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The main reason are natural gases and petroleum systems

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Gases that trap heat in the atmosphere are called greenhouse gases. This section provides information on emissions and removals of the main greenhouse gases to and from the atmosphere. For more information on the other climate forcers, such as black carbon, please visit the Climate Change Indicators: Climate Forcing page.

: Carbon dioxide enters the atmosphere through burning fossil fuels (coal, natural gas, and oil), solid waste, trees and wood products, and also as a result of certain chemical reactions (e.g., manufacture of cement). Carbon dioxide is removed from the atmosphere (or "sequestered") when it is absorbed by plants as part of the biological carbon cycle.

: Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.

: Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.

: Hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for stratospheric ozone-depleting substances (e.g., chlorofluorocarbons, hydrochlorofluorocarbons, and halons). These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases ("High GWP gases").

Each gas's effect on climate change depends on three main factors:

How much of these gases are in the atmosphere?

Concentration, or abundance, is the amount of a particular gas in the air. Larger emissions of greenhouse gases lead to higher concentrations in the atmosphere. Greenhouse gas concentrations are measured in parts per million, parts per billion, and even parts per trillion. One part per million is equivalent to one drop of water diluted into about 13 gallons of liquid (roughly the fuel tank of a compact car). To learn more about the increasing concentrations of greenhouse gases in the atmosphere, visit the Climate Change Indicators: Atmospheric Concentrations of Greenhouse Gases page.

How long do they stay in the atmosphere?

Each of these gases can remain in the atmosphere for different amounts of time, ranging from a few years to thousands of years. All of these gases remain in the atmosphere long enough to become well mixed, meaning that the amount that is measured in the atmosphere is roughly the same all over the world, regardless of the source of the emissions.

How strongly do they impact the atmosphere?

Some gases are more effective than others at making the planet warmer and "thickening the Earth's blanket."

For each greenhouse gas, a Global Warming Potential (GWP) has been calculated to reflect how long it remains in the atmosphere, on average, and how strongly it absorbs energy. Gases with a higher GWP absorb more energy, per pound, than gases with a lower GWP, and thus contribute more to warming Earth.

Note: All emission estimates are from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2016.

Carbon Dioxide Emissions

Carbon dioxide (CO ₂) is the primary greenhouse gas emitted through human activities. In 2016, CO ₂ accounted for about 81.6% of all U.S. greenhouse gas emissions from human activities. Carbon dioxide is naturally present in the atmosphere as part of the Earth's carbon cycle (the natural circulation of carbon among the atmosphere, oceans, soil, plants, and animals). Human activities are altering the carbon cycle-both by adding more CO ₂ to the atmosphere and by influencing the ability of natural sinks, like forests, to remove CO ₂ from the atmosphere. While CO ₂ emissions come from a variety of natural sources, human-related emissions are responsible for the increase that has occurred in the atmosphere since the industrial revolution.

The main human activity that emits CO ₂ is the combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation, although certain industrial processes and land-use changes also emit CO ₂. The main sources of CO ₂ emissions in the United States are described below.

Electricity. Electricity is a significant source of energy in the United States and is used to power homes, business, and industry. In 2016 the combustion of fossil fuels to generate electricity was the largest single source of CO₂ emissions in the nation, accounting for about 34 percent of total U.S. CO₂ emissions and 28 percent of total U.S. greenhouse gas emissions. The type of fossil fuel used to generate electricity will emit different amounts of CO₂. To produce a given amount of electricity, burning coal will produce more CO2 than oil or natural gas.

Transportation. The combustion of fossil fuels such as gasoline and diesel to transport people and goods was the second largest source of CO₂ emissions in 2016, accounting for about 34 percent of total U.S. CO₂ emissions and 27 percent of total U.S. greenhouse gas emissions. This category includes transportation sources such as highway vehicles, air travel, marine transportation, and rail.

Many industrial processes emit CO₂ through fossil fuel combustion. Several processes also produce CO₂ emissions through chemical reactions that do not involve combustion; for example, the production and consumption of mineral products such as cement, the production of metals such as iron and steel, and the production of chemicals. Fossil fuel combustion from various industrial processes accounted for about 15 percent of total U.S. CO₂ emissions and 12 percent of total U.S. greenhouse gas emissions in 2016. Note that many industrial processes also use electricity and therefore indirectly cause the emissions from the electricity production.

Carbon dioxide is constantly being exchanged among the atmosphere, ocean, and land surface as it is both produced and absorbed by many microorganisms, plants, and animals. However, emissions and removal of CO ₂ by these natural processes tend to balance. Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding CO ₂ and other heat-trapping gases to the atmosphere.

In the United States, since 1990, the management of forests and other land has acted as a net sink of CO ₂, which means that more CO ₂ is removed from the atmosphere, and stored in plants and trees, than is emitted. This carbon sink offset is about 12 percent of total emissions in 2016 and is discussed in more detail in the Land Use, Land-Use Change, and Forestry section.

