essay on green house gases

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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Greenhouse gases: Causes, sources and environmental effects

Greenhouse gases help keep the Earth at a habitable temperature — until there is too much of them.

Solar radiation and the "greenhouse effect"

• Global warming
• Future outlook

Additional resources

Behind the phenomena of global warming and climate change lies the increase in greenhouse gases in our atmosphere. A greenhouse gas is any gaseous compound in the atmosphere that is capable of absorbing infrared radiation , thereby trapping and holding heat in the atmosphere. By increasing the heat in the atmosphere, greenhouse gases are responsible for the greenhouse effect, which ultimately leads to global warming. (The effects of global warming can been seen across the globe.)

Related: 10 signs that Earth's climate is off the rails

Global warming isn't a recent scientific concept. The basics of the phenomenon were worked out well over a century ago by Swedish physicist and chemist Svante Arrhenius, in 1896. His paper, published in the Philosophical Magazine and Journal of Science , was the first to quantify the contribution of carbon dioxide to what scientists now call the " greenhouse effect ."

The greenhouse effect occurs because the sun bombards Earth with enormous amounts of radiation that strike Earth's atmosphere in the form of visible light, plus ultraviolet (UV), infrared (IR) and other types of radiation that are invisible to the human eye. UV radiation has a shorter wavelength and a higher energy level than visible light, while IR radiation has a longer wavelength and a weaker energy level. About 30% of the radiation that strikes Earth is reflected back out to space by clouds, ice and other reflective surfaces. The remaining 70% is absorbed by the oceans, the land and the atmosphere, according to NASA's Earth Observatory .

As they heat up, the oceans, land and atmosphere release heat in the form of IR thermal radiation, which passes out of the atmosphere and into space. It's this equilibrium of incoming and outgoing radiation that makes the Earth habitable, with an average temperature of about 59 degrees Fahrenheit (15 degrees Celsius), according to NASA . Without this atmospheric equilibrium, Earth would be as cold and lifeless as its moon, or as blazing hot as Venus. The moon, which has almost no atmosphere, is about minus 243 F (minus 153 C) on its dark side. Venus, on the other hand, has a very dense atmosphere that traps solar radiation; the average temperature on Venus is about 864 F (462 C).

The exchange of incoming and outgoing radiation that warms the Earth is often referred to as the greenhouse effect because an agricultural greenhouse works in much the same way. Incoming shortwave UV radiation easily passes through the glass walls of a greenhouse and is absorbed by the plants and hard surfaces inside. Weaker, longwave IR radiation, however, has difficulty passing through the glass walls and is thereby trapped inside, warming the greenhouse.

How greenhouse gases cause global warming

The gases in the atmosphere that absorb radiation are known as "greenhouse gases" (abbreviated as GHG) because they are largely responsible for the greenhouse effect. The greenhouse effect, in turn, is one of the leading causes of global warming. The most significant greenhouse gases, according to the Environmental Protection Agency (EPA), are: water vapor (H2O), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). 

"While oxygen (O2) is the second most abundant gas in our atmosphere, O2 does not absorb thermal infrared radiation," Michael Daley, an associate professor of environmental science at Lasell College in Massachusetts, told Live Science.

Global warming and the greenhouse gases that cause it occur naturally — without them, Earth's average surface temperature would be a gelid zero degrees F (minus 18 C). But the amount of greenhouse gases in the atmosphere has skyrocketed to detrimental levels in recent history. 

Related: Carbon dioxide soars to record breaking levels not seen in at least 800,000 years

During the 20,000-year period before the Industrial Revolution, atmospheric CO2 fluctuated between about 180 parts per million (ppm) during ice ages and 280 ppm during interglacial warm periods. However, since the beginning of the Industrial Revolution in the 1750s, the amount of CO2 has risen nearly 50%, according to NASA’s Global Climate Change portal . Today, CO2 levels stand at over 410 ppm.

Fluorinated gases — gases to which the element fluorine has been added — are created during industrial processes and are also considered greenhouse gases. These include hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride. Although they are present in the atmosphere in very small concentrations, they trap heat very effectively, making them high "global warming potential" (GWP) gases.

Chlorofluorocarbons (CFCs), once used as refrigerants and aerosol propellants until they were phased out by international agreement, are also greenhouse gases.

Related: Global warming vs. solar cooling: The showdown begins in 2020

There are three factors that affect the degree to which a greenhouse gas will influence global warming: Its abundance in the atmosphere, how long it stays in the atmosphere and its GWP. For example, water vapor is the most abundant greenhouse gas, but carbon dioxide has a more significant impact on global warming due to its abundance in the atmosphere plus its relatively long atmospheric lifetime of 300 to 1,000 years, according to NASA . Water vapor, on the other hand, has an atmospheric lifetime of no more than 10 days, according to a 2020 study published in the Journal of the Atmospheric Sciences .

Methane is about 21 times more efficient at absorbing radiation than CO2, giving it a higher GWP rating, even though it stays in the atmosphere for only about 12 years, according to the United Nations Framework Convention on Climate Change (UNFCCC) . Although methane and other GHGs are capable of trapping more heat than CO2, scientists still consider carbon dioxide to be the dominant greenhouse gas because its warming effect outlives the others' effects by centuries.

Sources of greenhouse gases

Some greenhouse gases, such as methane, are produced through agricultural practices, in the form of livestock manure, for example. Others, like CO2, largely result from natural processes like respiration, and from the burning of fossil fuels like coal, oil and gas.

Another primary source of CO2 is deforestation . When trees are felled to produce goods or heat, they release the carbon that is normally stored for photosynthesis . This process releases up to 4.8 billion metric tons of carbon into the atmosphere every year, according to the World Resources Institute .

Forestry and other land-use practices can offset some of these greenhouse gas emissions. "Replanting helps to reduce the buildup of carbon dioxide in the atmosphere as growing trees sequester carbon dioxide through photosynthesis," Daley told Live Science. "However, forests cannot sequester all of the carbon dioxide we are emitting to the atmosphere through the burning of fossil fuels, and a reduction in fossil fuel emissions is still necessary to avoid buildup in the atmosphere."

Worldwide, the output of greenhouse gases is a source of grave concern. According to NOAA’s Climate.gov , over the past 60 years, atmospheric CO2 has increased at an annual rate that's 100 times faster than previous natural increases. The last time global atmospheric CO2 amounts were this high was 3 million years ago, when temperatures were up to 5.4 degrees F (3 degrees C) higher than during the pre-industrial era. As a result of modern-day CO2-induced global warming, 2016 was the warmest year on record, with 2019 and 2020 ranking as the next warmest, respectively. In fact, the six hottest years on record have all occurred since 2015, according to the World Meteorological Organization .

"The warming we observe affects atmospheric circulation, which impacts rainfall patterns globally," said Josef Werne, an associate professor in the Department of Geology and Planetary Science at the University of Pittsburgh. "This will lead to big environmental changes, and challenges, for people all across the globe."

Our planet's future

If current trends continue, scientists, government officials and a growing number of citizens fear that the worst effects of global warming — extreme weather, rising sea levels , plant and animal extinctions, ocean acidification , major shifts in climate and unprecedented social upheaval — will be inevitable.

In an effort to combat GHG-induced global warming, the U.S. government created a climate action plan in 2013. And in April 2016, representatives from 73 countries signed the Paris Agreement , an international pact to combat climate change by investing in a sustainable, low-carbon future, according to the UNFCCC . Although the U.S. withdrew from the Paris Agreement in 2017, it rejoined in late-January 2021. President Biden's administration has also set a target of reducing U.S. emissions by 50-52% of 2005 levels by the year 2030. (Emissions are routinely compared to those in 2005 — the year U.S. emissions of CO2 peaked at nearly 6 billion tons.)

In 2020, global carbon dioxide emissions fell 6.4% (13% in the U.S. alone) — the first time in decades the annual rate hasn’t climbed, Nature reported . This was in part due to the decrease in fossil fuel combustion resulting from the switch to natural gas from coal , but largely because of the forced standstill in economic, social and transportation activities in response to the COVID-19 pandemic . Scientists expected the annual emissions decline to actually be larger than it was, but emissions rebounded as restrictions were lifted in some nations and activities recovered toward the end of 2020. 

Related: Global carbon emissions dropped an unprecedented 17% during the coronavirus lockdown — and it changes nothing

In order to limit global warming to the 2.7 degree F (1.5 degree C) target set by the Paris Agreement, the world still needs to cut its CO2 emissions by 7.6% for the next decade, according to the UN Environment Programme . 

Researchers around the world continue to work toward finding ways to lower greenhouse gas emissions and mitigate their effects. One potential solution scientists are examining is to suck some of the carbon dioxide out of the atmosphere and bury it underground indefinitely. Advocates argue that carbon capture and storage is technologically feasible , but market forces have prevented widespread adoption. 

Whether or not removing already-emitted carbon from the atmosphere is feasible, preventing future warming requires stopping the emissions of greenhouse gases. The most ambitious effort to forestall warming thus far is the 2016 Paris Agreement. This nonbinding international treaty aims to keep warming "well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius," according to the United Nations. Each signatory to the treaty agreed to set their own voluntary greenhouse gas emission limits and to make them stricter over time. Climate scientists said that the emissions limits committed under the agreement wouldn't keep warming as low as 1.5 or even 2 degrees C, but that it would be an improvement over the "business-as-usual" scenario.

• Find out the latest research and policy updates regarding global warming via NOAA's Climate.gov portal .
• Learn more about the Global Carbon Project .
• Read more about the impact of COVID-19 on 2020 CO2 emissions, according to Carbon Brief .

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Tiffany Means is a meteorologist turned science writer based in the Blue Ridge mountains of North Carolina. Her work has appeared in Yale Climate Connections, The Farmers' Almanac, and other publications. Tiffany has a bachelor's degree in atmospheric science from the University of North Carolina, Asheville, and she is earning a master's in science writing at Johns Hopkins University. 

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Greenhouse Effect

Global warming describes the current rise in the average temperature of Earth’s air and oceans. Global warming is often described as the most recent example of climate change.

Earth Science, Meteorology, Geography

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Global warming describes the current rise in the average temperature of Earth’s air and oceans. Global warming is often described as the most recent example of climate change . Earth’s climate has changed many times. Our planet has gone through multiple ice ages , in which ice sheets and glaciers covered large portions of Earth. It has also gone through warm periods when temperatures were higher than they are today. Past changes in Earth’s temperature happened very slowly, over hundreds of thousands of years. However, the recent warming trend is happening much faster than it ever has. Natural cycles of warming and cooling are not enough to explain the amount of warming we have experienced in such a short time—only human activities can account for it. Scientists worry that the climate is changing faster than some living things can adapt to it. In 1988, the World Meteorological Organization and the United Nations Environment Programme established a committee of climatologists , meteorologists , geographers , and other scientists from around the world. This Intergovernmental Panel on Climate Change (IPCC) includes thousands of scientists who review the most up-to-date research available related to global warming and climate change. The IPCC evaluates the risk of climate change caused by human activities. According to the IPCC’s most recent report (in 2007), Earth’s average surface temperatures have risen about 0.74 degrees Celsius (1.33 degrees Fahrenheit) during the past 100 years. The increase is greater in northern latitudes . The IPCC also found that land regions are warming faster than oceans. The IPCC states that most of the temperature increase since the mid-20th century is likely due to human activities. The Greenhouse Effect Human activities contribute to global warming by increasing the greenhouse effect. The greenhouse effect happens when certain gases—known as greenhouse gases —collect in Earth’s atmosphere . These gases, which occur naturally in the atmosphere, include carbon dioxide , methane , nitrogen oxide, and fluorinated gases sometimes known as chlorofluorocarbons (CFCs). Greenhouse gases let the sun’s light shine onto Earth’s surface, but they trap the heat that reflects back up into the atmosphere. In this way, they act like the insulating glass walls of a greenhouse. The greenhouse effect keeps Earth’s climate comfortable. Without it, surface temperatures would be cooler by about 33 degrees Celsius (60 degrees Fahrenheit), and many life forms would freeze . Since the Industrial Revolution in the late 1700s and early 1800s, people have been releasing large quantities of greenhouse gases into the atmosphere. That amount has skyrocketed in the past century. Greenhouse gas emissions increased 70 percent between 1970 and 2004. Emissions of carbon dioxide, the most important greenhouse gas, rose by about 80 percent during that time. The amount of carbon dioxide in the atmosphere today far exceeds the natural range seen over the last 650,000 years. Most of the carbon dioxide that people put into the atmosphere comes from burning fossil fuels such as oil , coal , and natural gas . Cars, trucks, trains, and planes all burn fossil fuels. Many electric power plants also burn fossil fuels. Another way people release carbon dioxide into the atmosphere is by cutting down forests . This happens for two reasons. Decaying plant material, including trees, releases tons of carbon dioxide into the atmosphere. Living trees absorb carbon dioxide. By diminishing the number of trees to absorb carbon dioxide, the gas remains in the atmosphere. Most methane in the atmosphere comes from livestock farming , landfills , and fossil fuel production such as coal mining and natural gas processing. Nitrous oxide comes from agricultural technology and fossil fuel burning. Fluorinated gases include chlorofluorocarbons, hydrochlorofluorocarbons , and hydrofluorocarbons. These greenhouse gases are used in aerosol cans and refrigeration. All of these human activities add greenhouse gases to the atmosphere, trapping more heat than usual and contributing to global warming. Effects of Global Warming Even slight rises in average global temperatures can have huge effects. Perhaps the biggest, most obvious effect is that glaciers and ice caps melt faster than usual. The meltwater drains into the oceans, causing sea levels to rise and oceans to become less salty. Ice sheets and glaciers advance and retreat naturally. As Earth’s temperature has changed, the ice sheets have grown and shrunk, and sea levels have fallen and risen. Ancient corals found on land in Florida, Bermuda, and the Bahamas show that the sea level must have been five to six meters (16-20 feet) higher 130,000 years ago than it is today. Earth doesn’t need to become oven-hot to melt the glaciers. Northern summers were just three to five degrees Celsius (five to nine degrees Fahrenheit) warmer during the time of those ancient fossils than they are today. However, the speed at which global warming is taking place is unprecedented . The effects are unknown. Glaciers and ice caps cover about 10 percent of the world’s landmass today. They hold about 75 percent of the world’s fresh water. If all of this ice melted, sea levels would rise by about 70 meters (230 feet). The IPCC reported that the global sea level rose about 1.8 millimeters (0.07 inches) per year from 1961 to 1993, and 3.1 millimeters (0.12 inches) per year since 1993. Rising sea levels could flood coastal communities, displacing millions of people in areas such as Bangladesh, the Netherlands, and the U.S. state of Florida. Forced migration would impact not only those areas, but the regions to which the “ climate refugees ” flee . Millions more people in countries like Bolivia, Peru, and India depend on glacial meltwater for drinking, irrigation , and hydroelectric power . Rapid loss of these glaciers would devastate those countries. Glacial melt has already raised the global sea level slightly. However, scientists are discovering ways the sea level could increase even faster. For example, the melting of the Chacaltaya Glacier in Bolivia has exposed dark rocks beneath it. The rocks absorb heat from the sun, speeding up the melting process. Many scientists use the term “climate change” instead of “global warming.” This is because greenhouse gas emissions affect more than just temperature. Another effect involves changes in precipitation like rain and snow . Patterns in precipitation may change or become more extreme. Over the course of 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 parts of southern Asia. Future Changes Nobody can look into a crystal ball and predict the future with certainty. However, scientists can make estimates about future population growth, greenhouse gas emissions, and other factors that affect climate. They can enter those estimates into computer models to find out the most likely effects of global warming. The IPCC predicts that greenhouse gas emissions will continue to increase over the next few decades . As a result, they predict the average global temperature will increase by about 0.2 degrees Celsius (0.36 degrees Fahrenheit) per decade. Even if we reduce greenhouse gas and aerosol emissions to their 2000 levels, we can still expect a warming of about 0.1 degree Celsius (0.18 degrees Fahrenheit) per decade. The panel also predicts global warming will contribute to some serious changes in water supplies around the world. By the middle of the 21st century, the IPCC predicts, river runoff and water availability will most likely increase at high latitudes and in some tropical areas. However, many dry regions in the mid-latitudes and tropics will experience a decrease in water resources. As a result, millions of people may be exposed to water shortages . Water shortages decrease the amount of water available for drinking, electricity , and hygiene . Shortages also reduce water used for irrigation. Agricultural output would slow and food prices would climb. Consistent years of drought in the Great Plains of the United States and Canada would have this effect. IPCC data also suggest that the frequency of heat waves and extreme precipitation will increase. Weather patterns such as storms and tropical cyclones will become more intense. Storms themselves may be stronger, more frequent, and longer-lasting. They would be followed by stronger storm surges , the immediate rise in sea level following storms. Storm surges are particularly damaging to coastal areas because their effects (flooding, erosion , damage to buildings and crops) are lasting. What We Can Do Reducing our greenhouse gas emissions is a critical step in slowing the global warming trend. Many governments around the world are working toward this goal. The biggest effort so far has been the Kyoto Protocol , which was adopted in 1997 and went into effect in 2005. By the end of 2009, 187 countries had signed and ratified the agreement. Under the protocol , 37 industrialized countries and the European Union have committed to reducing their greenhouse gas emissions. There are several ways that governments, industries, and individuals can reduce greenhouse gases. We can improve energy efficiency in homes and businesses. We can improve the fuel efficiency of cars and other vehicles. We can also support development of alternative energy sources, such as solar power and biofuels , that don’t involve burning fossil fuels. Some scientists are working to capture carbon dioxide and store it underground, rather than let it go into the atmosphere. This process is called carbon sequestration . Trees and other plants absorb carbon dioxide as they grow. Protecting existing forests and planting new ones can help balance greenhouse gases in the atmosphere. Changes in farming practices could also reduce greenhouse gas emissions. For example, farms use large amounts of nitrogen-based fertilizers , which increase nitrogen oxide emissions from the soil. Reducing the use of these fertilizers would reduce the amount of this greenhouse gas in the atmosphere. The way farmers handle animal manure can also have an effect on global warming. When manure is stored as liquid or slurry in ponds or tanks, it releases methane. When it dries as a solid, however, it does not. Reducing greenhouse gas emissions is vitally important. However, the global temperature has already changed and will most likely continue to change for years to come. The IPCC suggests that people explore ways to adapt to global warming as well as try to slow or stop it. Some of the suggestions for adapting include:

