estimation of the magnitude of the likely impacts of projected climatic changes. Table A.2 summarizes impacts to human and natural systems in the United States according to the sensitivity categories.

28. Can costs be calculated for the various impacts of projected climate changes?

Not directly. The climatic changes likely to occur in the future cannot be directly measured. The costs and benefits associated with some aspects of certain changes can be estimated, however. These can be used to produce very rough estimates of the costs of climatic impacts. However, these must be recognized as very imprecise indicators.

In general, the costs in the United States associated with the first category of sensitivity are low in relation to overall economic activity. The

TABLE A.2 The Sensitivity and Adaptability of Human Activities and Nature

costs associated with the second category are higher but still should not result in major disruption of the economy. Appropriate adjustments could probably be accomplished without replacing current systems. Costs associated with the third category are much larger, and the adjustments could involve disruption. Some type of anticipation for meeting them may be justified. The category of extremely adverse impacts would be associated with high potential costs and would disrupt most aspects of the system in question. These outcomes, however, are extremely difficult to assess. Table A.3 summarizes some "benchmark" costs illustrative of impacts similar to those that might be associated with climate change.

TABLE A.3 Illustrative Costs of Impacts and Adaptations

29. Are there possible consequences of greenhouse warming with highly adverse impacts?

Two have been identified.

• Deep ocean currents could be interrupted. Increased freshwater runoff in the Arctic might alter the salinity of northern oceans, thereby reducing or stopping the vertical flow of water into the deep ocean along Greenland and Iceland. This might interrupt a major deep ocean current running from the North Atlantic around the Cape of Good Hope and through the Indian Ocean to the Pacific. This could affect temperature and precipitation, with repercussions that might be catastrophic. Very little is currently known about the potential of this phenomenon.

• The West Antarctic Ice Sheet could surge. The Antarctic and Greenland ice sheets combined make up the world's largest reservoir of fresh water. The West Antarctic Ice Sheet alone contains enough water to raise global average sea level about 7 meters (23 feet). Warming could affect the speed at which the ice sheet flows to the sea and breaks off into icebergs. A large subsequent influx of fresh water could alter the salinity of the world's oceans, affecting currents and plant and animal populations alike. The ramifications are extreme, and it might lead to disruption of deep ocean currents and all that that entails. The timing of such a possibility is controversial. Current thinking is that it would take centuries, but there is little empirical evidence on which to base estimates.

30. What are appropriate responses to very uncertain, but highly adverse impacts?

Both individuals and societies must decide how to handle events that are very unlikely but which have severe consequences. Homeowners purchase insurance against the very unlikely event of fire. In essence, insurance is a cost today (the insurance premium) to avoid undesirable consequences later (losing one's possessions to fire). If we want to avoid unsure adverse impacts of possible climate change, we might want to spend money now that would reduce the likelihood that those things can happen. In principle, there are two different kinds of "climate insurance." We could do things that reduce the likelihood that the climate will change (mitigation options), or we could do things that reduce the sensitivity of affected human and natural systems to future climate change (adaptation options).

31. Does looking at potential impacts tell us where to set priorities for responding to greenhouse warming?

Partly. The examination of potential impacts can help provide rough estimates of the cost at which adaptation could be accomplished should climate change. This is an approximation of the "damage function" and can be used

to assess how much to spend on emission reductions or offsets. However, all estimates are approximations with very little precision. The amount to allocate to prevent additional greenhouse warming depends significantly on the preferred degree of risk aversion.

Preventing or Reducing Additional Greenhouse Warming

32. What are the sources of greenhouse gas emissions?

All of the major greenhouse gases except CFCs are produced by both natural processes and human activity. Table A.4 summarizes the principal sources of greenhouse gases associated with human activity.

33. What interventions could reduce greenhouse warming?

It is useful to examine two different aspects of reducing emissions or offsetting emissions:

• ''Direct" reduction or offsetting of emissions through altering equipment, products, physical processes, or behaviors

• "Indirect" reduction or offsetting of emissions through altering the behavior of people in their economic or private lives and thus affecting the overall level of activity leading to emissions

It is much easier to estimate potential effectiveness and costs of direct reductions than of indirect incentives on human behavior. This is mostly because of the many factors that affect behavior in addition to the incentives in any particular program.