To find out more about the role of CO ₂ warming the atmosphere and its sources, visit the Climate Change Indicators page.

Emissions and Trends

Carbon dioxide emissions in the United States increased by about 4 percent between 1990 and 2016. Since the combustion of fossil fuel is the largest source of greenhouse gas emissions in the United States, changes in emissions from fossil fuel combustion have historically been the dominant factor affecting total U.S. emission trends. Changes in CO ₂ emissions from fossil fuel combustion are influenced by many long-term and short-term factors, including population growth, economic growth, changing energy prices, new technologies, changing behavior, and seasonal temperatures. Between 1990 and 2016, the increase in CO ₂ emissions corresponded with increased energy use by an expanding economy and population, an overall growth in emissions from electricity generation, and increased demand for travel.

Reducing Carbon Dioxide Emissions

The most effective way to reduce CO emissions is to reduce fossil fuel consumption. Many strategies for reducing CO ₂ emissions from energy are cross-cutting and apply to homes, businesses, industry, and transportation.

EPA is taking common sense regulatory actions to reduce greenhouse gas emissions.

* Atmospheric CO is part of the global carbon cycle, and therefore its fate is a complex function of geochemical and biological processes. Some of the excess carbon dioxide will be 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.

References

¹ NRC (2010). Advancing the Science of Climate Change. Exit National Research Council. The National Academies Press, Washington, DC, USA.

Methane Emissions

In 2016, methane (CH ₄) accounted for about 10 percent of all U.S. greenhouse gas emissions from human activities. Human activities emitting methane include leaks from natural gas systems and the raising of livestock. Methane is also emitted by natural sources such as natural wetlands. In addition, natural processes in soil and chemical reactions in the atmosphere help remove CH ₄ from the atmosphere. Methane's lifetime in the atmosphere is much shorter than carbon dioxide (CO ₂), but CH ₄ is more efficient at trapping radiation than CO ₂. Pound for pound, the comparative impact of CH ₄ is more than 25 times greater than CO ₂ over a 100-year period.

Globally, over 60 percent of total CH ₄ emissions come from human activities. Methane is emitted from industry, agriculture, and waste management activities, described below.

Energy and Industry. Natural gas and petroleum systems are the largest source of CH₄ emissions in the United States. Methane is the primary component of natural gas. Methane is emitted to the atmosphere during the production, processing, storage, transmission, and distribution of natural gas. Because gas is often found alongside petroleum, the production, refinement, transportation, and storage of crude oil is also a source of CH₄ emissions. For more information, see the Inventory of U.S. Greenhouse Gas Emissions and Sinks sections on Natural Gas Systems and Petroleum Systems.

Agriculture. Domestic livestock such as cattle, buffalo, sheep, and goats produce large amounts of CH₄ as part of their normal digestive process. Also, when animals' manure is stored or managed in lagoons or holding tanks, CH₄ is produced. Because humans raise these animals for food and other products, the emissions are considered human-related. When livestock and manure emissions are combined, the Agriculture sector is the primary source of CH₄ emissions. For more information, see the Inventory of U.S. Greenhouse Gas Emissions and Sinks Agriculture chapter.

Waste from Homes and Businesses. Methane is generated in landfills as waste decomposes and in the treatment of wastewater. Landfills are the third largest source of CH₄ emissions in the United States. For more information, see the Inventory of U.S. Greenhouse Gas Emissions and Sinks Waste chapter.

Methane is also emitted from a number of natural sources. Natural wetlands are the largest source, emitting CH ₄ from bacteria that decompose organic materials in the absence of oxygen. Smaller sources include termites, oceans, sediments, volcanoes, and wildfires.

To find out more about the role of CH ₄ in warming the atmosphere and its sources, visit the Climate Change Indicators page.

Emissions and Trends

Methane emissions in the United States decreased by 16 percent between 1990 and 2016. During this time period, emissions increased from sources associated with agricultural activities, while emissions decreased from sources associated with landfills, coal mining, and the exploration through distribution of natural gas and petroleum products.

Reducing Methane Emissions

There are a number of ways to reduce CH4 emissions. Some examples are discussed below. EPA has a series of voluntary programs for reducing CH ₄ emissions, in addition to regulatory initiatives. EPA also supports the Global Methane Initiative Exit, an international partnership encouraging global methane reduction strategies.

References

IPCC (2007) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press. Cambridge, United Kingdom 996 pp.

² EPA (2010). Methane and Nitrous Oxide Emissions from Natural Sources. U.S. Environmental Protection Agency, Washington, DC, USA.