• Expanding water supplies through rain catchment , conservation , reuse, and desalination .
• Adjusting crop locations, variety, and planting dates.
• Building seawalls and storm surge barriers and creating marshes and wetlands as buffers against rising sea levels .
• Creating heat-health action plans , boosting emergency medical services, and improving disease surveillance and control.
• Diversifying tourism attractions, because existing attractions like ski resorts and coral reefs may disappear.
• Planning for roads and rail lines to cope with warming and/or flooding.
• Strengthening energy infrastructure , improving energy efficiency, and reducing dependence on single sources of energy.

Barking up the Wrong Tree Spruce bark beetles in the U.S. state of Alaska have had a population boom thanks to 20 years of warmer-than-average summers. The insects have managed to chew their way through 1.6 million hectares (four million acres) of spruce trees.

Disappearing Penguins Emperor penguins ( Aptenodytes forsteri ) made a showbiz splash in the 2005 film March of the Penguins . Sadly, their encore might include a disappearing act. In the 1970s, an abnormally long warm spell caused these Antarctic birds' population to drop by 50 percent. Some scientists worry that continued global warming will push the creatures to extinction by changing their habitat and food supply.

Shell Shock A sudden increase in the amount of carbon dioxide in the atmosphere does more than change Earth's temperature. A lot of the carbon dioxide in the air dissolves into seawater. There, it forms carbonic acid in a process called ocean acidification. Ocean acidification is making it hard for some sea creatures to build shells and skeletal structures. This could alter the ecological balance in the oceans and cause problems for fishing and tourism industries.

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Understanding Global Change

Discover why the climate and environment changes, your place in the Earth system, and paths to a resilient future.

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.

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• 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

• ENVIRONMENT

Carbon dioxide levels are at a record high. Here's what you need to know.

Carbon dioxide, a key greenhouse gas that drives global climate change, continues to rise every month. Find out the dangerous role it and other gases play.

By trapping heat from the sun, greenhouse gases have kept Earth's climate habitable for humans and millions of other species. But those gases are now out of balance and threaten to change drastically which living things can survive on this planet—and where.

Atmospheric levels of carbon dioxide—the most dangerous and prevalent greenhouse gas—are at the highest levels   ever recorded. Greenhouse gas levels are so high primarily because humans have released them into the air by burning fossil fuels. The gases absorb solar energy and keep heat close to Earth's surface, rather than letting it escape into space. That trapping of heat is known as the greenhouse effect.

The roots of the greenhouse effect concept lie in the 19th century, when French mathematician Joseph Fourier calculated in 1824 that the Earth would be much colder if it had no atmosphere. In 1896, Swedish scientist Svante Arrhenius was the first to link a rise in carbon dioxide gas from burning fossil fuels with a warming effect . Nearly a century later, American climate scientist James E. Hansen testified to Congress that “The greenhouse effect has been detected and is changing our climate now."

Today, climate change is the term scientists use to describe the complex shifts, driven by greenhouse gas concentrations, that are now affecting our planet’s weather and climate systems . Climate change encompasses not only the rising average temperatures we refer to as global warming but also extreme weather events, shifting wildlife populations and and habitats, rising seas , and a range of other impacts.

Governments and organizations around the world such as the Intergovernmental Panel on Climate Change (IPCC), the United Nations body that tracks the latest climate change science, are measuring greenhouse gases, tracking their impacts, and implementing solutions .

Major greenhouse gases and sources

Carbon dioxide (CO 2 ):   Carbon dioxide is the primary greenhouse gas, responsible for about three-quarters of emissions . It can linger in the atmosphere for thousands of years . In 2018, carbon dioxide levels reached 411 parts per million at Hawaii's Mauna Loa Atmospheric Baseline Observatory, the highest monthly average ever recorded . Carbon dioxide emissions mainly come from burning organic materials: coal, oil, gas, wood, and solid waste.

Methane (CH 4 ):   The main component of natural gas, methane is released from landfills, natural gas and petroleum industries, and agriculture (especially from the digestive systems of grazing animals). A molecule of methane doesn't stay in the atmosphere as long as a molecule of carbon dioxide—about 12 years—but it is at least 84 times more   potent over two decades. It accounts for about 16 percent of all greenhouse gas emissions.

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Nitrous Oxide (N 2 O):   Nitrous oxide occupies a relatively small share of global greenhouse gas emissions—about six percent—but it is 264 times more powerful than carbon dioxide over 20 years, and its lifetime in the atmosphere exceeds a century, according to the IPCC. Agriculture and livestock, including fertilizer, manure, and burning of agricultural residues, along with burning fuel, are the biggest sources of nitrous oxide emissions .

Industrial gases:   Fluorinated gases such as hydrofluorocarbons, perfluorocarbons, chlorofluorocarbons, sulfur hexafluoride (SF 6 ), and nitrogen trifluoride (NF 3 ) have heat-trapping potential thousands of times greater than CO 2 and stay in the atmosphere for hundreds to thousands of years. Accounting for about 2 percent of all emissions, they're used as refrigerants, solvents, and in manufacturing, sometimes occurring as byproducts.

Other greenhouse gases include water vapor and ozone (O 3 ). Water vapor is actually the world's most abundant greenhouse gas, but it is not tracked the same way as other greenhouse gases because it is not directly emitted by human activity and its effects are not well understood. Similarly, ground-level or tropospheric ozone (not to be confused with the protective stratospheric ozone layer higher up) is not emitted directly but emerges from complex reactions among pollutants in the air.

Effects of greenhouse gases

Greenhouse gases have far-ranging environmental and health effects. They cause climate change by trapping heat, and they also contribute to respiratory disease from smog and air pollution . Extreme weather, food supply disruptions, and increased wildfires are other effects of climate change caused by greenhouse gases. The typical weather patterns we've grown to expect will change ; some species will disappear; others will migrate or grow . ( Read more about greenhouse gas effects via climate change here . )

How to reduce greenhouse gas emissions

Virtually every sector of the global economy, from manufacturing to agriculture to transportation to power production, contributes greenhouse gases to the atmosphere, so all of them must evolve away from fossil fuels if we are to avoid the worst effects of climate change. Countries around the world acknowledged this reality with the Paris Climate Agreement of 2015. The changes will be most important among the biggest emitters: Twenty countries are responsible for at least three-quarters of the world's greenhouse gas emissions, with China, the United States, and India leading the way .

The technologies for ramping down greenhouse gas emissions already exist, for the most part. They include swapping fossil fuels for renewable sources, boosting energy efficiency, and discouraging carbon emissions by putting a price on them. ( Read more about such solutions here . )

The world technically has only one-fifth of its "carbon budget" —the total is 2.8 trillion metric tons—remaining in order to avoid warming the Earth more than 1.5 degrees Celsius. Halting the trends in motion will require more than just phasing out fossil fuels. In fact, the paths to halting global temperature increases of 1.5 or 2 degrees C, the two goals outlined by the IPCC, rely in some way on adopting methods of sucking CO2 from the sky . Those include planting trees, conserving existing forests and grasslands, and capturing CO 2 from power plants and factories.

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Carbon dioxide isn’t the only one that matters, and the gases vary widely in potency and duration.

When hearing the words “greenhouse gas,” most people think immediately of carbon dioxide. This is indeed the greenhouse gas that is currently producing the greatest impact on the Earth’s rapidly changing climate. But it is far from the only one making its mark, and for mitigating climate change it’s important to be able to compare the effects of the various gases that contribute to warming the planet.

But that’s not easy to do.

Greenhouse gases vary in not only their sources and the measures needed to control them, but also in how intensely they trap solar heat, how long they last once they’re in the atmosphere, and how they react with other gases and ultimately get flushed out of the air. The differences make it impossible to do the very thing researchers and policymakers want most to do: come up with a simple conversion factor to allow exact comparisons among them.

Let’s take a look at the most extreme case: chlorofluorocarbons (CFCs). Compared to carbon dioxide, CFCs can produce more than 10,000 times as much warming, pound for pound, once they are in the air. Fortunately, CFCs were banned by an international agreement called the Montreal Protocol in 1987 — not because of their dramatic warming potential, although that was a secondary reason recognized at the time, but because they were found to be the primary cause of the rapidly escalating destruction of the Earth’s ozone layer, which protects the planet from dangerous, cancer-causing levels of ultraviolet radiation.

Out of the picture

CFCs “would be a major player by now” in contributing to global warming if they hadn’t been phased out, says Susan Solomon, the Lee and Geraldine Martin Professor of Environmental Studies at MIT. By now, if they were still being used at the same rate as before the phaseout, CFCs would be contributing about one-third as much to the Earth’s greenhouse effect as carbon dioxide, which remains by far the biggest contributor, she says.

For comparison, she says, the Kyoto Protocol (now superceded by the Paris Agreement of 2015), which called for a series of measures to reduce greenhouse gas emissions around the world, produced a total reduction of about 2 gigatons of “carbon equivalent” emissions per year, while the phaseout of CFCs has already eliminated five times as much — an estimated 10 gigatons of carbon equivalent gas per year.

Today, the number-two producer of human-caused greenhouse effects is methane, the main constituent of natural gas. When initially released, methane is about 100 times more potent than carbon dioxide, but its lifetime in the atmosphere is much shorter — about a decade, unlike carbon dioxide’s residence time of centuries. When averaged over a 20-year period, methane’s “greenhouse gas equivalency” is about 72 times that of carbon dioxide, but when looked at on a timescale of 100 years, that equivalency drops to just 25 times.

Methane comes from multiple sources, some of which are relatively hard to measure. For example, leakage from natural gas wells, storage facilities, and distribution systems is a significant source. But because such leaks are highly variable and depend on factors such as well construction methods and maintenance systems for infrastructure — which in some cases are proprietary information — there has been a great deal of controversy over the extent of such leaks. Other sources, such as emissions related to wetlands, deforestation, and cattle, are difficult to measure accurately.

Accounting for dynamics

Jessika Trancik, the Atlantic Richfield Career Development Associate Professor in Energy Studies at MIT’s Institute for Data, Systems, and Society, says that because of the very different dynamics of methane in the atmosphere compared to carbon dioxide, it can be misleading to rely on the conventional single-factor comparisons that are often used. Instead, she and collaborators suggested in a 2014 research paper — and further expanded on the idea in 2016 — that a measure of the relative effects of different gases based on specific climate mitigation goals should be used, for example where the time horizon for the comparison is based on a specific stabilization goal.

The usual way of comparing greenhouse gases is through a single conversion factor, called the global warming potential, which uses a somewhat arbitrarily chosen time horizon of 100 years. For methane, this is usually given as a factor of 25 (that is, methane is 25 times more potent than carbon dioxide). But Trancik suggests that it is more meaningful to use “ goal-inspired metrics ,” which incorporate the different residence times of different gases over a time span that depends on when the emissions occur relative to a mitigation goal: an instantaneous climate impact (ICI) and a cumulative climate impact (CCI). She says that how much weight to give the different factors “comes down to how much you care about the rate of change in the short term, as opposed to the equilibrium state” that the climate will ultimately settle in to — which may not be reached for centuries.