34. How can specific mitigation options be compared?

Mitigation options can be compared quantitatively in terms of their cost-effectiveness and qualitatively in terms of the obstacles to their implementation and in terms of other benefits and costs.

The standard quantitative unit used to compare mitigation options is the cost per metric ton of carbon emissions reduced or per metric ton of carbon removed from the atmosphere. The amount of carbon can be converted to the amount of CO 2 in the atmosphere by multiplying by 3.67, which is the ratio of the molecular weights of carbon and CO 2 . Other greenhouse gases can be "translated" to CO 2 equivalency by using two calculations. First, the amount of radiative forcing caused by a specific concentration of the gas is estimated in terms of the change in energy reaching the surface (in watts per square meter). This estimate accounts for atmospheric chemistry, atmospheric lifetime of the gas, and other relevant factors affecting the total contribution of that gas to greenhouse warming. Second, the amount of

TABLE A.4 Estimated 1985 Global Greenhouse Gas Emissions from Human Activities

CO 2 that would produce the same amount of forcing at the surface is calculated. This is the CO 2 equivalent for that specific concentration of the other greenhouse gas. The respective costs per ton for different options can then be compared directly. It is important to recognize, however, that these calculations allow comparison only of initial contributions. They do not account for changes in energy-trapping effectiveness over the various lifetimes of these gases in the atmosphere.

35. What mitigation options are most cost-effective?

The panel ranks options for reducing greenhouse gas emissions or removing greenhouse gases from the atmosphere according to their cost-effectiveness. Some of these options have net savings or very low net implementation costs compared to other investments. The options range from net savings to more than $100 per metric ton of CO 2 -equivalent emissions avoided or removed from the atmosphere. The most cost-effective mitigation options are presented in Table A.5.

36. What are examples of options with large potential to reduce or offset emissions?

The so-called geoengineering options have the potential of substantially affecting atmospheric concentrations of greenhouse gases. They have the ability to screen incoming sunlight, stimulate uptake of CO 2 by plants and animals in the oceans, or remove CO 2 from the atmosphere. Although they appear feasible, they require additional investigation because of their potential environmental impacts.

37. How much would it cost to significantly reduce current U.S. greenhouse gas emissions?

It depends on the level of emission reduction desired and how it is done. The most cost-effective options are those that enhance efficient use of energy: efficiency improvements in lighting and appliances, white roofs and paving to enhance reflectivity, and improvement in building and construction practices.

Figure A.4 compares mitigation options, and Table A.5 gives the panel's estimates of net cost and emission reductions for several options. It must be emphasized that the table presents the panel's estimates of the maximum technical potential for each option. The calculation of cost-effectiveness of lighting efficiency, for example, does not consider whether the supply of light bulbs could meet the demand with current production capacities. Nor does it consider the trade-off between expenditures on light bulbs and on health care, education, or basic shelter for low-income families. In addition, there is a danger of some "double counting." For example, in the area of energy supply both nuclear and natural gas energy options assume replacement

TABLE A.5 Comparison of Selected Mitigation Options in the United States

of the same coal-fired power plants. Table A.5, however, presents only options that avoid double counting. Finally, although there is evidence that efficiency programs can pay, there is no field evidence showing success with programs on the massive scale suggested here. Thus there may be very good reasons why "negative cost options" on the figure are not implemented today.

The United States could reduce its greenhouse gas emissions by between 10 and 40 percent of the 1990 levels at low cost, or perhaps some net savings, if proper policies are implemented.

FIGURE A.4 Comparison of mitigation options. Total potential reduction of CO 2 equivalent emissions is compared to the cost in dollars per ton of CO 2 reduction. Options are ranked from left to right in CO 2 emissions according to cost. Some options show the possibility of reductions of CO 2 emissions at a net savings.

Adapting to Additional Greenhouse Warming

38. Will human and natural systems adapt without assistance?

Farmers adjust their crops and cultivation practices in response to weather patterns over time. Natural ecosystems also adapt to changing conditions. The real issue is the rate at which human and natural systems will be able to adjust.