Nitrous Oxide Emissions

In 2016, nitrous oxide (N ₂O) accounted for about 6 percent of all U.S. greenhouse gas emissions from human activities. Human activities such as agriculture, fuel combustion, wastewater management, and industrial processes are increasing the amount of N ₂O in the atmosphere. Nitrous oxide is also naturally present in the atmosphere as part of the Earth's nitrogen cycle, and has a variety of natural sources. Nitrous oxide molecules stay in the atmosphere for an average of 114 years before being removed by a sink or destroyed through chemical reactions. The impact of 1 pound of N ₂ O on warming the atmosphere is almost 300 times that of 1 pound of carbon dioxide.

Globally, about 40 percent of total N ₂ O emissions come from human activities. Nitrous oxide is emitted from agriculture, transportation, and industry activities, described below.

Agriculture. Nitrous oxide can result from various agricultural soil management activities, such as synthe t ic and organic fertilizer application and other cropping practices, the management of manure, or burning of agricultural residues. Agricultural soil management is the largest source of N₂O emissions in the United States, accounting for about 77 percent of total U.S. N₂O emissions in 2016.

Fuel Combustion. Nitrous oxide is emitted when fuels are burned. The amount of N₂O emitted from burning fuels depends on the type of fuel and combustion technology, maintenance, and operating practices.

Industry. Nitrous oxide is generated as a byproduct during the production of nitric acid, which is used to make synthetic commercial fertilizer, and in the production of adipic acid, which is used to make fibers, like nylon, and other synthetic products.

Nitrous oxide emissions occur naturally through many sources associated with the nitrogen cycle, which is the natural circulation of nitrogen among the atmosphere, plants, animals, and microorganisms that live in soil and water. Nitrogen takes on a variety of chemical forms throughout the nitrogen cycle, including N ₂O. Natural emissions of N ₂ O are mainly from bacteria breaking down nitrogen in soils and the oceans. Nitrous oxide is removed from the atmosphere when it is absorbed by certain types of bacteria or destroyed by ultraviolet radiation or chemical reactions.

To find out more about the sources of N ₂O and its role in warming the atmosphere, visit the Climate Change Indicators page.

Emissions and Trends

Nitrous oxide emissions in the United States have increased by about 4 percent between 1990 and 2016. This increase in emissions is due in part to an increase in emissions from mobile combustion resulting from emission control standards for on-road vehicles. Nitrous oxide emissions from agricultural soils have varied during this period and were about 13 percent higher in 2016 than in 1990.

Reducing Nitrous Oxide Emissions

There are a number of ways to reduce emissions of N ₂ O, discussed below.

References

¹ IPCC (2007) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press. Cambridge, United Kingdom 996 pp.
² EPA (2010). Methane and Nitrous Oxide Emissions from Natural Sources. U.S. Environmental Protection Agency, Washington, DC, USA.
³ EPA (2005). Greenhouse Gas Mitigation Potential in U.S. Forestry and Agriculture. U.S. Environmental Protection Agency, Washington, DC, USA.

Emissions of Fluorinated Gases

Unlike many other greenhouse gases, fluorinated gases have no natural sources and only come from human-related activities. They are emitted through a variety of industrial processes such as aluminum and semiconductor manufacturing. Many fluorinated gases have very high global warming potentials (GWPs) relative to other greenhouse gases, so small atmospheric concentrations can have proportionately large effects on global temperatures. They can also have long atmospheric lifetimes-in some cases, lasting thousands of years. Like other long-lived greenhouse gases, most fluorinated gases are well-mixed in the atmosphere, spreading around the world after they are emitted. Many fluorinated gases are removed from the atmosphere only when they are destroyed by sunlight in the far upper atmosphere. In general, fluorinated gases are the most potent and longest lasting type of greenhouse gases emitted by human activities.

There are four main categories of fluorinated gases-hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF ₆), and nitrogen trifluoride (NF ₃). The largest sources of fluorinated gas emissions are described below.

Substitution for Ozone-Depleting Substances. Hydrofluorocarbons are used as refrigerants, aerosol propellants, foam blowing agents, solvents, and fire retardants. The major emissions source of these compounds is their use as refrigerants-for example, in air conditioning systems in both vehicles and buildings. These chemicals were developed as a replacement for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) because they do not deplete the stratospheric ozone layer. Chlorofluorocarbons and HCFCs are being phased out under an international agreement, called the Montreal Protocol. HFCs are potent greenhouse gases with high GWPs, and they are released into the atmosphere during manufacturing processes and through leaks, servicing, and disposal of equipment in which they are used. The Kigali Amendment to the Montreal Protocol calls for a phasedown in the production and consumption of the most potent HFCs. Newly developed hydrofluoroolefins (HFOs) are a subset of HFCs and are characterized by short atmospheric lifetimes and low GWPs. HFOs are currently being introduced as refrigerants, aerosol propellants and foam blowing agents.