Solomon’s research has recently shown that some of the effects of greenhouse gases can persist for centuries , even after the gases that initially triggered those changes are no longer being emitted at all. Specifically, the expansion of water as it warms, combined with the melting of polar and glacier ice, can lead to significant sea-level rise that would last for centuries even if all new greenhouse gas emissions were stopped altogether. That’s because these gases will remain in the atmosphere and continue to trap heat long after their sources are eliminated — a fact that’s sometimes overlooked in discussions of mitigating climate change. If all carbon dioxide emissions were eliminated by 2050, Solomon and her co-authors found, as much as half of the emissions would still be in the air 750 years later, and still warming the planet.

“There’s no question that carbon dioxide is the biggest contributor to human-caused climate change,” Trancik says, “so that’s the big focus of mitigation efforts. But there are a number of others that are also significant. These non-carbon dioxide emissions often come from some sort of leakage in the supply system, unlike the direct emissions of carbon dioxide that result from combusting carbon-containing fossil fuels. There are opportunities to clean these systems up to reduce leakage, though it’s not always easy.”

Also, she says, “there’s a challenge in understanding the atmospheric lifetimes of all these greenhouse gases and how the radiative forcing changes as the concentration changes. There are interactive effects that change the radiative efficiencies of all these gases.”

Gases are not the only contributors to the greenhouse effect: Black carbon, otherwise known as soot, as well as some other particulate matter can also play a role. But such materials have even shorter residence times, typically just days or weeks, as they tend to be flushed out of the air by the next rainfall.

Which brings us to the biggest greenhouse gas of all: water vapor. There’s no doubt that water vapor is responsible for more greenhouse warming than any other atmospheric constituent. But water vapor’s behavior depends on the climate, so it is not a driver of climate change but rather an amplifying feedback, since the water cycle is a constant part of the atmospheric circulation. As the air gets warmer, it can hold more water vapor, so a warming climate leads to more vapor in the air, providing a feedback effect — and potentially leading to dramatic changes in rainfall patterns. But, water vapor only stays around until the next rainfall. “Water vapor is a slave to the climate system, it’s not a master,” Solomon says.

So when it comes to changing the planet’s climate, carbon dioxide really is the number one factor — and will be so for the foreseeable future, even if all emissions were to stop right now. Much of the carbon dioxide emitted over the last century will still be there centuries in the future — and will still be warming the planet and causing sea level to rise. “Some of our carbon dioxide will still be there in 1,000 years,” Solomon says. So for all practical purposes, she says, on a human timescale, carbon dioxide emitted into the air leads to “the irreversibility of carbon dioxide-induced warming.”

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Human activity affects global surface temperatures by changing Earth ’s radiative balance—the “give and take” between what comes in during the day and what Earth emits at night. Increases in greenhouse gases —i.e., trace gases such as carbon dioxide and methane that absorb heat energy emitted from Earth’s surface and reradiate it back—generated by industry and transportation cause the atmosphere to retain more heat, which increases temperatures and alters precipitation patterns.

Global warming, the phenomenon of increasing average air temperatures near Earth’s surface over the past one to two centuries, happens mostly in the troposphere , the lowest level of the atmosphere, which extends from Earth’s surface up to a height of 6–11 miles. This layer contains most of Earth’s clouds and is where living things and their habitats and weather primarily occur.

Continued global warming is expected to impact everything from energy use to water availability to crop productivity throughout the world. Poor countries and communities with limited abilities to adapt to these changes are expected to suffer disproportionately. Global warming is already being associated with increases in the incidence of severe and extreme weather, heavy flooding , and wildfires —phenomena that threaten homes, dams, transportation networks, and other facets of human infrastructure. Learn more about how the IPCC’s Sixth Assessment Report, released in 2021, describes the social impacts of global warming.

Polar bears live in the Arctic , where they use the region’s ice floes as they hunt seals and other marine mammals . Temperature increases related to global warming have been the most pronounced at the poles, where they often make the difference between frozen and melted ice. Polar bears rely on small gaps in the ice to hunt their prey. As these gaps widen because of continued melting, prey capture has become more challenging for these animals.

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global warming , the phenomenon of increasing average air temperatures near the surface of Earth over the past one to two centuries. Climate scientists have since the mid-20th century gathered detailed observations of various weather phenomena (such as temperatures, precipitation , and storms) and of related influences on climate (such as ocean currents and the atmosphere’s chemical composition). These data indicate that Earth’s climate has changed over almost every conceivable timescale since the beginning of geologic time and that human activities since at least the beginning of the Industrial Revolution have a growing influence over the pace and extent of present-day climate change .

Giving voice to a growing conviction of most of the scientific community , the Intergovernmental Panel on Climate Change (IPCC) was formed in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP). The IPCC’s Sixth Assessment Report (AR6), published in 2021, noted that the best estimate of the increase in global average surface temperature between 1850 and 2019 was 1.07 °C (1.9 °F). An IPCC special report produced in 2018 noted that human beings and their activities have been responsible for a worldwide average temperature increase between 0.8 and 1.2 °C (1.4 and 2.2 °F) since preindustrial times, and most of the warming over the second half of the 20th century could be attributed to human activities.

AR6 produced a series of global climate predictions based on modeling five greenhouse gas emission scenarios that accounted for future emissions, mitigation (severity reduction) measures, and uncertainties in the model projections. Some of the main uncertainties include the precise role of feedback processes and the impacts of industrial pollutants known as aerosols , which may offset some warming. The lowest-emissions scenario, which assumed steep cuts in greenhouse gas emissions beginning in 2015, predicted that the global mean surface temperature would increase between 1.0 and 1.8 °C (1.8 and 3.2 °F) by 2100 relative to the 1850–1900 average. This range stood in stark contrast to the highest-emissions scenario, which predicted that the mean surface temperature would rise between 3.3 and 5.7 °C (5.9 and 10.2 °F) by 2100 based on the assumption that greenhouse gas emissions would continue to increase throughout the 21st century. The intermediate-emissions scenario, which assumed that emissions would stabilize by 2050 before declining gradually, projected an increase of between 2.1 and 3.5 °C (3.8 and 6.3 °F) by 2100.

Many climate scientists agree that significant societal, economic, and ecological damage would result if the global average temperature rose by more than 2 °C (3.6 °F) in such a short time. Such damage would include increased extinction of many plant and animal species, shifts in patterns of agriculture , and rising sea levels. By 2015 all but a few national governments had begun the process of instituting carbon reduction plans as part of the Paris Agreement , a treaty designed to help countries keep global warming to 1.5 °C (2.7 °F) above preindustrial levels in order to avoid the worst of the predicted effects. Whereas authors of the 2018 special report noted that should carbon emissions continue at their present rate, the increase in average near-surface air temperature would reach 1.5 °C sometime between 2030 and 2052, authors of the AR6 report suggested that this threshold would be reached by 2041 at the latest.

The AR6 report also noted that the global average sea level had risen by some 20 cm (7.9 inches) between 1901 and 2018 and that sea level rose faster in the second half of the 20th century than in the first half. It also predicted, again depending on a wide range of scenarios, that the global average sea level would rise by different amounts by 2100 relative to the 1995–2014 average. Under the report’s lowest-emission scenario, sea level would rise by 28–55 cm (11–21.7 inches), whereas, under the intermediate emissions scenario, sea level would rise by 44–76 cm (17.3–29.9 inches). The highest-emissions scenario suggested that sea level would rise by 63–101 cm (24.8–39.8 inches) by 2100.

The scenarios referred to above depend mainly on future concentrations of certain trace gases, called greenhouse gases , that have been injected into the lower atmosphere in increasing amounts through the burning of fossil fuels for industry, transportation , and residential uses. Modern global warming is the result of an increase in magnitude of the so-called greenhouse effect , a warming of Earth’s surface and lower atmosphere caused by the presence of water vapour , carbon dioxide , methane , nitrous oxides , and other greenhouse gases. In 2014 the IPCC first reported that concentrations of carbon dioxide, methane, and nitrous oxides in the atmosphere surpassed those found in ice cores dating back 800,000 years.

Of all these gases, carbon dioxide is the most important, both for its role in the greenhouse effect and for its role in the human economy. It has been estimated that, at the beginning of the industrial age in the mid-18th century, carbon dioxide concentrations in the atmosphere were roughly 280 parts per million (ppm). By the end of 2022 they had risen to 419 ppm, and, if fossil fuels continue to be burned at current rates, they are projected to reach 550 ppm by the mid-21st century—essentially, a doubling of carbon dioxide concentrations in 300 years.

A vigorous debate is in progress over the extent and seriousness of rising surface temperatures, the effects of past and future warming on human life, and the need for action to reduce future warming and deal with its consequences. This article provides an overview of the scientific background related to the subject of global warming. It considers the causes of rising near-surface air temperatures, the influencing factors, the process of climate research and forecasting, and the possible ecological and social impacts of rising temperatures. For an overview of the public policy developments related to global warming occurring since the mid-20th century, see global warming policy . For a detailed description of Earth’s climate, its processes, and the responses of living things to its changing nature, see climate . For additional background on how Earth’s climate has changed throughout geologic time , see climatic variation and change . For a full description of Earth’s gaseous envelope, within which climate change and global warming occur, see atmosphere .

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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

Supported by

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.

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

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

+0.75°C

–0.25°

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.

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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

© 2020 UCAR

• Solving Climate Change
• Why Earth Is Warming
• The Greenhouse Effect
• What's Your Carbon Footprint?
• Classroom Activity: Mitigation or Adaptation?
• Classroom Activity: Solving the Carbon Dioxide Problem
• Stabilization Wedges (Activity and Resources)

MSU Extension

Greenhouse gases: their impact on climate change.

Terry Gibb <[email protected]> , Michigan State University Extension - December 21, 2015

Many scientists are predicting the Earth is heading toward a global warming danger zone and humans are responsible for most of it by upsetting the balance in the natural ecosystem.

Climate change has been debated for a while on whether it’s real or a myth. With the signing of the Paris climate agreement this month, it appears that everyone finally agrees it’s real and we need start working on it before the planet reaches the danger zone from where it can’t recover. But what is causing most of this climate change?

In a word: Humans, or more precisely, the human actions that increase greenhouse gas emissions. How does burning coal, driving our cars and cutting down trees for development result in melting glaciers, rising water levels and warmer water temperatures?

Greenhouse gases are gases in the Earth's atmosphere that produce the greenhouse effect. Changes in the concentration of certain greenhouse gases, from human activity (such as burning fossil fuels), increase the risk of global climate change. Greenhouse gases include water vapor, carbon dioxide (CO 2 ), methane, nitrous oxide, halogenated fluorocarbons, ozone, perfluorinated carbons, and hydro fluorocarbons. 

These gases surround and insulate the Earth like a blanket. They allow the sun to reach and warm the Earth’s surface then block the warmth from escaping back into space. Human activities, including those mentioned above, have continued to increase and have upset the balance of the natural system for several greenhouse gases: methane, nitrous oxide, fluorinated gases and especially carbon dioxide. As these gases continue to be emitted into the atmosphere, they form a thicker layer. And just like the blanket, the thicker it is, the more heat it holds.

The United States and China are the biggest contributors of these gases with the European Union coming in third. The major culprit of gas emissions is the burning of fossil fuels (coal, oil and natural gas). Other pollution activities include agriculture, deforestation and livestock breeding.

Carbon dioxide is the biggest contributor to the problem. It occurs naturally from a number of sources. In the natural system, CO 2 can be absorbed by oceans and plants taking it in during photosynthesis. The man made increases of CO 2 have tipped the balance and these natural intake sources cannot absorb these additional amounts. Some CO 2 can remain in the atmosphere for thousands of year.

Even if the oceans could absorb more CO 2 from the atmosphere , it increases the acidity of the oceans and turns it into carbonic acid. This acid uses up the carbonate in the water so it’s not available for sea creatures who need it for skeleton and shell building and coral reefs.

Methane is the second largest culprit of the greenhouse gases. In addition to the same industrial sources that emit CO 2 , landfills and rice cultivation are additional sources of methane gas. While methane does not last as long in the atmosphere as CO 2 , it is much better at trapping heat.

Nitrous oxide (N 2 O), another greenhouse gas, is part of the Earth’s nitrogen cycle. Unfortunately 40 percent of the nitrous oxide worldwide comes from human activities, including fossil fuel burning, agriculture and wastewater management. It can last in the atmosphere approximately 114 years before it is removed or destroyed by chemical reaction. According the U.S. EPA , “the impact of 1 pound of N 2 O on warming the atmosphere is almost 300 times that of 1 pound of carbon dioxide.”

Since weather information has been accurately recorded, the temperature trend has been increasing. For example, the ten warmest years on record have occurred since 1998 and so far 2015 is the hottest year on record and with only a few weeks left in the year, it should end that way.

Unfortunately, it appears the blanket is doing its job.

For more information on the Paris agreement, see the Michigan State University Extension news article “ Paris climate agreement ”

For more information on greenhouse gases, visit the U.S. EPA website . 

This article was published by Michigan State University Extension . For more information, visit https://extension.msu.edu . To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters . To contact an expert in your area, visit https://extension.msu.edu/experts , or call 888-MSUE4MI (888-678-3464).

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November 3, 2023

Earth Reacts to Greenhouse Gases More Strongly Than We Thought

Climate scientists, including pioneer James Hansen, are pinning down a fundamental factor that drives how hot Earth will get

By Chelsea Harvey & E&E News

A 'Blue Marble' image of the Earth taken from the VIIRS instrument aboard NASA's Earth-observing satellite – Suomi NPP.

NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring

CLIMATEWIRE |  Climate scientist James Hansen is frustrated. And he’s worried.

For nearly 40 years, Hansen has been warning the world of the dangers of global warming. His testimony at a groundbreaking 1988 Senate hearing on the greenhouse effect helped inject the coming climate crisis into the public consciousness. And it helped make him one of the most influential climate scientists in the world.

Hansen has spent several decades as director of NASA’s Goddard Institute for Space Studies, and now at 82, he directs Columbia University’s  Climate Science, Awareness and Solutions program .

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In the years since his seminal testimony, many of Hansen’s basic scientific predictions about the Earth’s climate future have come true. Greenhouse gas emissions have grown, and global temperatures have continued to rise. The world’s glaciers and ice sheets are melting and sea level rise is accelerating.

But Hansen has been disappointed with the scientific community’s response to some of his more recent projections about the future of the warming Earth, which some researchers have characterized as unrealistically dire.