39. At what rates can human and natural systems adapt?

Many human systems have decision and investment cycles that are shorter than the time in which impacts of climate change would become manifest. These systems in the United States should be able to adjust to climate change without governmental intervention, as long as it is gradual and information about the rates of change is widely available. This applies to agriculture, commercial forestry, and most of industry. Industrial sectors with extremely long investment cycles (e.g., transport systems, urban infrastructure, and major structures and facilities) or requiring high volumes of water may require special attention. Coastal urban settlements would be

able to react quickly (within 3 to 5 years) if sea level rises. Response would be much more difficult, however, where financial and other resources are limited, such as in many developing countries.

Some natural systems adjust at rates an order of magnitude or more slower than those anticipated for global-scale temperature changes. For example, the observed and theoretical migration of large trees with heavy seeds is an order of magnitude slower than the anticipated change in climate zones. Furthermore, natural ecosystems cannot anticipate climate change but must wait until after conditions have changed to respond.

40. What is the value of the vulnerable natural ecosystems?

Natural ecosystems contribute commercial products, but their value is generally considered to exceed this contribution to the economy. For example, genetic resources are generally undervalued because people cannot capture the benefits of investments they might make in preserving biodiversity. Many species are unlikely to ever have commercial value, and it is virtually impossible to predict which ones will become marketable.

In addition, some people value natural systems regardless of their economic value. Loss of species, in their view, is undesirable whether or not those species have any commercial value. They generally hold that preservation of the potential for evolutionary change is a desirable goal in and of itself. Humanity, they claim, should not do things that alter the course of natural evolution. This view is sometimes also applied to humanity's cultural heritage—to buildings, music, art, and other cultural artifacts.

41. How much would it cost to adapt to the anticipated climatic changes?

The panel's analysis suggests that some human and natural systems are not very sensitive to the anticipated climatic changes. These include most sectors of industry. Other systems are sensitive to climatic changes but can be adapted at a cost whose present value is small in comparison to the overall level of economic activity. These include agriculture, commercial forestry, urban coastal infrastructure, and tourism. Some systems are sensitive, and their adaptation is questionable. The unmanaged systems of plants and animals that occupy much of our lands and oceans adapt at a pace slower than the anticipated rate of climatic change. Their future under climate change would be problematic. Poor nations may also adapt painfully. Finally, some possible climatic changes like shifts in ocean currents have consequences that could be extremely severe, and thus the costs of adaptation might be very large. However, it is not currently possible to assess the likelihood of such cataclysmic changes.

No attempt has been made to comprehensively assess the costs of anticipated climatic changes on a global basis.

42. How much should be spent in response to greenhouse warming?

The answer depends on the estimated costs of prevention and the estimated damages from greenhouse warming. In addition, the likelihood and severity of extreme events, the discount rate, and the degree of risk aversion will modify this first-order approximation.

The appropriate level of expenditure depends on the value attached to the adverse outcomes compared to other allocations of available funds, human resources, and so on. In essence, the answer depends on the degree of risk aversion attached to adverse outcomes of climate change. The fact that less is known about the more adverse outcomes makes this a classic example of dealing with high-consequence, low-probability events. Programs that truly increase our knowledge and monitor relevant changes are especially needed.

Implementing Response Programs

43. What policy instruments could be used to implement response options?

A wide array of policy instruments of two different types are available: regulation and incentives. Regulatory instruments mandate action, and include controls on consumption (bans, quotas, required product attributes), production (quotas on products or substances), factors in design or production (efficiency, durability, processes), and provision of services (mass transit, land use). Incentive instruments are designed to influence decisions by individuals and organizations and include taxes and subsidies on production factors (carbon tax, fuel tax), on products and other outputs (emission taxes, product taxes), financial inducements (tax credits, subsidies), and transferable emission rights (tradable emission reductions, tradable credits). The choice of policy instrument depends on the objective to be served.

44. At what level of society should actions be taken?

Interventions at all levels of human aggregation could effectively reduce greenhouse warming. For example, individuals could reduce energy consumption, recycle goods, and reduce consumption of deleterious materials. Local governments could control emissions from buildings, transport fleets, waste processing plants, and landfill dumps. State governments could restructure electric utility pricing structures and stimulate a variety of efficiency incentives. National governments could pursue action in most of the policy areas of relevance. International organizations could coordinate programs in various parts of the world, manage transfers of resources and technologies, and facilitate exchange of monitoring and other relevant data.