Industry. Perfluorocarbons are compounds produced as a byproduct of various industrial processes associated with aluminum production and the manufacturing of semiconductors. PFCs generally have long atmospheric lifetimes and high GWPs. Sulfur hexafluoride is used in magnesium processing and semiconductor manufacturing, as well as a tracer gas for leak detection. HFC-23 is produced as a byproduct of HCFC-22 production.

Transmission and Distribution of Electricity. Sulfur hexafluoride is used in electrical transmission equipment, including circuit breakers. The GWP of SF₆ is 22,800, making it the most potent greenhouse gas that the Intergovernmental Panel on Climate Change has evaluated.

To find out more about the role of fluorinated gases in warming the atmosphere and their sources, visit the Climate Change Indicators page.

Emissions and Trends

Overall, fluorinated gas emissions in the United States have increased by about 74 percent between 1990 and 2016. This increase has been driven by a 249 percent increase in emissions of hydrofluorocarbons (HFCs) since 1990, as they have been widely used as a substitute for ozone-depleting substances. Emissions of perfluorocarbons (PFCs) and sulfur hexafluoride (SF ₆) have actually declined during this time due to emission reduction efforts in the aluminum production industry (PFCs) and the electricity transmission and distribution industry (SF ₆).

Reducing Fluorinated Gas Emissions

Because most fluorinated gases have a very long atmospheric lifetime, it will take many years to see a noticeable decline in current concentrations. However, there are a number of ways to reduce emissions of fluorinated gases, described below.

References

¹ IPCC (2007) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press. Cambridge, United Kingdom 996 pp.

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Gases that trap heat in the atmosphere are called greenhouse gases. This section provides information on emissions and removals of the main greenhouse gases to and from the atmosphere. For more information on the other climate forcers, such as black carbon, please visit the Climate Change Indicators: Climate Forcing page.

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NOAA

A changing Gulf Stream off the East Coast has destabilized frozen methane deposits trapped under nearly 4,000 square miles of seafloor, scientists reported Wednesday. And since methane is even more potent than carbon dioxide as a global warming gas, the researchers said, any large-scale release could have significant climate impacts.

Temperature changes in the Gulf Stream are "rapidly destabilizing methane hydrate along a broad swathe of the North American margin," the experts said in a study published Wednesday in the peer-reviewed journal Nature.

Using seismic records and ocean models, the team estimated that 2.5 gigatonnes of frozen methane hydrate are being destabilized and could separate into methane gas and water.

It is not clear if that is happening yet, but that methane gas would have the potential to rise up through the ocean and into the atmosphere, where it would add to the greenhouse gases warming Earth.

The 2.5 gigatonnes isn't enough to trigger a sudden climate shift, but the team worries that other areas around the globe might be seeing a similar destabilization.

USGS

"It is unlikely that the western North Atlantic margin is the only area experiencing changing ocean currents," they noted. "Our estimate ... may therefore represent only a fraction of the methane hydrate currently destabilizing globally."

The wider destabilization evidence, co-author Ben Phrampus told NBC News, includes data from the Arctic and Alaska's northern slope in the Beaufort Sea.

And it's not just under the seafloor that methane has been locked up. Some Arctic land area are seeing permafrost thaw, which could release methane stored there as well.

An expert who was not part of the study said it suggests that methane could become a bigger climate factor than carbon dioxide.

"We may approach a turning point" from a warming driven by man-made carbon dioxide to a warming driven by methane, Jurgen Mienert, the geology department chair at Norway's University of Tromso, told NBC News.

"The interactions between the warming Arctic Ocean and the potentially huge methane-ice reservoirs beneath the Arctic Ocean floor point towards increasing instability," he added.

He also noted, however, that "one of the big unknowns is the magnitude of rapid methane escape from the ocean floor, and how natural filter systems react and affect the future ocean, its environment and the climate."
Relate: Thawing Arctic permafrost is releasing methane
Another unknown is what caused the Gulf Stream changes, said Phrampus, an earth sciences PhD candidate at Southern Methodist University in Dallas, Texas.

"Multiple events can play a factor, such as changing sea level or an addition of cold/fresh water from the north," Phrampus said, adding he was hopeful that the changes might be "reversible under their own influence."

But, he added, "we need more data to resolve this, and we are currently investigating this process."
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Climate-changing methane 'rapidly destabilizing' off East Coast, study finds

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A changing Gulf Stream off the East Coast has destabilized frozen methane deposits trapped under nearly 4,000 square miles of seafloor, scientists reported Wednesday. And since methane is even more potent than carbon dioxide as a global warming gas, the

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What are the main sources of methane emissions?