In particular, he was discouraged by the response to a paper he published in 2016, suggesting catastrophic ice melt in Greenland and Antarctica, with widespread global effects, may be possible with relatively modest future warming.

Many researchers said such outcomes were unlikely. But Hansen described the paper as some of his most important work and a warning about the need for more urgent action.

Now he’s bracing himself for a similar reaction to his  latest paper , published Thursday morning.

“I expect the response to be characterized by scientific reticence,” he said in an email to E&E News.

The new paper, published in the research journal  Oxford Open Climate Change , addresses a central question in modern climate science: How much will the Earth warm in response to future carbon emissions? It’s a metric known as “climate sensitivity,” or how sensitive the planet is to greenhouse gases in the atmosphere.

Hansen’s findings suggest the planet may warm faster than previous estimates have indicated. And while some experts say it’s possible, others suggest that he’s taken the results too far.

In studies, scientists often tackle the climate sensitivity question by investigating how much the Earth would warm if atmospheric carbon dioxide concentrations doubled their preindustrial levels. Prior to the industrial era, global CO2 levels hovered around 280 parts per million, meaning a doubling would land around 560 ppm.

Today’s CO2 levels have already climbed above 400 ppm, giving the question a growing relevance.

Climate sensitivity is a difficult metric to estimate. It hinges on a wide variety of feedback loops in the Earth’s climate system, which can speed up or slow down the planet’s warming.

As the Earth’s reflective glaciers and ice sheets melt, for instance, the planet can absorb more sunlight and warm at a faster rate. Forests and other natural ecosystems may absorb different amounts of carbon as the planet warms. Different types of clouds can both speed up or slow down global warming, and it’s still unclear how they will change as the Earth heats up.

The uncertainties around these factors have made it challenging for scientists to pin down an exact estimate for climate sensitivity. But they’ve chipped away at it in recent years.

For decades, studies generally suggested that the Earth should experience anywhere from 1.5 to 4.5 degrees Celsius of warming with a doubling of CO2. But a  2020 paper narrowed the range  to between 2.6 and 3.9 C, using multiple lines of evidence including climate models, the Earth’s response to recent historical emissions and the Earth’s ancient climate history.

The latest assessment report from the U.N.’s Intergovernmental Panel on Climate Change adopted a similar estimate, suggesting a likely range of 2.5 to 4 C with a central estimate around 3 C.

Hansen’s new paper, published with an international group of co-authors, significantly ups the numbers. It suggests a central estimate of around 4.8 C, nearly 2 degrees higher than the IPCC’s figure.

The paper relies largely on evidence from Earth’s ancient climate history. One reason? It’s unclear whether current climate models accurately represent all the relevant feedback effects that may affect climate sensitivity, Hansen and his co-authors argue. The planet’s past provides a clearer view of how the Earth has responded to previous shifts in atmospheric carbon dioxide concentrations.

The paper also suggests that global warming is likely to proceed faster in the near term than previous studies have suggested.

Under the international Paris climate agreement, world leaders are striving to keep global warming well below 2 C and below 1.5 C if at all possible. The new paper warns that warming could exceed 1.5 C by the end of the 2020s and 2 C by 2050.

A gradual global decline in air pollution, driven by tightening environmental regulations, is part of the reasoning. Some types of air pollution are known to have a cooling effect on the climate, which may mask some of the impact of greenhouse gas emissions. As these aerosols decline in the atmosphere, some research suggests, this masking effect may fall away and global temperatures may rise at faster rates.

Hansen and his co-authors argue that better accounting for the declines in global aerosols should accelerate estimates of near-term global warming. Studies suggest that warming between 1970 and 2010 likely proceeded at around 0.18 C per decade. Post-2010, the new paper argues, that figure should rise to 0.27 C.

The findings should motivate greater urgency to not only cut greenhouse gas emissions but to eventually lower global temperatures closer to their preindustrial levels, Hansen suggests. That means using natural resources and technological means to remove carbon dioxide from the atmosphere.

Hansen also suggests that a controversial form of geoengineering, known as solar radiation management, is likely warranted. SRM, in theory, would use reflective aerosols to beam sunlight away from the Earth and lower the planet’s temperatures. The practice has not been tested at any large scale, and scientists have raised a variety of concerns about its ethics and potential unintended side effects.

Yet Hansen believes scientists and activists “should raise concerns about the safety and ethics of NOT doing SRM,” he said by email.

Climate change, caused by human greenhouse gas emissions, is in itself a form of planetary geoengineering, he added.

“My suggestion is to reduce human geoengineering of the planet,” he said.

Yet some scientists say the new paper’s findings — again — are overblown.

The paper “adds very little to the literature,” said Piers Forster, director of the Priestly International Centre for Climate at Leeds University in the U.K. and a lead chapter author of the IPCC’s latest assessment report, in an email to E&E News.

It presents high-end estimates of climate sensitivity based on ancient climate records from the Earth’s past — but those findings aren’t necessarily new, he said. Forster also suggested that some of the methods the new paper used to arrive at those high estimates were “quite subjective and not justified by observations, model studies or literature.”

Forster also took issue with the new paper’s treatment of previous climate sensitivity estimates, including the widely cited 2020 study, which the authors suggested were far too low. The 2020 study presented a careful analysis, using multiple lines of high-quality evidence, Forster said. And yet the authors of the new paper “dismiss it, on spurious grounds.”

Michael Oppenheimer, a climate scientist and director of the Center for Policy Research on Energy and Environment at Princeton University, said the uncertainties around the effects of declining aerosols were important to pay attention to. And he suggested that the new paper’s climate sensitivity estimates were possible.

But added that he regards them as “a worst-worst-case” scenario.

“I think it’s perfectly legitimate to have a worst-worst-case out there,” he added. “They help people think about what the boundaries of the possible are, and they are necessary for risk management against the climate problem.”

But there are still so many uncertainties about the kinds of feedback factors affecting the Earth’s climate sensitivity, he said, that “you can’t really nail it down with the kind of precision that [Hansen’s] provided.”

But Hansen says the new paper’s lines of evidence are based on the most up-to-date research on the Earth’s ancient history.

“[T]here is no basis whatever for the claim that our results are ‘unlikely,’” he said by email. “It is the IPCC sensitivity that is unlikely, less than 1 percent chance of being right, as we show quantitatively in our (peer-reviewed) paper.”

Hansen and 'scientific reticence'

Hansen has been into the deep end of climate debates for much of his career.

In 1988, at the time of his Senate testimony, scientists were still discussing whether the fingerprint of human-caused global warming could yet be detected above the “noise” of the Earth’s natural climate variations.

“When I first got into this, and when Jim and I were testifying, we were arguing about whether there's a global signal,” said Oppenheimer, the Princeton scientist, who testified alongside Hansen in 1988. “All the information we had was about global mean temperature, global mean sea level. We couldn’t talk in the language of things that people cared about.”

But even with the limitations of climate science at the time, the scientists warned the world of the dangers to come.

Hansen has co-authored dozens of papers on climate change in the years since, many of which have been highly regarded by the scientific community.

“Over time, he’s got a pretty damn good track record of turning out to be right about things that other people thought differently about,” Oppenheimer said.

Forster, the Leeds University scientist, agreed that “some of Hansen’s papers are brilliant and his work and deeds helped establish this IPCC in the first place.”

But he added that he still thought the new paper misses the mark.

The reception is similar to a major paper Hansen published in 2016, widely known as the  “Ice Melt” paper.

The Ice Melt paper, published in the journal  Atmospheric Chemistry and Physics , provided a grim, sweeping vision of the Earth’s climate future, focused on the consequences of the melting Greenland and Antarctic ice sheets. Drawing largely on ancient climate data — similar to the new paper — it warned of rapid melting and sea-level rise on the order of several meters within the next century.

It also suggested that the rapid influx of cold, fresh meltwater into the sea could affect ocean circulation patterns and even cause a giant Atlantic current to shut down. That’s a controversial prediction  deemed unlikely by the IPCC , one that would have severe impacts on global weather and climate patterns if it actually happened.

The paper received mixed reactions from other climate scientists upon publication. Some praised the paper, while many suggested the findings were unrealistic.

Another 2016 paper , published by a different group of scientists, later found that the likelihood of an Atlantic current shutdown was relatively small and suggested that Hansen’s paper relied on “unrealistic assumptions.”

In his new paper, Hansen referred to that study as an “indictment” of Ice Melt. He also noted that the IPCC’s latest assessment report did not include Ice Melt’s predictions, an omission he likened in the new paper to a form of censorship.

“Science usually acknowledges alternative views and grants ultimate authority to nature,” the new paper states. “In the opinion of our first author (Hansen), IPCC does not want its authority challenged and is comfortable with gradualism. Caution has merits, but the delayed response and amplifying feedbacks of climate make excessive reticence a danger.”

Responding to critiques of his new paper, Hansen again suggested that “scientific reticence” — or a kind of resistance to new findings — is at play. He pointed to a  1961 paper by sociologist Bernard Barber  suggesting that scientists themselves can be resistant to scientific discovery.

Claims that his new findings are unrealistic, Hansen said, are “a perfect example of the category of scientific reticence that Barber describes as ‘resistance to discovery.’ It takes a long time for new results to sink into the community.”

Resistance to scientific findings is nothing new to Hansen. His 1988 testimony initially shook the political establishment — yet decades later, global climate action is still proceeding too slowly to meet the Paris climate targets.

When he first testified to Congress in the 1980s, Oppenheimer said, he expected that world governments would have started meaningful emissions reduction programs by the year 2000 or so.

“We didn’t get ahead of the impacts,” he said. “And that’s probably because people weren't willing to support strong governmental action in most countries … until they were getting clobbered by unusual and highly damaging, and in some cases unprecedented, climate events.”

He regards the current state of global climate action now with a mix of skepticism and optimism.

“We’re in the process of muddling through — we’re in a period where climate change is gonna be painful for a while, it’s gonna hurt a lot of people in a lot of places, but we can get out the other side,” he said. “I think we can get there. But will we?”

Hansen echoed his sentiments in starker terms.

He wrote that he’s been surprised by “the increase of anti-science no-nothing thinking in our politics.”

“That's why I focus on young people,” he added. “They need to understand the situation and take control.”

Reprinted from E&E News with permission from POLITICO, LLC. Copyright 2022. E&E News provides essential news for energy and environment professionals.

5 things you should know about the greenhouse gases warming the planet

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News stories about the climate crisis often contain mentions of greenhouse gases, and the greenhouse effect. Whilst most will find the analogy easy to understand, what exactly are these gases, and why are they contributing to the warming of the Earth?

1. What is the greenhouse effect?

In a greenhouse, sunlight enters, and heat is retained. The greenhouse effect describes a similar phenomenon on a planetary scale but, instead of the glass of a greenhouse,  certain gases are increasingly raising global temperatures.

The surface of the Earth absorbs just under half of the sun’s energy, while the atmosphere absorbs 23 per cent, and the rest is reflected back into space. Natural processes ensure that the amount of incoming and outgoing energy is equal, keeping the planet’s temperature stable.

However, human activity is resulting in the increased emission of so-called greenhouse gases (GHGs) which, unlike other atmospheric gases such as oxygen and nitrogen, becomes trapped in the atmosphere, unable to escape the planet. This energy returns to the surface, where it is reabsorbed.

Because more energy enters than exits the planet, surface temperatures increase until a new balance is achieved. 

2. Why does the warming matter?

This temperature increase has long-term, adverse effects on the climate, and affects a myriad of natural systems. Effects include increases in the frequency and intensity of extreme weather events – including flooding, droughts, wildfires and hurricanes – that affect millions of people and cause trillions in economic losses.

“Human-caused greenhouse gas emissions endanger human and environmental health,” says Mark Radka, Chief of the UN Environment Programme’s ( UNEP ) Energy and Climate Branch. “And the impacts will become more widespread and severe without strong climate action.”

GHG emissions are critical to understanding and addressing the climate crisis: despite an initial dip due to COVID-19 , the latest UNEP Emissions Gap Report shows a rebound, and forecasts a disastrous global temperature rise of at least 2.7 degrees this century, unless countries make much greater efforts to reduce emissions.

The report found that GHG emissions need to be halved by 2030, if we are to limit global warming to 1.5°C compared to pre-industrial levels by the end of the century.

3. What are the major greenhouse gases?

Water vapour is the biggest overall contributor to the greenhouse effect. However, almost all the water vapour in the atmosphere comes from natural processes.

Carbon dioxide (CO2), methane and nitrous oxide are the major GHGs to worry about. CO2 stays in the atmosphere for up to 1,000 years, methane for around a decade, and nitrous oxide for approximately 120 years.

Measured over a 20-year period, methane is 80 times more potent than CO2 in causing global warming, while nitrous oxide is 280 times more potent.

4. How is human activity producing these greenhouse gases?

Coal, oil, and natural gas continue to power many parts of the world. Carbon is the main element in these fuels and, when they’re burned to generate electricity, power transportation, or provide heat, they produce CO2.

Oil and gas extraction, coal mining, and waste landfills account for 55 per cent of human-caused methane emissions. Approximately 32 per cent of human-caused methane emissions are attributable to cows, sheep and other ruminants that ferment food in their stomachs. Manure decomposition is another agricultural source of the gas, as is rice cultivation. 

Human-caused nitrous oxide emissions largely arise from agriculture practices. Bacteria in soil and water naturally convert nitrogen into nitrous oxide, but fertilizer use and run-off add to this process by putting more nitrogen into the environment.

Fluorinated gases – such as hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride – are GHGs that do not occur naturally. Hydrofluorocarbons are refrigerants used as alternatives to chlorofluorocarbons (CFCs), which, having depleted the ozone layer,were phased out thanks to the Montreal Protocol. The others have industrial and commercial uses.

While fluorinated gases are far less prevalent than other GHGs and do not deplete the ozone layer like CFCs, they are still very powerful. Over a 20-year period, the global warming potential of some fluorinated gases is up to 16,300 times greater than that of CO2.

5. What can we do to reduce GHG emissions?

Shifting to renewable energy, putting a price on carbon, and phasing out coal are all important elements in reducing GHG emissions. Ultimately, stronger emission-reduction targets are necessary for the preservation of long-term human and environmental health.

“We need to implement strong policies that back the raised ambitions,” says Mr. Radka. “We cannot continue down the same path and expect better results. Action is needed now.”

During COP26, the European Union and the United States launched the Global Methane Pledge, which will see over 100 countries aim to reduce 30 per cent of methane emissions in the fuel, agriculture and waste sectors by 2030.

Despite the challenges, there is reason to be positive. From 2010 to 2021, policies were put in place  to lower annual emissions by 11 gigatons by 2030 compared to what would have otherwise happened. Individuals can also join the UN’s #ActNow campaign for ideas to take climate-positive actions.