45. Is international action necessary?

The greenhouse phenomenon is global. Unilateral actions can contribute significantly, but national efforts alone would not be sufficient to eliminate the problem. The United States is the largest contributor of CO 2 emissions (with estimates ranging from 17 to 21 percent of the global total). But even if this country were to totally eliminate or offset its emissions, the effect on overall greenhouse warming might be lost if no other countries acted in concert with that aim.

46. What about differences between rich and poor countries?

Poor and developing countries are likely to be the most vulnerable to climate change. In addition, many developing countries today are sorely pressed in a variety of other ways. They may conclude that other issues have more immediate consequences for their citizens. Incentives in all parts of the world for intervention in the area of greenhouse warming may thus draw heavily on the industrialized nations. They may be called upon to help poor countries stimulate economic development and thus become better able to cope with climate change. They may also be asked to provide expertise and technologies to help poor countries adapt to the conditions they face.

Actions to be Taken

47. Do scientific assessments of greenhouse warming tell us what to do?

Current scientific understanding of greenhouse warming is both incomplete and uncertain. Response depends in part on the degree of risk aversion attached to poorly understood, low-probability events with extremely adverse outcomes. Lack of scientific understanding should not be used as a justification for avoiding reasoned decisions about responses to possible additional greenhouse warming.

48. Is it better to prevent greenhouse warming now or wait and adapt to the consequences ?

This complicated question has several parts.

• First, will it be possible to live with the consequences if nothing is done now? The panel's analysis suggests that advanced, industrialized countries will be able to adapt to most of the anticipated consequences of additional greenhouse warming without great economic hardship. In some regions, climate and related conditions may be noticeably worse, but in other regions better. Countries that currently face difficulty coping with extreme climatic events, or whose traditional coping mechanisms are breaking down, may be sorely pressed by the climatic changes accompanying an equivalent doubling of atmospheric CO 2 concentrations. It is important to recognize that there may be dramatic improvement or disastrous deterioration in specific locales. In addition, this analysis applies to the next 30 to 50 years. The situation may be different beyond that time horizon.

Natural communities of plants and animals, however, face much greater difficulties. Greenhouse warming would likely stress such ecosystems sufficiently to break them apart, resulting in a restructuring of the community in any given locale. New species would be likely to gain dominance, with a different overall mix of species. Some individual species would migrate to new, more livable locations. Greenhouse warming would most likely change the face of the natural landscape. Similar changes would occur in lakes and oceans.

In addition, there are possible extremely adverse consequences, such as changing ocean currents, that are poorly understood today. The response to such possibilities depends on the degree of risk aversion concerning those outcomes. The greater the degree of risk aversion, the greater the impetus for preventive action.

• Second, does it matter when interventions are made? Yes, for three different kinds of reasons. Because greenhouse gases have relatively long lifetimes in the atmosphere, and because of lags in the response of the system, their effect builds up over time. These time-dependent phenomena lead to the long-term "equilibrium" warming being greater than the "realized" warming at any given point in time. These dynamic aspects of the climate system show the importance of acting now to change traditional patterns of behavior that we have recently recognized to be detrimental, such as heavy reliance on fossil fuels. In addition, the implications of intervention programs for the overall economy vary with time. Gradual imposition of restraints is much less disruptive to the overall economy than their sudden application. Finally, the length of investment cycles can be crucial in determining the costs of intervention. In addition, some investments can be thought of as insurance, or payments now to avoid undesirable outcomes in the future. The choice is made more complicated by the fact that the outcomes are highly uncertain.

• Third, what discount rate should be used? The selection of a discount rate is very controversial. Macroeconomic calculations for the United States show a return on capital investment of 12 percent. The choice of discount rate reflects time preference. The panel has used discount rates of 3, 6, and 10 percent in its analysis. Finally, consumers often behave as if they have used a discount rate closer to 30 percent. The panel has also included this rate for comparison when options involve individual action.