There are both natural and human sources of methane emissions. The main natural sources include wetlands, termites and the oceans. Natural sources create 36% of methane emissions. Important human sources come from landfills, livestock farming, as well as the production, transportation and use of fossil fuels. Human-related sources create the majority of methane emissions, accounting for 64% of the total.1

Methane levels have more than doubled over the last 150 years because of human activities like fossil fuel use and intensive farming.2 Before the Industrial Revolution, the atmospheric concentration of methane was maintained in a safe range by natural sinks.

But for a long time now human activities have been creating methane emissions much more rapidly than the Earth can remove them, increasing global methane levels. During the last 800,000 years, methane concentrations naturally varied between 350-800 ppb but since the Industrial Revolution, methane levels have become much higher and are now 2.5 times larger.3 4

Human Sources

Since the Industrial Revolution, human sources of methane emissions have been growing. Human activities such as fossil fuel production and intensive livestock farming are the primary cause of the increased methane concentrations in the atmosphere. Together these two sources are responsible for 60% of all human methane emissions. Other sources include landfills and waste (16%), biomass burning (11%), rice agriculture (9%) as well as biofuels (4%).1

Figure 1: Source: Bousquet, P. et al. (2006). Contribution of anthropogenic and natural sources to atmospheric methane variability.

Fossil fuel production, distribution and use

The largest human source of methane emissions is from the production, distribution and combustion of fossil fuels. This is responsible for 33% of human methane emissions.1

Methane emissions are produced wherever there are fossil fuels. It is released whenever fossil fuels are extracted from the earth whether it is natural gas (which is mostly methane), coal or petroleum. There are additional methane emission that are created during any type of handling, transportation (pipeline, truck delivery, etc.) or refinement of fossil fuels. Finally some methane is also produced during fossil fuel combustion.

By simply buying and using any fossil fuel whether it is coal, natural gas or petroleum you contribute to the most important source of methane emissions worldwide. Fossil fuel production, distribution and use creates 110 million tonnes of methane annually.1

A large portion of methane emissions are caused by the extraction, processing and transportation of natural gas. Methane is the main component of natural gas, so leakage throughout the different levels of this industry releases methane directly into the atmosphere.

Coal is another important source of methane emissions. During the geological process of coal formation, pockets of methane get trapped around and within the rock. Coal mining related activities (extraction, crushing, distribution, etc.) release some of this trapped methane. Methane is emitted from active underground and surface mines as well as abandoned ones.

As with coal, the geological formation of oil can also create large methane deposits that get released during drilling and extraction. The production, refinement, transportation and storage of oil is also a source of methane emissions.

Incomplete combustion of fossil fuels also produces methane emissions. No combustion process is one hundred percent efficient, so when fossil fuels are used to create electricity, heat or power vehicles these all contribute as sources.

Livestock farming

An important source of methane emissions is from enteric fermentation in farm animals. This is responsible for 27% of human methane emissions.1 Animals like cows, sheep and goats are examples of ruminant animals and during their normal digestion process they create large amounts of methane. Enteric fermentation occurs because of microorganisms in the stomach of these animals, which creates methane as a by-product that is either exhaled by the animal or released via flatus.

Because humans raise these animals for food, their emissions are considered human-related. The meat that we eat everyday contributes significantly to total methane emissions because of this. Livestock farming creates 90 million tonnes of methane annually.1

Livestock related emissions have grown mainly because of the large growth of livestock populations worldwide over the last fifty years. Global livestock production has increased substantially since the 1960s with beef production more than doubling during this time.5

Landfills and waste

Another important human source of methane emissions is from landfills and waste. Methane is generated by the decomposition of biodegradable solid waste in landfills as well as animal and human waste streams. This accounts for 16% of human methane emissions. Landfills and waste produces 55 million tonnes of methane annually.1

Landfills and open garbage dumps are full of organic matter from our garbage (things like food scraps, newspapers, cut grass and leaves). Every time new garbage comes in it is pilled over the old garbage that was already there. The organic matter in our garbage gets trapped in anaerobic conditions where there is no oxygen. This provides excellent conditions for methane producing microbes to break down the waste, which produces large amounts of methane emissions. Even after a landfill is closed, the bacteria will continue to decompose the buried waste and emit methane for years.

Wastewater from domestic, municipal and industrial sources can also produce methane emissions. Wastewater can be either released, stored or sent for treatment to remove contaminants. As with landfills, if at any of these steps the decay of organic material in wastewater happens in an anaerobic environment then this will create methane.

Livestock farming at even a modest scale will have to deal with the manure that gets produced by farm animals everyday. Livestock manure management is usually done by using large waste treatment systems and holding tanks. In many of these systems methane is produced because they promote anaerobic conditions.