By making choices that have less harmful effects on the environment, everyone can be a part of the solution and influence change. Speaking up is one way to multiply impact and create change on a much bigger scale.  

UNEP’s role in reducing GHGs

• UNEP has outlined its six-sector solution, which can reduce 29–32 gigatons of carbon dioxide by 2030 to meet the 1.5°C warming limit. The six sectors identified are: energy; industry; agricultureand food; forests andland use; transport; and buildings and cities.
• UNEP also maintains an online “Climate Note,” a tool that visualizes the changing state of the climate with a baseline of 1990.
• Through its other multilateral environmental agreements and reports, UNEP raises awareness and advocates for effective environmental action. UNEP will continue to work closely with its 193 Member States and other stakeholders to set the environmental agenda and advocate for a drastic reduction in GHG emissions.
• greenhouse gas emissions

The Causes of Climate Change

Human activities are driving the global warming trend observed since the mid-20th century.

• The greenhouse effect is essential to life on Earth, but human-made emissions in the atmosphere are trapping and slowing heat loss to space.
• Five key greenhouse gases are carbon dioxide, nitrous oxide, methane, chlorofluorocarbons, and water vapor.
• While the Sun has played a role in past climate changes, the evidence shows the current warming cannot be explained by the Sun.

Increasing Greenhouses Gases Are Warming the Planet

Scientists attribute the global warming trend observed since the mid-20 th century to the human expansion of the "greenhouse effect" 1 — warming that results when the atmosphere traps heat radiating from Earth toward space.

Life on Earth depends on energy coming from the Sun. About half the light energy reaching Earth's atmosphere passes through the air and clouds to the surface, where it is absorbed and radiated in the form of infrared heat. About 90% of this heat is then absorbed by greenhouse gases and re-radiated, slowing heat loss to space.

Four Major Gases That Contribute to the Greenhouse Effect

Carbon dioxide.

A vital component of the atmosphere, carbon dioxide (CO 2 ) is released through natural processes (like volcanic eruptions) and through human activities, such as burning fossil fuels and deforestation.

Like many atmospheric gases, methane comes from both natural and human-caused sources. Methane comes from plant-matter breakdown in wetlands and is also released from landfills and rice farming. Livestock animals emit methane from their digestion and manure. Leaks from fossil fuel production and transportation are another major source of methane, and natural gas is 70% to 90% methane.

Nitrous Oxide

A potent greenhouse gas produced by farming practices, nitrous oxide is released during commercial and organic fertilizer production and use. Nitrous oxide also comes from burning fossil fuels and burning vegetation and has increased by 18% in the last 100 years.

Chlorofluorocarbons (CFCs)

These chemical compounds do not exist in nature – they are entirely of industrial origin. They were used as refrigerants, solvents (a substance that dissolves others), and spray can propellants.

FORCING:  Something acting upon Earth's climate that causes a change in how energy flows through it (such as long-lasting, heat-trapping gases - also known as greenhouse gases). These gases slow outgoing heat in the atmosphere and cause the planet to warm.

Another Gas That Contributes to the Greenhouse Effect:

Water vapor.

Water vapor is the most abundant greenhouse gas, but because the warming ocean increases the amount of it in our atmosphere, it is not a direct cause of climate change. Credit:  John Fowler  on  Unsplash

FEEDBACKS:  A process where something is either amplified or reduced as time goes on, such as water vapor increasing as Earth warms leading to even more warming.

Human Activity Is the Cause of Increased Greenhouse Gas Concentrations

Over the last century, burning of fossil fuels like coal and oil has increased the concentration of atmospheric carbon dioxide (CO 2 ). This increase happens because the coal or oil burning process combines carbon with oxygen in the air to make CO 2 . To a lesser extent, clearing of land for agriculture, industry, and other human activities has increased concentrations of greenhouse gases.

The industrial activities that our modern civilization depends upon have raised atmospheric carbon dioxide levels by nearly 50% since 1750 2 . This increase is due to human activities, because scientists can see a distinctive isotopic fingerprint in the atmosphere.

In its Sixth Assessment Report, the Intergovernmental Panel on Climate Change, composed of scientific experts from countries all over the world, concluded that it is unequivocal that the increase of CO 2 , methane, and nitrous oxide in the atmosphere over the industrial era is the result of human activities and that human influence is the principal driver of many changes observed across the atmosphere, ocean, cryosphere and biosphere.

"Since systematic scientific assessments began in the 1970s, the influence of human activity on the warming of the climate system has evolved from theory to established fact."

Intergovernmental Panel on Climate Change

The panel's AR6 Working Group I (WGI) Summary for Policymakers report is online at https://www.ipcc.ch/report/ar6/wg1/ .

Evidence Shows That Current Global Warming Cannot Be Explained by Solar Irradiance

Scientists use a metric called Total Solar Irradiance (TSI) to measure the changes in energy the Earth receives from the Sun. TSI incorporates the 11-year solar cycle and solar flares/storms from the Sun's surface.

Studies show that solar variability has played a role in past climate changes. For example, a decrease in solar activity coupled with increased volcanic activity helped trigger the Little Ice Age.

But several lines of evidence show that current global warming cannot be explained by changes in energy from the Sun:

• Since 1750, the average amount of energy from the Sun either remained constant or decreased slightly 3 .
• If a more active Sun caused the warming, scientists would expect warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere and a warming at the surface and lower parts of the atmosphere. That's because greenhouse gases are slowing heat loss from the lower atmosphere.
• Climate models that include solar irradiance changes can’t reproduce the observed temperature trend over the past century or more without including a rise in greenhouse gases.

1. IPCC 6 th Assessment Report, WG1, Summary for Policy Makers, Sections A, “ The Current State of the Climate ”

IPCC 6 th Assessment Report, WG1, Technical Summary, Sections TS.1.2, TS.2.1 and TS.3.1

2. P. Friedlingstein, et al., 2022: “Global Carbon Budget 2022”, Earth System Science Data ( 11 Nov 2022): 4811–4900. https://doi.org/10.5194/essd-14-4811-2022

3. IPCC 6 th Assessment Report, WG1, Chapter 2, Section 2.2.1, “ Solar and Orbital Forcing ” IPCC 6 th Assessment Report, WG1, Chapter 7, Sections 7.3.4.4, 7.3.5.2, Figure 7.6, “ Solar ” M. Lockwood and W.T. Ball, Placing limits on long-term variations in quiet-Sun irradiance and their contribution to total solar irradiance and solar radiative forcing of climate,” Proceedings of the Royal Society A , 476, issue 2228 (24 June 2020): https://doi 10.1098/rspa.2020.0077

Header image credit: Pixabay/stevepb Four Major Gases image credit: Adobe Stock/Ilya Glovatskiy

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Climate Change and the Impact of Greenhouse Gasses: CO 2 and NO, Friends and Foes of Plant Oxidative Stress

Here, we review information on how plants face redox imbalance caused by climate change, and focus on the role of nitric oxide (NO) in this response. Life on Earth is possible thanks to greenhouse effect. Without it, temperature on Earth’s surface would be around -19°C, instead of the current average of 14°C. Greenhouse effect is produced by greenhouse gasses (GHG) like water vapor, carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxides (N x O) and ozone (O 3 ). GHG have natural and anthropogenic origin. However, increasing GHG provokes extreme climate changes such as floods, droughts and heat, which induce reactive oxygen species (ROS) and oxidative stress in plants. The main sources of ROS in stress conditions are: augmented photorespiration, NADPH oxidase (NOX) activity, β-oxidation of fatty acids and disorders in the electron transport chains of mitochondria and chloroplasts. Plants have developed an antioxidant machinery that includes the activity of ROS detoxifying enzymes [e.g., superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), glutathione peroxidase (GPX), and peroxiredoxin (PRX)], as well as antioxidant molecules such as ascorbic acid (ASC) and glutathione (GSH) that are present in almost all subcellular compartments. CO 2 and NO help to maintain the redox equilibrium. Higher CO 2 concentrations increase the photosynthesis through the CO 2 -unsaturated Rubisco activity. But Rubisco photorespiration and NOX activities could also augment ROS production. NO regulate the ROS concentration preserving balance among ROS, GSH, GSNO, and ASC. When ROS are in huge concentration, NO induces transcription and activity of SOD, APX, and CAT. However, when ROS are necessary (e.g., for pathogen resistance), NO may inhibit APX, CAT, and NOX activity by the S-nitrosylation of cysteine residues, favoring cell death. NO also regulates GSH concentration in several ways. NO may react with GSH to form GSNO, the NO cell reservoir and main source of S-nitrosylation. GSNO could be decomposed by the GSNO reductase (GSNOR) to GSSG which, in turn, is reduced to GSH by glutathione reductase (GR). GSNOR may be also inhibited by S-nitrosylation and GR activated by NO. In conclusion, NO plays a central role in the tolerance of plants to climate change.

Introduction

Life on Earth, as it is, relies on the natural atmospheric greenhouse effect. This is the result of a process in which a planet’s atmosphere traps the sun radiation and warms the planet’s surface.

Greenhouse effect occurs in the troposphere (the lower atmosphere layer), where life and weather occur. In the absence of greenhouse effect, the average temperature on Earth’s surface is estimated around -19°C, instead of the current average of 14°C ( Le Treut et al., 2007 ). Greenhouse effect is produced by greenhouse gasses (GHG). GHG are those gaseous constituents of the atmosphere that absorb and emit radiation in the thermal infrared range ( IPCC, 2014 ). Traces of GHG, both natural and anthropogenic, are present in the troposphere. The most abundant GHG in increasing order of importance are: water vapor, carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxides (N x O) and ozone (O 3 ) ( Kiehl and Trenberth, 1997 ). GHG percentages vary daily, seasonally, and annually.

GHG Contribute Differentially to Greenhouse Effect

Water vapor.

Water is present in the troposphere both as vapor and clouds. Water vapor was reported by Tyndal in 1861 as the most important gaseous absorber of variations in infrared radiation (cited in Held and Souden, 2000 ). Further accurate calculation estimate that water vapor and clouds are responsible for 49 and 25%, respectively, of the long wave (thermal) absorption ( Schmidt et al., 2010 ). However, atmospheric lifetime of water vapor is short (days) compared to other GHG as CO 2 (years) ( IPCC, 2014 ).

Water vapor concentrations are not directly influenced by anthropogenic activity and vary regionally. However, human activity increases global temperatures and water vapor formation indirectly, amplifying the warming in a process known as water vapor feedback ( Soden et al., 2005 ).

Carbon Dioxide (CO 2 )

Carbon dioxide is responsible for 20% of the thermal absorption ( Schmidt et al., 2010 ).

Natural sources of CO 2 include organic decomposition, ocean release and respiration. Anthropogenic CO 2 sources are derived from activities such as cement manufacturing, deforestation, fossil fuels combustion such as coal, oil and natural gas, etc. Surprisingly, 24% of direct CO 2 emission comes from agriculture, forestry and other land use, and 21% comes from industry ( IPCC, 2014 ).

Atmospheric CO 2 concentrations climbed up dramatically in the past two centuries, rising from around 270 μmol.mol -1 in 1750 to present concentrations higher than 385 μmol.mol -1 ( Mittler and Blumwald, 2010 ; IPCC, 2014 ). Around 50% of cumulative anthropogenic CO 2 emissions between 1750 and 2010 have taken place since the 1970s ( IPCC, 2014 ). It is calculated that the temperature rise produced by high CO 2 concentrations, plus the water positive feedback, would increase by 3–5°C the global mean surface temperature in 2100 ( IPCC, 2014 ).

Methane (CH 4 )

Methane (CH 4 ) is the main atmospheric organic trace gas. CH 4 is the primary component of natural gas, a worldwide fuel source. Significant emissions of CH 4 result from cattle farming and agriculture, but mainly as a consequence of fossil fuel use. Concentrations of CH 4 were multiplied by two since the pre-industrial era. The present worldwide-averaged concentration is of 1.8 μmol.mol -1 ( IPCC, 2014 ).

Although its concentration represents only 0.5% that of CO 2 , concerns arise regarding a jump in CH 4 atmospheric release. Indeed, it is 30 times more powerful than CO 2 as GHG ( IPCC, 2014 ). CH 4 generates O 3 (see below), and along with carbon monoxide (CO), contributes to control the amount of OH in the troposphere ( Wuebbles and Hayhoe, 2002 ).

Nitrous Oxides (NxO)

Nitrous oxide (N 2 O) and nitric oxide (NO) are GHG. During the last century, their global emissions have rised, due mainly to human intervention ( IPCC, 2014 ). The soil emits both N 2 O and NO. N 2 O is a strong GHG, whereas NO contributes indirectly to O 3 synthesis. As GHG, N 2 O is potentially 300 times stronger than CO 2 . Once in the stratosphere, the former catalyzes the elimination of O 3 ( IPCC, 2014 ). In the atmosphere, N 2 O concentrations are climbing up due mainly to microbial activity in nitrogen (N)-rich soils related with agricultural and fertilization practices ( Hall et al., 2008 ).

Anthropogenic emissions (from combustion of fossil fuels) and biogenic emissions from soils are the main sources of NO in the atmosphere ( Medinets et al., 2015 ). In the troposphere, NO quickly oxidizes to nitrogen dioxide (NO 2 ). NO and NO 2 (termed as NO x ) may react with volatile organic compounds (VOCs) and hydroxyl, resulting in organic nitrates and nitric acid, respectively. They access ecosystems through atmospheric deposition that has an impact on the N cycle as a result of acidification or N enrichment ( Pilegaard, 2013 ).

NO Sources and Chemical Reactions in Plants

Two major pathways for NO production have been described in plants: the reductive and the oxidative pathways. The reductive pathway involves the reduction of nitrite to NO by NR under conditions such as acidic pH, anoxia, or an increase in nitrite levels ( Rockel et al., 2002 ; Meyer et al., 2005 ). NR-dependent NO formation has been involved in processes such as stomatal closure, root development, germination and immune responses. In plants, nitrite may also be reduced enzymatically by other molybdenum enzymes such as, xanthine oxidase, aldehyde oxidase, and sulfite oxidase, in animals ( Chamizo-Ampudia et al., 2016 ) or via the electron transport system in mitochondria ( Gupta and Igamberdiev, 2016 ).