49. Are there special attributes of programs appropriate for response to greenhouse warming?

Yes. The uncertainties present in all aspects of climate change and our understanding of response to potential greenhouse warming place a high premium on information. Small-scale interventions that are both reversible and yield information about key aspects of the relevant phenomena are especially attractive for both mitigation and adaptation options. Monitoring of emission rates, climatic changes, and human and ecologic responses should yield considerable payoffs.

Perhaps the most important attribute of preferred policies is that they be able to accommodate surprises. They should be constructed so that they are flexible and can change if the nature or speed of stress is different than anticipated.

50. What should be done now?

The panel developed a set of recommended options in five areas: reducing or offsetting emissions, enhancing adaptation to greenhouse warming, improving knowledge for future decisions, evaluating geoengineering options, and exercising international leadership. The panel recommends moving decisively to undertake all of the actions described under questions 51 through 55 below.

51. What can be done to reduce or offset emissions of greenhouse gases?

Three areas dominate the panel's analysis of reducing or offsetting current emissions: eliminating CFC emissions and developing substitutes that minimize or eliminate greenhouse gas emissions, changing energy policy, and utilizing forest offsets. Eliminating CFC emissions has the biggest single contribution. Recommendations concerning energy policy are to examine how to make the price of energy reflect all health, environmental, and other social costs with a goal of gradual introduction of such a system; to make conservation and efficiency the chief element in energy policy; and to consider the full range of supply, conversion, end use, and external effects in planning future energy supply. Global deforestation should be reduced, and a moderate domestic reforestation program should be explored.

52. What can be done now to help people and natural systems of plants and animals adapt to future greenhouse warming?

Most of the actions that can be taken today improve the capability of the affected systems to deal with current climatic variability. Options include maintaining agricultural basic, applied, and experimental research; making water supplies more robust by coping with present variability; taking into consideration possible climate change in the margins of safety for long-lived structures; and reducing present rates of loss in biodiversity.

53. What can be done to improve knowledge for future decisions?

Action is needed in several areas. Collection and dissemination of data that provide an uninterrupted record of the evolving climate and of data that are needed for the improvement and testing of climate models should be expanded. Weather forecasts should be improved, especially of extremes, for weeks and seasons to ease adaptation to climate change. The mechanisms that play a significant role in the responses of the climate to changing concentrations of greenhouse gases need further identification, and quantification at scales appropriate for climate models. Field research should be conducted on entire systems of species over many years to learn how CO 2 enrichment and other facets of greenhouse warming alter the mix of species and changes in total production or quality of biomass. Research on social and economic aspects of global change and greenhouse warming should be strengthened.

54. Do geoengineering options really have potential?

Preliminary assessments of these options suggest that they have large potential to mitigate greenhouse warming and are relatively cost-effective in comparison to other mitigation options. However, their feasibility and especially the side-effects associated with them need to be carefully examined. Because the geoengineering options have the potential to affect greenhouse warming on a substantial scale, because there is convincing evidence that some of these cause or alter a variety of chemical reactions in the atmosphere, and because the climate system is poorly understood, such options must be considered extremely carefully. If greenhouse warming occurs, and the climate system turns out to be highly sensitive to radiative forcing, they may be needed.

55. What should the United States do at the international level?

The United States should resume full participation in international programs to slow population growth and contribute its share to their financial and other support. In addition, the United States should participate fully in international agreements and programs to address greenhouse warming, including representation by officials at an appropriate level.

Global warming continues to gain importance on the international agenda and calls for action are heightening. Yet, there is still controversy over what must be done and what is needed to proceed.

Policy Implications of Greenhouse Warming describes the information necessary to make decisions about global warming resulting from atmospheric releases of radiatively active trace gases. The conclusions and recommendations include some unexpected results. The distinguished authoring committee provides specific advice for U.S. policy and addresses the need for an international response to potential greenhouse warming.

It offers a realistic view of gaps in the scientific understanding of greenhouse warming and how much effort and expense might be required to produce definitive answers.

The book presents methods for assessing options to reduce emissions of greenhouse gases into the atmosphere, offset emissions, and assist humans and unmanaged systems of plants and animals to adjust to the consequences of global warming.

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