Biomass burning

Biomass burning causes a substantial amount of methane emissions. Biomass is material from living or dead organic matter. Incomplete burning of biomass creates methane emissions and huge amounts can be produced during large scale burning. This is responsible for 11% of human methane emissions.1

Large open fires are mainly used by humans to destroy crop waste and clear land for agricultural or other uses. While natural wildfires can contribute to this, the great majority of biomass burning is caused by human beings. Biomass burning creates 38 million tonnes of methane annually.1

Rice agriculture

Another substantial human source of methane emissions is from rice agriculture. Paddy fields for rice production are essentially man-made wetlands. They are characterized by high moisture content, oxygen depletion and ample organic material. This creates a great environment for methane producing microbes that decompose the organic matter.

Though some of the methane produced is absorbed by methane-consuming microorganisms, the vast majority is released into the atmosphere. Due to the swamp-like environment of rice fields, this crop is responsible for 9% of human methane emissions. Rice agriculture creates 31 million tonnes of methane annually.1

Biofuels

Annually 12 million tonnes of methane are produced by biofuels making it a significant source. Biomass that is used to produce energy for domestic, industrial or transportation purposes are called biofuels. Incomplete biofuel combustion leads to the production of methane. This is responsible for 4% of human methane emissions.1

An estimated 80% of biofuels are used for domestic cooking, heating, and lighting mostly in open cooking fires burning wood, agricultural waste, or animal dung. This is the single largest contributor to global biofuel emissions.6 Almost half of the world's population, about 2.7 billion people, use solid biofuels to cook and heat their homes on a daily basis. Most are poor, and live in developing countries.7

18% of biofuels are used by low-technology, largely unregulated, micro-enterprises such as brick or tile making kilns, restaurants, etc. The balance of the biofuels are consumed for transportation uses.6

Natural Sources

Apart from being created by human activities, methane is also released into the atmosphere by natural processes. Wetlands, termites and the oceans are all natural sources of methane emissions.

The amount of methane produced by natural sources is completely offset by natural methane sinks and has been for thousands of years. Before the influence of humans, methane levels were quite steady because of this natural balance. Today, human-related sources create the majority of total methane emissions which has upset the natural balance that existed before the Industrial Revolution.

Wetlands are an important source of methane, accounting for 78% of all naturally produced emissions. Other natural methane sources include termites (12%) and the oceans (10%).1

Figure 2: Source: Bousquet, P. et al. (2006). Contribution of anthropogenic and natural sources to atmospheric methane variability.

Wetlands

Wetlands are the largest natural source of methane. This produces 78% of natural methane emissions.1 The water-logged conditions of wetlands are perfect for microbes who require environments with no oxygen and abundant organic matter.

While part of wetland related emissions is absorbed by methane-consuming microbes, a large percentage escapes into the atmosphere. Wetlands create 147 million tonnes of methane each year.1

Termites

Termites are a significant natural source of methane. During the normal digestion process of a termite, methane is produced. Termites eat cellulose but rely on micro-organisms in their gut to digest it which produces methane during the process. This is responsible for 12% of natural methane emissions.1

Each termite produces very small amounts of methane on a daily basis. However, when this is multiplied by the world population of termites, their emissions add up to 23 million tonnes of methane annually.1

Oceans

Another significant natural source of methane comes from the oceans. Methane producing microbes living in the ocean create these emissions. This creates 10% of natural methane emissions. Globally, oceans create 19 million tonnes of methane annually.1

The majority of oceanic methane emissions gets produced in deeper sediment layers of productive coastal areas. This accounts for 75% of the oceans methane emissions.8 The methane created by these microbes mixes with the surrounding water and is emitted to the atmosphere from the ocean surface.

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it alters the energy of the climate system

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The effect of natrual gas supply on US renewable energy and co2 emission.

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Natural gas may not be a "bridge fuel" to a clean energy future, according to a new study by researchers at UC Irvine, Stanford University and the energy nonprofit Near Zero. Their modeling found that having a high supply of natural gas does little to reduce greenhouse gas emissions in the future, largely because the use of gas slowed the transition to renewables.

The study, published on Sept. 24 in the journal Environmental Research Letters, modeled the impacts of high and low supplies of natural gas on overall emissions from the energy sector. Researchers studied the impacts of abundant natural gas over a range of potential future climate policies, including no limit on carbon emissions, a moderate carbon tax, a strict carbon cap and a federal requirement that 50 percent of energy come from renewable sources by 2050. They found that though increased availability of gas led to less use of coal, it also slowed the adoption of renewable technologies under all scenarios but the requirement to use renewables.

They also found that, if there were no limits on carbon pollution, more abundant and cheaper natural gas led to increased electricity use. "The availability of low-cost, abundant gas lowers energy prices, which increases energy consumption and [greenhouse gas] emissions," the study's lead author, Christine Shearer, told The Huffington Post.

Burning natural gas produces roughly half as much carbon dioxide per unit of energy as burning coal, so it is often touted as a way to reduce greenhouse gas emissions while transitioning to renewable energy. But because abundant natural gas competes with renewable energy, greenhouse gas emissions were not significantly reduced in the models.