The oxidative pathway produces NO through the oxidation of organic compounds such as polyamines, hydroxylamine and arginine. In animals, NOS catalyzes arginine oxidation to citrulline and NO. Many efforts were made to find the arginine-dependent NO formation in plants, as well as of plant NOS ( Frohlich and Durner, 2011 ). The identification of NOS in the green alga Ostreococcus tauri ( Foresi et al., 2010 ) led to high-throughput bioinformatic analysis in plant genomes. This study shows that NOS homologs were not present in over 1,000 genomes of higher plants analyzed, but only in few photosynthetic microorganisms, such as algae and diatoms ( Di Dato et al., 2015 ; Kumar et al., 2015 ; Jeandroz et al., 2016 ). In summary, although an arginine-dependent NO production is found in higher plants, the specific enzyme/s involved in the oxidative pathways remain elusive.

Ozone (O 3 )

Ozone (O 3 ) is mainly found in the stratosphere, but a little amount is generated in the troposphere. Stratospheric ozone (namely the ozone layer) is formed naturally by chemical reactions involving solar ultraviolet (UV) radiation and O 2 . Solar UV radiation breaks one O 2 molecule, producing two oxygen atoms (2 O). Then, each of these highly reactive atoms combines with O 2 to produce an (O 3 ) molecule. Almost 99% of the Sun’s medium-frequency UV light (from about 200 to 315 nm wavelength) is absorbed by the (O 3 ) layer. Otherwise, they could damage exposed life forms near the Earth surface 1 .

The majority of tropospheric O 3 appears when NOx, CO and VOCs, react in the presence of sunlight. However, it was reported that NOx may scavenge O 3 in urban areas ( Gregg et al., 2003 ). This dual interaction between NOx and O 3 is influenced by light, season, temperature and VOC concentration ( Jhun et al., 2015 ).

Besides, the oxidation of CH 4 by OH in the troposphere gives way to formaldehyde (CH 2 O), CO, and O 3 , in the presence of high amounts of NOx 1 .

Tropospheric O 3 is harmful to both plants and animals (including humans). O 3 affects plants in several ways. Stomata are the cells, mostly on the underside of the plant leaves, that allow CO 2 and water to diffuse into the tissue. High concentrations of O 3 cause plants to close their stomata ( McAdam et al., 2017 ), slowing down photosynthesis and plant growth. O 3 may also provoke strong oxidative stress, damaging plant cells ( Vainonen and Kangasjärvi, 2015 ).

Global Climate Change: an Integrative Balance of the Impact on Plants

Anthropogenic activity alters global climate by interfering with the flows of energy through changes in atmospheric gasses composition, more than the actual generation of heat due to energy usage ( Karl and Trenberth, 2003 ). Short-term consequences of GHG increase in plants are mainly associated with the rise in atmospheric CO 2 . Plants respond directly to elevated CO 2 increasing net photosynthesis, and decreasing stomatal opening ( Long et al., 2004 ). To a lesser extent, O 3 uptake by plants may reduce photosynthesis and induce oxidative stress. In the middle and long term, prognostic consensus about climate change signal a rise in CO 2 concentration and temperature on the Earth’s surface, unexpected variations in rainfall, and more recurrent and intense weather conditions, e.g., heat waves, drought and flooding events ( Mittler and Blumwald, 2010 ; IPCC, 2014 ). These brief episodes bring plants beyond their capacity of adaptation; decreasing crop and tree yield ( Ciais et al., 2005 ; Zinta et al., 2014 ).

Here we will not discuss plants capacity of adaptation to novel environmental conditions when considering large scales and long-term periods. Ecosystems are being affected by climate change at all levels (terrestrial, freshwater, and marine), and it was already reported that species are under evolutionary adaptation to human-caused climate change (for a review see Scheffers et al., 2016 ). Migration and plasticity are two biological mechanisms to cope with these changes. Data indicate that each population of a species has limited tolerance to sharp climate variations, and they could migrate to find more favorable environments. Habitat fragmentation limits plant movement, being other big threat for adaptation ( Stockwell et al., 2003 ; Leimu et al., 2010 ). Despite the fact that individual plants are immobile, plant populations move when seeds are dispersed, resulting in differences in the general distribution of the species ( Corlett and Westcott, 2013 ). In this sense, anthropogenic activities also contribute to seed dispersal.

Plasticity is a characteristic related to phenology and phenotype. Phenology is the timing of phases occurrence in the life cycle, and phenotypic plasticity is the range of phenotypes that a single genotype may express depending on its environment ( Nicotra et al., 2010 ). Plasticity is adaptive when the phenotype changes occur in a direction favored by selection in the new environment.

Climate Change and ROS

Reactive Oxygen Species (ROS) are continuously generated by plants under normal conditions. However, they are increased in response to different abiotic stresses. One of the most important effects of climate change-related stresses at the molecular level is the increase of ROS inside the cells ( Farnese et al., 2016 ). Among ROS, the most studied are superoxide anion ( O 2 •– ), H 2 O 2 and the hydroxyl radical (⋅OH - ).

Reactive Oxygen Species cause damage to proteins, lipids and DNA, affecting cell integrity, morphology, physiology, and, consequently, the growth of plants ( Frohnmeyer and Staiger, 2003 ). The main sources of ROS in stress conditions are: augmented photorespiration, NADPH oxidase (NOX) activity, β-oxidation of fatty acids and disorders in the electron transport chains of mitochondrias and chloroplasts ( Apel and Hirt, 2004 ; AbdElgawad et al., 2015 ). Hence, higher plants have evolved in the presence of ROS and have acquired pathways to protect themselves from its toxicity. Plant antioxidant system (AS) includes the activity of ROS detoxifying enzymes [e.g., superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), glutathione peroxidase (GPX), and peroxiredoxin (PRX)], as well as antioxidant molecules such as ascorbic acid (ASC) and glutathione (GSH) that are present in almost all subcellular compartments (reviewed by Choudhury et al., 2017 ).

In this context, plants have also developed a tight interaction between ROS and NO as a mechanism to reduce the deleterious consequences of these ROS-induced oxidative injuries. NO orchestrates a wide range of mechanisms leading to the preservation of redox homeostasis in plants. Consequently, NO at low concentration is considered a broad-spectrum anti-stress molecule ( Lamattina et al., 2003 ; Tossi et al., 2009 ; Correa-Aragunde et al., 2015 ). Figure ​ Figure1 1 shows the relationship among the different GHG and their impact on plants.

Simplified scheme showing greenhouse gasses (GHG) and their effects on plants. GHG (H 2 O vapor, clouds, CO 2 , CH 4 , N 2 O, and NO) have both natural and anthropogenic origin, contributing to greenhouse effect. Short-term effects of GHG increase is mainly CO 2 rise, that activates photosynthesis (PS) and inhibits stomatal opening (SO). Long-term effects of GHG increase are extreme climate changes such as floods, droughts, heat. All of them induce the generation of reactive oxygen species (ROS) and oxidative stress in plants. Nitric oxide (NO) could alleviate oxidative stress by scavenging ROS and/or regulating the antioxidant system (AS). GHG and volatile organic compounds (VOC) react in presence of sunlight (E#) to give tropospheric O 3 . Although tropospheric O 3 is prejudicial for life, stratospheric O 3 is beneficial, because filters harmful UV-B radiation. The size of arrows are representative of the GHG concentration.

CO 2 and NO Contribute to Regulate Redox Homeostasis in Plants

Co 2 increasing: advantages and disadvantages.

Increased CO 2 was suggested to have a “fertilization” effect, because crops would increase their photosynthesis and stomatal conductance in response to elevated CO 2 . This belief was supported by studies performed in greenhouses, laboratory controlled-environment chambers, and transparent field chambers, where emitted CO 2 may be held back and readily controlled ( Drake et al., 1997 ; Markelz et al., 2014 ). However, more realistic results, obtained by Free-Air Concentration Enrichment (FACE) technology, suggest that the fertilization response due to CO 2 increase is probably dependent on genetic and environmental factors, and the duration of the study ( Smith and Dukes, 2013 ). An extensive review of the literature in this field made by Xu et al. (2015) concluded that augmented CO 2 normally increases photosynthesis in C3 species such as rice, soybean and wheat. On the other hand, they pointed out that a negative feedback of photosynthesis could take place in augmented CO 2 , as a result of overload of chemical and reactive generated substrates, leading to an imbalance in the sink:source carbon ratio. Moreover, the energetic cost of carbohydrate exportation increases in elevated CO 2 level.

The most important photosynthetic enzyme is the ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO). Rubisco is located in mesophyll cells of C3 plants, in direct contact with the intercellular air space linked to the atmosphere by epidermal stomatal pores. Photosynthesis increases at high CO 2 , because Rubisco is not CO 2 saturated and CO 2 inhibits the oxygenation reactions and photorespiration ( Long et al., 2006 ). However, long-term high concentration of CO 2 may down regulate Rubisco activity because ribulose-1,5-bisphosphate is not regenerated. Hexokinase (HXK), a sensor of extreme photosynthate, may participate in the down regulation of Rubisco concentration ( Xu et al., 2015 ). Moreover, severe abiotic stresses, such as temperature and drought, may restrain Rubisco carboxylation and foster oxygenation ( Xu et al., 2015 ).

In C4 crops, such as maize and sorghum, the elevated concentration of CO 2 inside the bundle sheath cells could prevent a large increase of Rubisco activity at higher atmospheric CO 2 and, thereby, photosynthetic activity is not augmented. However, at high CO 2 levels, the water status of C4 plants under drought conditions is improved, increasing photosynthesis and biomass accumulation ( Long et al., 2006 ; Mittler and Blumwald, 2010 ). That envisages potential advantages for the C4 species in future climatic change scenarios, particularly in arid and semiarid areas.

In addition, high CO 2 has the benefit of reducing stomatal conductance, decreasing 10% evapotranspiration in both C3 and C4 plants. Simultaneously, the cooling decreased resulting from reduced transpiration causes elevated canopy temperatures of around 0.7°C for most crops. Biomass and yield rise due to high CO 2 in all C3 plants, but not in C4 plants exception made when water is a restraint. Yields of C3 grain crops jump around 19% on average at high CO 2 ( Kimball, 2016 ).

Some reports analyze the contribution of CO 2 in the responses of plants to the combination of multiple stresses. For Arabidopsis thaliana , the combination of heat and drought induces photosynthesis inhibition of 62% under ambient CO 2 , but the drop in photosynthesis is just 40% at high CO 2 . Moreover, the protein oxidation increases significantly during a heat wave and drought, and this effect is repressed by increased CO 2 . Photorespiration is also reduced by high CO 2 ( Zinta et al., 2014 ).

Studying grasses ( Lolium perenne, Poa pratensis ) and legumes ( Medicago lupulina, Lotus corniculatus ) exposed to drought, high temperature and augmented CO 2 , AbdElgawad et al. (2015) demonstrated that drought suppresses plant growth, photosynthesis and stomatal conductance, and promotes in all species the synthesis of osmolytes and antioxidants. Instead, oxidative damage is more markedly observed in legumes than in grasses. In general, warming amplifies drought consequences. In contrast, augmented CO 2 diminishes stress impact. Reduction in photosynthesis and chlorophyll, as a result of drought and elevated temperature, were avoided by high CO 2 in the grasses. Noxious effects of oxidative stress, i.e., lipid peroxidation, are phased down in all species by augmented CO 2 . Normally, a reduced impact of oxidative stress is due to decreased photorespiration and diminished NOX activity. In legumes, a rise in levels of antioxidant molecules (flavonoids and tocopherols) contribute as well to the stress mitigation caused by augmented CO 2 . The authors draw the conclusion that these different responses point at an unequal future impact of climate change on the production of agricultural-scale legumes and grass crops.

Kumari et al. (2015) assessed the impact of various levels of CO 2 , ambient (382 ppm) and augmented (570 ppm), and O 3 , ambient (50 ppb) and augmented (70 ppb) on the potato physiological and biochemical responses ( Solanum tuberosum ). They observed that augmented CO 2 cut down O 3 uptake, enhanced carbon assimilation, and curbed oxidative stress. Elevated CO 2 also mitigated the noxious effect of high O 3 on photosynthesis.

Although some molecular mechanisms underpinning CO 2 actions are unknown, the results presented highlight the importance of CO 2 as a regulator that mitigates the potential climate change-induced deleterious consequences in plants. Recent reports suggest that some CO 2 -associated responses may be mediated by NO.

Du et al. (2016) determined that 800 μmol.mol -1 of CO 2 increased the NO concentration in Arabidopsis leaves, through a mechanism related to nitrate availability. Moreover, NO increase, as a consequence of high CO 2 levels, was reported as a general procedure to improve iron (Fe) nutrition in response to Fe deficiency in tomato roots ( Jin et al., 2009 ).

The gas exchange between the atmosphere and plants is mainly regulated by stomata. But structure and physiology of stomata are also influenced by gasses ( García-Mata and Lamattina, 2013 ). Elevated CO 2 regulate stomatal density and conductance. Moreover, there is increasing evidence that this response is modified by interaction of CO 2 with other environmental factors ( Xu et al., 2016 ; Yan et al., 2017 ). Wang et al. (2015) reported that 800 μmol.mol -1 of CO 2 increases the NO concentration in A. thaliana guard cells, inducing stomatal closure. Both NR and NO synthase (NOS)-like activities are necessary for CO 2 -induced NO accumulation. Comprehensive pharmacological and genetic results obtained in Arabidopsis by Chater et al. (2015) , show that when CO 2 concentration is around 700–1000 ppm, stomatal density and closure are reduced. They also illustrate that those elements necessary for this process are: activation of both ABA biosynthesis genes and the PYR/RCAR ABA receptor, and ROS increase. However, Shi et al. (2015) provide genetic and pharmacological evidence that high CO 2 concentration induces stomatal closure by an ABA-independent mechanism in tomato. They show that 800 μmol.mol -1 of CO 2 increase the expression of the protein kinase OPEN STOMATA 1 (OST1), NOX, and nitrate reductase (NR) genes. They also show that the sequential production of NOX-dependent H 2 O 2 and NR-produced NO are mainly dependent of OST1, and are involved in the CO 2 -induced stomatal closure.

In ABA-dependent mechanisms, ABA is increased by CO 2. The binding of ABA to its receptor (PYR/RCAR) inactivates PP2C, activating OST1. In ABA-independent mechanism, OST1 will be transcriptionally induced by CO 2 . Once activated, OST1 along with Ca 2 + , activates NOX, increasing ROS ( Kim et al., 2010 ). The rise of guard cells ROS enhances NO, cytosolic free Ca 2 + , and pH ( Song et al., 2014 ; Xie et al., 2014 ). ROS and NO release Ca 2 + from internal reservoirs, or influx external Ca 2 + through plasma membrane Ca 2 + in channels. Cytosolic free Ca 2 + inactivate inward K + channels (K + in ) to prevent K + uptake and activate outward K + channels (K + out ) and Cl - (anion) channels (Cl - ) at the plasma membrane ( Blatt, 2000 ; García-Mata et al., 2003 ). Ca 2 + also activates slow anion channel homolog 3 (SLAH3), slow anion channel-associated 1 (SLAC1) and aluminum activated malate transporters (ALMT) ( Roelfsema et al., 2012 ). The consequence of the regulation of cation/anion channels is the net efflux of K + /Cl - /malate and influx of Ca 2 + , making guard cells lose turgor by water outlet, causing stomatal closure.

All together, the results discussed here suggest that CO 2 -induced NO increase is a common plant physiological response to oxidative stresses. Figure ​ Figure2 2 shows the importance of CO 2 and NO in these processes.

Interplay between CO 2 and NO in plant redox physiology: CO 2 enters to the leaves by stomata. Once in mesophyll cells, CO 2 increase photosynthesis (PS) through the CO 2 -unsaturated Rubisco activity. When plants are in stress environments, ROS could be augmented by Rubisco-induced photorespiration and NADPH oxidase (NOX) activities. NOX- induced O 2 •– , in the apoplast is immediately transformed to H 2 O 2 by the superoxide dismutase (SOD). Plasma membrane is permeable to H 2 O 2 . CO 2 moderates oxidative stress in mesophyll cells by inhibiting both Rubisco photorespiration (PR) and NOX activities. Besides, NO is induced by CO 2 and ROS, alleviating the consequences of oxidative stress by scavenging ROS and activating or inhibiting the antioxidant system (AS). In guard cells, CO 2 increases the expression and activity of OPEN STOMATA 1 (OST1), in both ABA-dependent and independent mechanisms. OST1 activates NOX, producing ROS and consequently NO increase by nitrate reductase (NR), and NOS-like activities. NO prevents ROS increase by direct scavenging, and inhibiting NOX. NO-dependent Ca 2 + regulated ion channels induces stomatal closure, modulating O 3 and CO 2 uptake, decreasing evapotranspiration, and rising leaf temperature.

Abiotic Stress, ROS Generation, and Redox Balance: The Key Role of NO

Reactive oxygen species are generated in apoplast, plasma membrane, chloroplasts, mitochondria, and peroxisomes ( Farnese et al., 2016 ). It was proposed that each stress produces its own “ROS signature” ( Choudhury et al., 2017 ). For instance, drought may reduce the activity of Rubisco, decreasing CO 2 fixation and NADP+ regeneration by the Calvin cycle. As a consequence, chloroplast electron transport is altered, generating ROS by electron leakage to O 2 ( Carvalho, 2008 ). In drought stress, ROS increase is produced by NOX activity ( Farnese et al., 2016 ). In flooding, ROS generation is an ethylene-promoted process that involves calcium (Ca 2+ ) flux, and NOX activity ( Voesenek and Bailey-Serres, 2015 ).

In heat stress, a NOX-dependent transient ROS rise is an early event ( Königshofer et al., 2008 ). Then, endogenous ROS are sensed through histidine kinases, and an Arabidopsis heat stress factor (HsfA4a) appears to sense exogenous ROS. As a result, the MAPK signal pathway is activated ( Qu et al., 2013 ). Moreover, functional decrease in photosynthetic light reaction induces ROS concentration by high electron leakage from the thylakoid membrane ( Hasanuzzaman et al., 2013 ). In this process, O 2 is the acceptor, generating O 2 •– .

Thus, individual stresses or their different combinations may produce particular “ROS signatures.” Besides their deleterious effects, ROS are recognized as a signal in the plant reaction to biotic and abiotic stressors. ROS may induce programed cell death (PCD) to avoid pathogen spread ( Mur et al., 2008 ), trigger a systemic defense response signal ( Dubiella et al., 2013 ), or avoid the chloroplast antenna overloading by electrons divert ( Choudhury et al., 2017 ).

Whatever the origin and function, ROS concentration must be adequately regulated to avoid excessive concentration and consequent cellular damages. Depending on NO and ROS concentrations, NO has the dual capacity to activate or inhibit the ROS production, and is a key molecule for keeping cellular redox homeostasis under control ( Beligni and Lamattina, 1999a ; Correa-Aragunde et al., 2015 ). NO has a direct ROS-scavenging activity because it holds an unpaired electron, reaching elevated reactivity with O 2 , O 2 •– , and redox active metals. NO can mitigate OH formation by scavenging either Fe or O 2 •– ( Lamattina et al., 2003 ). However, NO reacting with ROS (mainly O 2 •– ) may generate reactive nitrogen species (RNS). An excess of RNS originates a nitrosative stress ( Corpas et al., 2011 ). To avoid the toxicity of nitrosative stress, NO is stored as GSNO in the cell.

GSH as a Redox Buffer. GSNO as NO Reservoir. SNO and S-Nitrosylation

Glutathione (GSH) is a small peptide with the sequence γ-l-glutamyl-l-cysteinyl-glycine that has a cell redox homeostatic impact in most plant tissues. It is a soluble small thiol considered a non-enzymatic antioxidant. It exists in the reduced (GSH) or oxidized state (GSSG), in which two GSH molecules are joined by a disulfide bond ( Rouhier et al., 2008 ). GSH alleviates oxidative damages in plants generated by abiotic stresses, including salinity, drought, higher, low temperature, and heavy metals. GSH is precursor of phytochelatins, polymers that chelate toxic metals and transport them to the vacuole ( Grill et al., 1989 ). Studies shown that GSH contributes to tolerate nickel, cadmium, zinc, mercury, aluminum and arsenate heavy metals in plants ( Asgher et al., 2017 ). Moreover, GSH has a role in the detoxification of ROS both directly, interacting with them, or indirectly, participating of enzymatic pathways. GSH is involved in glutathionylation, a posttranslational modification that causes a mixed disulfide bond between a Cys residue and GSH.

GSH can be oxidized to GSSG by H 2 O 2 and can react with NO to form the nitrosoglutathione (GSNO) derivative. GSNO is an intracellular NO reservoir. It is also a vehicle of NO throughout the cell and organs, spreading NO biological function. GSNO is the largest low-molecular-mass S-nitrosothiol (SNO) in plant cells ( Corpas et al., 2013 ). GSNO metabolism and its reaction with other molecules involve S-nitrosylation and S-transnitrosation which consist of the binding of a NO molecule to a cysteine residue in proteins. Thioredoxin produces protein denitrosylation ( Correa-Aragunde et al., 2013 ). GSNO could be decomposed by the GSNO reductase (GSNOR) to GSSG which, in turn, is reduced to GSH by glutathione reductase (GR).

Glutathione also participates in the GSH/ASC cycle, a series of enzymatic reactions that degrade H 2 O 2 . APX degrades H 2 O 2 using ASC, the other major antioxidant in plants, as cofactor. The oxidized ASC is reduced by monodehydroascorbate reductase (MDHAR) in an NAD(P)H-dependent manner and by dehydroascorbate reductase (DHAR) employing GSH as electron donor. The resulting GSSG is reduced in turn to GSH by GR ( Foyer and Noctor, 2011 ).

Different Effects of NO in the Regulation of Antioxidant Enzymes

The application of NO donors alleviates oxidative stress in plants challenged to abiotic and/or biotic stresses ( Laxalt et al., 1997 ; Beligni and Lamattina, 1999b , 2002 ; Shi et al., 2007 ; Xue et al., 2007 ; Leitner et al., 2009 ).

Besides the direct ROS-scavenging activity of NO, its beneficial effect is exerted by the regulation of the antioxidant enzymes activity that controls toxic levels of ROS and RNS ( Uchida et al., 2002 ; Shi et al., 2005 ; Song et al., 2006 ; Romero-Puertas et al., 2007 ; Bai et al., 2011 ). NO can modulate cell redox balance in plants through the regulation of gene expression, posttranslational modification or by its binding to the heme prosthetic group of some antioxidant enzymes.

SOD catalyzes the dismutation of stress-generated O 2 •– in one of two less harmful species: either molecular oxygen (O 2 ) or hydrogen peroxide (H 2 O 2 ). APX and CAT are the most important enzymes degrading H 2 O 2 in plants. They transform H 2 O 2 to H 2 O and O 2 . APX isoforms are primarily found in the cytosol and chloroplasts, while the CAT isoforms are found in peroxisomes. APX has strong affinity for H 2 O 2 and uses ASC as an electron donor. In contrast, CAT removes H 2 O 2 generated in the peroxisomal respiratory pathway without the need to reduce power. Even though CAT affinity for H 2 O 2 is low, its elevated rate of reaction offers an effective way to detoxify H 2 O 2 inside the cell. PRX may reduce both hydroperoxide and peroxynitrite.

Many reports on different plant species demonstrate that NO induces the transcription and activity of antioxidative enzymes in response to oxidative stress. The tolerance to drought and salt-induced oxidative stress in tobacco is related to the ABA-triggered production of H 2 O 2 and NO. In turn, they induce transcripts and activities of SOD, CAT, APX, and GR ( Zhang et al., 2009 ). UV-B-produced oxidative stress in Glycine max was alleviated by NO donors, which induced transcription and activities of SOD, CAT, and APX ( Santa-Cruz et al., 2014 ). Furthermore, in bean leaves, SOD, CAT, and APX activities are increased by NO donors, and protected from the oxidative stress generated by UV-B irradiation ( Shi et al., 2005 ). Drought tolerance in bermudagrass is improved by ABA-dependent SOD and CAT activities. This effect is regulated by H 2 O 2 and NO, NO acting downstream H 2 O 2 ( Lu et al., 2009 ).

Several antioxidant enzymes have been identified as target of S-nitrosylation, resulting in a change of their biological activity ( Romero-Puertas et al., 2008 ; Bai et al., 2011 ; Fares et al., 2011 ). For instance, NO reinforces recalcitrant seed desiccation tolerance in Antiaris toxicaria by activating the ascorbate-glutathione cycle through S-nitrosylation to control H 2 O 2 accumulation. Desiccation treatment reduced the level of S-nitrosylated APX, GR, and DHAR proteins. Instead, NO gas exposure activated them by S-nitrosylation ( Bai et al., 2011 ). Furthermore, APX was S-nitrosylated at Cys32 during saline stress and biotic stress, enhancing its enzymatic activity ( Begara-Morales et al., 2014 ; Yang et al., 2015 ). In addition, auxin-induced denitrosylation of cytosolic APX provoked inhibition of its activity, followed by an increase of H 2 O 2 concentration and the consequent lateral root formation in Arabidopsis ( Correa-Aragunde et al., 2013 ). Moreover, an inhibitory impact of S-nitrosylation on APX activity was also reported during programmed cell death in Arabidopsis ( de Pinto et al., 2013 ). CAT was identified to be S-nitrosylated in a proteomic study of isolated peroxisomes ( Ortega-Galisteo et al., 2012 ). A decrease of S-nitrosylated CAT under Cd treatment was reported. In addition, in vitro experiments demonstrated a reversible inhibitory effect of APX and CAT activities by NO binding to the Fe of the heme cofactor ( Brown, 1995 ; Clark et al., 2000 ). In addition, NOXs have been involved in plant defense, development, hormone biosynthesis and signaling ( Marino et al., 2012 ). Whereas S-nitrosylation did not affect SOD activities, nitration inhibited Mn-SOD1, Fe-SOD3, and CuZn-SOD3 activity to different degrees ( Holzmeister et al., 2015 ). SOD isoforms could also regulate endogenous NO availability by competing for the common substrate, O 2 •– , and it was demonstrated that bovine SOD may release NO from GSNO ( Singh et al., 1999 ). When GSNO is decomposed by GSNOR, it produces GSSG. GSNOR is also regulated by NO. Frungillo et al. (2014) demonstrated that NO-derived from nitrate assimilation in Arabidopsis inhibited GSNOR1 by S-nitrosylation, preventing GSNO degradation. They proposed that (S)NO controls its own generation and scavenging by modulating nitrate assimilation and GSNOR1 activity. It was also shown that chilling treatment in poplar increased S-nitrosylation of NR, along with a significant decrease of its activity ( Cheng et al., 2015 ).

The dual activity of Prx, suggests a role for this enzyme both in ROS and RNS regulation. S-nitrosylation of Arabidopsis PrxIIE inhibits its peroxynitrite activity, increasing peroxynitrite-mediated tyrosine nitration ( Romero-Puertas et al., 2007 ). Pea mitochondrial PrxIIF was S-nitrosylated under salt stress, and its peroxidase activity was reduced by 5 mM GSNO ( Camejo et al., 2013 ).

An interesting study demonstrated that NO controls hypersensitive response (HR) through S-nitrosylation of NOX, inhibiting ROS synthesis. This triggers a feedback loop limiting HR ( Yun et al., 2011 ).

Other proteins related to abiotic stress response are regulated by S-nitrosylation (For a review see Fancy et al., 2017 ).

Figure ​ Figure3 3 is a simplified diagram that illustrates the main oxidative and nitrosative effects that modulate the activities of key cell components, thus maintaining cell redox balance. Note the feedback and positive-negative regulatory processes occurring in the main pathways. They involve posttranslational modifications that activate and inhibit the components involved in cell antioxidant system.

Molecules and mechanisms involved in NO-mediated redox balance. H 2 O 2 is generated mainly by NOX and SOD as a response to (a)biotic stress. APX and CAT are the main H 2 O 2 -degrading enzymes. NO is increased by H 2 O 2 through the induction of NR/NOS-like activities, and may scavenge ROS or induce both the transcription and activity of SOD, CAT, and APX. In parallel, NO is combined with GSH to form nitrosoglutathione GSNO. GSNO regulates many enzymatic activities by the posttranslational modification of cysteine residues through S-Nitrosylation. NOX and CAT activities are inhibited by S-nitrosylation, whereas APX is either activated or inhibited by S-nitrosylation. NO also inhibits APX by binding to heme group. GSNO is degraded by GSNOR, which could be inhibited by H 2 O 2 and S-nitrosylation.NR could be inhibited by S-nitrosylation. GR reduces GSSG to GSH, and it is activated by S-nitrosylation. Ascorbate (ASC) is a cofactor of APX. Reduced ASC is generated by MDHAR and DHAR, using GSH as electron donor. Both enzymes are inhibited by S-nitrosylation. Reactive Nitrogen Species (RNS) may be originated by NO and O 2 •– reaction. SOD regulate RNS dismutating O 2 •– . Peroxiredoxins (Prx) reduce both ROS AND RNS. RNS are degraded by PrxIIe, and H 2 O 2 by PrxIIF. Both enzymes are inhibited by S-nitrosylation. Red lines: H 2 O 2 -regulated reactions. Purple lines: NO-regulated reactions. Green lines: GSNO-regulated reactions.