Greenhouse gas emissions from the power sector, under various climate policies and under high or low supply of natural gas. The difference in greenhouse gas emissions between high and low supply of natural gas is small in all scenarios, but is the largest under the renewable portfolio standard (RPS).

President Barack Obama said during his 2014 State of the Union address that natural gas, "if extracted safely," can be "the 'bridge fuel' that can power our economy with less of the carbon pollution that causes climate change." Much of the growth in natural gas production in the United States comes from shale gas, which is projected to continue to grow in the future. This growth is largely due to increased use of hydraulic fracturing, or fracking, to tap into gas contained in shale formations.

"Our study suggests fracking is a poor strategy for lowering [greenhouse gas] emissions," Shearer told HuffPost. "In our results, increased gas use lowered U.S. [greenhouse gas] emissions, at most, 13 percent through 2055." Even that reduction, Shearer said, was possible only if the federal government also implemented a policy requiring 50 percent of U.S. electricity to be drawn from renewable sources by 2050 (a policy often referred to as a renewable electricity standard or renewable portfolio standard). "Otherwise, increased gas consumption had very little effect on [greenhouse gas] emissions," Shearer said.

Shearer also said their models found this to be true regardless of the level of anticipated methane gas leakage. Methane, which is the primary component of natural gas, is a potent greenhouse gas. Some research has found that methane leakage from gas wells could also negate the benefits of lower carbon dioxide emissions that come from burning natural gas.

Ultimately, "the biggest driver of emissions reductions was a climate policy, not the amount of natural gas," Shearer explains in a video accompanying the study. "For gas to be effective in lowering emissions," Shearer told HuffPost, "we should strengthen and expand existing state renewable portfolio standards, which would help minimize the competition between gas and renewable electricity."

"Cutting greenhouse gas emissions by burning natural gas is like dieting by eating reduced-fat cookies," said professor Steven Davis, the study's principal investigator, in a statement. "It may be better than eating full-fat cookies, but if you really want to lose weight, you probably need to avoid cookies altogether."

BEFORE YOU GO

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Natural Gas Not The Solution To Reducing Emissions, Study Says

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Natural gas may not be a "bridge fuel" to a clean energy future, according to a new study by researchers at UC Irvine, Stanford University and the energy nonprofit Near Zero. Their modeling found that having a high supply of natural gas does little to reduce greenhouse gas emissions in the future, largely because the use of gas slowed the transition to renewables.

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question 39-

The main causes of increased methane emissions are human activities. Methane is emitted through natural sources like wetlands. Human activities that produce a lot of methane are leakages from natural gas systems and the raising of livestock.

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Gases that trap heat in the atmosphere are called greenhouse gases. This section provides information on emissions and removals of the main greenhouse gases to and from the atmosphere. For more information on the other climate forcers, such as black carbon, please visit the Climate Change Indicators: Climate Forcing page.

: Carbon dioxide enters the atmosphere through burning fossil fuels (coal, natural gas, and oil), solid waste, trees and wood products, and also as a result of certain chemical reactions (e.g., manufacture of cement). Carbon dioxide is removed from the atmosphere (or "sequestered") when it is absorbed by plants as part of the biological carbon cycle.

: Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.

: Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.

: Hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for stratospheric ozone-depleting substances (e.g., chlorofluorocarbons, hydrochlorofluorocarbons, and halons). These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases ("High GWP gases").

Each gas's effect on climate change depends on three main factors:

How much of these gases are in the atmosphere?

Concentration, or abundance, is the amount of a particular gas in the air. Larger emissions of greenhouse gases lead to higher concentrations in the atmosphere. Greenhouse gas concentrations are measured in parts per million, parts per billion, and even parts per trillion. One part per million is equivalent to one drop of water diluted into about 13 gallons of liquid (roughly the fuel tank of a compact car). To learn more about the increasing concentrations of greenhouse gases in the atmosphere, visit the Climate Change Indicators: Atmospheric Concentrations of Greenhouse Gases page.

How long do they stay in the atmosphere?

Each of these gases can remain in the atmosphere for different amounts of time, ranging from a few years to thousands of years. All of these gases remain in the atmosphere long enough to become well mixed, meaning that the amount that is measured in the atmosphere is roughly the same all over the world, regardless of the source of the emissions.

How strongly do they impact the atmosphere?

Some gases are more effective than others at making the planet warmer and "thickening the Earth's blanket."

For each greenhouse gas, a Global Warming Potential (GWP) has been calculated to reflect how long it remains in the atmosphere, on average, and how strongly it absorbs energy. Gases with a higher GWP absorb more energy, per pound, than gases with a lower GWP, and thus contribute more to warming Earth.

Note: All emission estimates are from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2016.