Conclusions and Perspectives

The accelerating rate of climate change, together with habitat fragmentation caused by human activity, are part of the selective pressures building a new Earth’s landscape.

Climate change is a multidimensional and simultaneous variation in duration, frequency and intensity of parameters like temperature and precipitation, altering the seasons and life on the Earth. In this scenario, plant species with increased adaptive plasticity will be better equipped to tolerate changes in the frequency of extreme weather events. GHG are one of the forces driving climate change. However, CO 2 and NO may contribute to maintaining the cell redox homeostasis, regulating the amount of ROS, GSH, GSNO, and SNO.

In this manuscript, we summarize the available evidence supporting the presence of broad spectrum anti-stress molecules, as NO in plants, for coping with unprecedented changes in environmental conditions. Future research should focus in better understanding the influence of GHG on plant physiology.

Author Contributions

RC conceived the project and wrote the manuscript. MN drew figures and collaborated in writing the manuscript. NC-A and LL supervised and complemented the drafting. All the persons entitled to authorship have been named and have approved the final version of the submitted manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer MCR-P and handling Editor declared their shared affiliation.

Acknowledgments

We thank ANPCYT for MN fellowship. We also thank Marta Terrazo for helping with the language revision of the manuscript.

Funding. This work was supported by grants from the Consejo Nacional de Investigaciones Cientificas y Tecnicas, the Agencia Nacional de Promoción Científica y Tecnológica, and the Universidad Nacional de Mar del Plata, Argentina. NC-A, LL, and RC are permanent members of the Scientific Research career of CONICET. MN is doctoral fellow of the ANPCYT.

1 https://ozonewatch.gsfc.nasa.gov/facts/ozone.html

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Climate Change Indicators: Greenhouse Gases

View indicators:.

Greenhouse gases from human activities are the most significant driver of observed climate change since the mid-20th century. 1 The indicators in this chapter characterize emissions of the major greenhouse gases resulting from human activities, the concentrations of these gases in the atmosphere, and how emissions and concentrations have changed over time. When comparing emissions of different gases, these indicators use a concept called “global warming potential” to convert amounts of other gases into carbon dioxide equivalents.

Why does it matter?

As greenhouse gas emissions from human activities increase, they build up in the atmosphere and warm the climate, leading to many other changes around the world—in the atmosphere, on land, and in the oceans. The indicators in other chapters of this report illustrate many of these changes, which have both positive and negative effects on people, society, and the environment—including plants and animals. Because many of the major greenhouse gases stay in the atmosphere for tens to hundreds of years after being released, their warming effects on the climate persist over a long time and can therefore affect both present and future generations.

Summary of Key Points

• Sources of Data on U.S. Greenhouse Gas Emissions. EPA has two key programs that provide data on greenhouse gas emissions in the United States: the Inventory of U.S. Greenhouse Gas Emissions and Sinks and the Greenhouse Gas Reporting Program . The programs are complementary, providing both a higher-level perspective on the nation’s total emissions and detailed information about the sources and types of emissions from individual facilities.
• Global Greenhouse Gas Emissions . Worldwide, net emissions of greenhouse gases from human activities increased by 44 percent from 1990 to 2015. Emissions of carbon dioxide, which account for about three-fourths of total emissions, increased by 50 percent over this period. As with the United States, the majority of the world’s emissions result from transportation, electricity generation, and other forms of energy production and use.
• Atmospheric Concentrations of Greenhouse Gases . Concentrations of carbon dioxide and other greenhouse gases in the atmosphere have increased since the beginning of the industrial era. Almost all of this increase is attributable to human activities. 2 Historical measurements show that the current global atmospheric concentrations of carbon dioxide are unprecedented compared with the past 800,000 years, even after accounting for natural fluctuations.
• Climate Forcing . Climate forcing refers to a change in the Earth’s energy balance, leading to either a warming or cooling effect over time. An increase in the atmospheric concentrations of greenhouse gases produces a positive climate forcing, or warming effect. From 1990 to 2019, the total warming effect from greenhouse gases added by humans to the Earth’s atmosphere increased by 45 percent. The warming effect associated with carbon dioxide alone increased by 36 percent.

Major Greenhouse Gases Associated with Human Activities

This table shows 100-year global warming potentials, which describe the effects that occur over a period of 100 years after a particular mass of a gas is emitted. Global warming potentials and lifetimes come from Tables 7.15 and 7.SM.7 of the Intergovernmental Panel on Climate Change’s Sixth Assessment Report, Working Group I contribution. 3

*  Carbon dioxide’s lifetime cannot be represented with a single value because the gas is not destroyed over time, but instead moves among different parts of the ocean–atmosphere–land system. Some of the excess carbon dioxide is absorbed quickly (for example, by the ocean surface), but some will remain in the atmosphere for thousands of years, due in part to the very slow process by which carbon is transferred to ocean sediments.

**  Methane's global warming potential is shown as a range that includes methane from both fossil and non-fossil sources.

See Understanding Global Warming Potentials to learn more about the numbers in the table above and the versions EPA uses for various calculations.  

Sources of Data on U.S. Greenhouse Gas Emissions

EPA has two key programs that provide data on greenhouse gas emissions in the United States: the Inventory of U.S. Greenhouse Gas Emissions and Sinks and the Greenhouse Gas Reporting Program. The programs are complementary, providing both a higher-level perspective on the nation’s total emissions and detailed information about the sources and types of emissions from individual facilities. The data in EPA’s U.S. Greenhouse Gas Emissions indicator come from the national inventory.

EPA's Inventory of Greenhouse Gas Emissions and Sinks

EPA develops an annual report called the Inventory of U.S. Greenhouse Gas Emissions and Sinks (or the GHG Inventory). This report tracks trends in total annual U.S. emissions by source (or sink), economic sector, and greenhouse gas going back to 1990. EPA uses national energy data, data on national agricultural activities, and other national statistics to provide a comprehensive accounting of total greenhouse gas emissions for all man-made sources in the United States. This inventory fulfills the nation’s obligation to provide an annual emissions report under the United Nations Framework Convention on Climate Change.

Learn more about the inventory and explore the data  using interactive tools.

EPA's Greenhouse Gas Reporting Program

Since 2010, EPA’s Greenhouse Gas Reporting Program has been collecting annual emissions data from industrial sources that directly emit large amounts of greenhouse gases. Generally, facilities that emit more than 25,000 metric tons of carbon dioxide equivalents per year are required to report. The program also collects data from entities known as "suppliers" that supply certain fossil fuels and industrial gases that will emit greenhouse gases into the atmosphere if burned or released—for example, refineries that supply petroleum products such as gasoline. The Greenhouse Gas Reporting Program only requires reporting; it is not an emissions control program. This program helps EPA and the public understand where greenhouse gas emissions are coming from, and will improve our ability to make informed policy, business, and regulatory decisions.

Learn more about the Greenhouse Gas Reporting Program and explore data  by facility, industry, location, or gas using a data visualization and mapping tool called FLIGHT . You can also review state- or Tribal-specific emissions using interactive fact sheets and download detailed data via EPA's Envirofacts database. 

1 IPCC (Intergovernmental Panel on Climate Change). 2021. Climate change 2021: The physical science basis. Working Group I contribution to the IPCC Sixth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/assessment-report/ar6 .

2 IPCC (Intergovernmental Panel on Climate Change). 2021. Climate change 2021: The physical science basis. Working Group I contribution to the IPCC Sixth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/assessment-report/ar6 . 

3 IPCC (Intergovernmental Panel on Climate Change). 2021. Climate change 2021: The physical science basis. Working Group I contribution to the IPCC Sixth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/assessment-report/ar6 .

• Indicators Home
• U.S. Greenhouse Gas Emissions
• Global Greenhouse Gas Emissions
• Atmospheric Concentrations of Greenhouse Gases
• Climate Forcing
• Weather and Climate
• Snow and Ice
• Health and Society
• Dig into the Data
• Indicator Stories
• About the Indicators

Climate Change: Evidence and Causes: Update 2020 (2020)

Chapter: conclusion, c onclusion.

This document explains that there are well-understood physical mechanisms by which changes in the amounts of greenhouse gases cause climate changes. It discusses the evidence that the concentrations of these gases in the atmosphere have increased and are still increasing rapidly, that climate change is occurring, and that most of the recent change is almost certainly due to emissions of greenhouse gases caused by human activities. Further climate change is inevitable; if emissions of greenhouse gases continue unabated, future changes will substantially exceed those that have occurred so far. There remains a range of estimates of the magnitude and regional expression of future change, but increases in the extremes of climate that can adversely affect natural ecosystems and human activities and infrastructure are expected.

Citizens and governments can choose among several options (or a mixture of those options) in response to this information: they can change their pattern of energy production and usage in order to limit emissions of greenhouse gases and hence the magnitude of climate changes; they can wait for changes to occur and accept the losses, damage, and suffering that arise; they can adapt to actual and expected changes as much as possible; or they can seek as yet unproven “geoengineering” solutions to counteract some of the climate changes that would otherwise occur. Each of these options has risks, attractions and costs, and what is actually done may be a mixture of these different options. Different nations and communities will vary in their vulnerability and their capacity to adapt. There is an important debate to be had about choices among these options, to decide what is best for each group or nation, and most importantly for the global population as a whole. The options have to be discussed at a global scale because in many cases those communities that are most vulnerable control few of the emissions, either past or future. Our description of the science of climate change, with both its facts and its uncertainties, is offered as a basis to inform that policy debate.

A CKNOWLEDGEMENTS

The following individuals served as the primary writing team for the 2014 and 2020 editions of this document:

• Eric Wolff FRS, (UK lead), University of Cambridge
• Inez Fung (NAS, US lead), University of California, Berkeley
• Brian Hoskins FRS, Grantham Institute for Climate Change
• John F.B. Mitchell FRS, UK Met Office
• Tim Palmer FRS, University of Oxford
• Benjamin Santer (NAS), Lawrence Livermore National Laboratory
• John Shepherd FRS, University of Southampton
• Keith Shine FRS, University of Reading.
• Susan Solomon (NAS), Massachusetts Institute of Technology
• Kevin Trenberth, National Center for Atmospheric Research
• John Walsh, University of Alaska, Fairbanks
• Don Wuebbles, University of Illinois

Staff support for the 2020 revision was provided by Richard Walker, Amanda Purcell, Nancy Huddleston, and Michael Hudson. We offer special thanks to Rebecca Lindsey and NOAA Climate.gov for providing data and figure updates.

The following individuals served as reviewers of the 2014 document in accordance with procedures approved by the Royal Society and the National Academy of Sciences:

• Richard Alley (NAS), Department of Geosciences, Pennsylvania State University
• Alec Broers FRS, Former President of the Royal Academy of Engineering
• Harry Elderfield FRS, Department of Earth Sciences, University of Cambridge
• Joanna Haigh FRS, Professor of Atmospheric Physics, Imperial College London
• Isaac Held (NAS), NOAA Geophysical Fluid Dynamics Laboratory
• John Kutzbach (NAS), Center for Climatic Research, University of Wisconsin
• Jerry Meehl, Senior Scientist, National Center for Atmospheric Research
• John Pendry FRS, Imperial College London
• John Pyle FRS, Department of Chemistry, University of Cambridge
• Gavin Schmidt, NASA Goddard Space Flight Center
• Emily Shuckburgh, British Antarctic Survey
• Gabrielle Walker, Journalist
• Andrew Watson FRS, University of East Anglia

The Support for the 2014 Edition was provided by NAS Endowment Funds. We offer sincere thanks to the Ralph J. and Carol M. Cicerone Endowment for NAS Missions for supporting the production of this 2020 Edition.

F OR FURTHER READING

For more detailed discussion of the topics addressed in this document (including references to the underlying original research), see:

• Intergovernmental Panel on Climate Change (IPCC), 2019: Special Report on the Ocean and Cryosphere in a Changing Climate [ https://www.ipcc.ch/srocc ]
• National Academies of Sciences, Engineering, and Medicine (NASEM), 2019: Negative Emissions Technologies and Reliable Sequestration: A Research Agenda [ https://www.nap.edu/catalog/25259 ]
• Royal Society, 2018: Greenhouse gas removal [ https://raeng.org.uk/greenhousegasremoval ]
• U.S. Global Change Research Program (USGCRP), 2018: Fourth National Climate Assessment Volume II: Impacts, Risks, and Adaptation in the United States [ https://nca2018.globalchange.gov ]
• IPCC, 2018: Global Warming of 1.5°C [ https://www.ipcc.ch/sr15 ]
• USGCRP, 2017: Fourth National Climate Assessment Volume I: Climate Science Special Reports [ https://science2017.globalchange.gov ]
• NASEM, 2016: Attribution of Extreme Weather Events in the Context of Climate Change [ https://www.nap.edu/catalog/21852 ]
• IPCC, 2013: Fifth Assessment Report (AR5) Working Group 1. Climate Change 2013: The Physical Science Basis [ https://www.ipcc.ch/report/ar5/wg1 ]
• NRC, 2013: Abrupt Impacts of Climate Change: Anticipating Surprises [ https://www.nap.edu/catalog/18373 ]
• NRC, 2011: Climate Stabilization Targets: Emissions, Concentrations, and Impacts Over Decades to Millennia [ https://www.nap.edu/catalog/12877 ]
• Royal Society 2010: Climate Change: A Summary of the Science [ https://royalsociety.org/topics-policy/publications/2010/climate-change-summary-science ]
• NRC, 2010: America’s Climate Choices: Advancing the Science of Climate Change [ https://www.nap.edu/catalog/12782 ]

Much of the original data underlying the scientific findings discussed here are available at:

• https://data.ucar.edu/
• https://climatedataguide.ucar.edu
• https://iridl.ldeo.columbia.edu
• https://ess-dive.lbl.gov/
• https://www.ncdc.noaa.gov/
• https://www.esrl.noaa.gov/gmd/ccgg/trends/
• http://scrippsco2.ucsd.edu
• http://hahana.soest.hawaii.edu/hot/

Climate change is one of the defining issues of our time. It is now more certain than ever, based on many lines of evidence, that humans are changing Earth's climate. The Royal Society and the US National Academy of Sciences, with their similar missions to promote the use of science to benefit society and to inform critical policy debates, produced the original Climate Change: Evidence and Causes in 2014. It was written and reviewed by a UK-US team of leading climate scientists. This new edition, prepared by the same author team, has been updated with the most recent climate data and scientific analyses, all of which reinforce our understanding of human-caused climate change.

Scientific information is a vital component for society to make informed decisions about how to reduce the magnitude of climate change and how to adapt to its impacts. This booklet serves as a key reference document for decision makers, policy makers, educators, and others seeking authoritative answers about the current state of climate-change science.

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