Carbon Dioxide Emissions

Carbon dioxide (CO ₂) is the primary greenhouse gas emitted through human activities. In 2016, CO ₂ accounted for about 81.6% of all U.S. greenhouse gas emissions from human activities. Carbon dioxide is naturally present in the atmosphere as part of the Earth's carbon cycle (the natural circulation of carbon among the atmosphere, oceans, soil, plants, and animals). Human activities are altering the carbon cycle-both by adding more CO ₂ to the atmosphere and by influencing the ability of natural sinks, like forests, to remove CO ₂ from the atmosphere. While CO ₂ emissions come from a variety of natural sources, human-related emissions are responsible for the increase that has occurred in the atmosphere since the industrial revolution.

The main human activity that emits CO ₂ is the combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation, although certain industrial processes and land-use changes also emit CO ₂. The main sources of CO ₂ emissions in the United States are described below.

Electricity. Electricity is a significant source of energy in the United States and is used to power homes, business, and industry. In 2016 the combustion of fossil fuels to generate electricity was the largest single source of CO₂ emissions in the nation, accounting for about 34 percent of total U.S. CO₂ emissions and 28 percent of total U.S. greenhouse gas emissions. The type of fossil fuel used to generate electricity will emit different amounts of CO₂. To produce a given amount of electricity, burning coal will produce more CO2 than oil or natural gas.

Transportation. The combustion of fossil fuels such as gasoline and diesel to transport people and goods was the second largest source of CO₂ emissions in 2016, accounting for about 34 percent of total U.S. CO₂ emissions and 27 percent of total U.S. greenhouse gas emissions. This category includes transportation sources such as highway vehicles, air travel, marine transportation, and rail.

Many industrial processes emit CO₂ through fossil fuel combustion. Several processes also produce CO₂ emissions through chemical reactions that do not involve combustion; for example, the production and consumption of mineral products such as cement, the production of metals such as iron and steel, and the production of chemicals. Fossil fuel combustion from various industrial processes accounted for about 15 percent of total U.S. CO₂ emissions and 12 percent of total U.S. greenhouse gas emissions in 2016. Note that many industrial processes also use electricity and therefore indirectly cause the emissions from the electricity production.

Carbon dioxide is constantly being exchanged among the atmosphere, ocean, and land surface as it is both produced and absorbed by many microorganisms, plants, and animals. However, emissions and removal of CO ₂ by these natural processes tend to balance. Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding CO ₂ and other heat-trapping gases to the atmosphere.

In the United States, since 1990, the management of forests and other land has acted as a net sink of CO ₂, which means that more CO ₂ is removed from the atmosphere, and stored in plants and trees, than is emitted. This carbon sink offset is about 12 percent of total emissions in 2016 and is discussed in more detail in the Land Use, Land-Use Change, and Forestry section.

To find out more about the role of CO ₂ warming the atmosphere and its sources, visit the Climate Change Indicators page.

Emissions and Trends

Carbon dioxide emissions in the United States increased by about 4 percent between 1990 and 2016. Since the combustion of fossil fuel is the largest source of greenhouse gas emissions in the United States, changes in emissions from fossil fuel combustion have historically been the dominant factor affecting total U.S. emission trends. Changes in CO ₂ emissions from fossil fuel combustion are influenced by many long-term and short-term factors, including population growth, economic growth, changing energy prices, new technologies, changing behavior, and seasonal temperatures. Between 1990 and 2016, the increase in CO ₂ emissions corresponded with increased energy use by an expanding economy and population, an overall growth in emissions from electricity generation, and increased demand for travel.

Reducing Carbon Dioxide Emissions

The most effective way to reduce CO emissions is to reduce fossil fuel consumption. Many strategies for reducing CO ₂ emissions from energy are cross-cutting and apply to homes, businesses, industry, and transportation.

EPA is taking common sense regulatory actions to reduce greenhouse gas emissions.

* Atmospheric CO is part of the global carbon cycle, and therefore its fate is a complex function of geochemical and biological processes. Some of the excess carbon dioxide will be 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.

References

¹ NRC (2010). Advancing the Science of Climate Change. Exit National Research Council. The National Academies Press, Washington, DC, USA.

Methane Emissions

In 2016, methane (CH ₄) accounted for about 10 percent of all U.S. greenhouse gas emissions from human activities. Human activities emitting methane include leaks from natural gas systems and the raising of livestock. Methane is also emitted by natural sources such as natural wetlands. In addition, natural processes in soil and chemical reactions in the atmosphere help remove CH ₄ from the atmosphere. Methane's lifetime in the atmosphere is much shorter than carbon dioxide (CO ₂), but CH ₄ is more efficient at trapping radiation than CO ₂. Pound for pound, the comparative impact of CH ₄ is more than 25 times greater than CO ₂ over a 100-year period.

Globally, over 60 percent of total CH ₄ emissions come from human activities. Methane is emitted from industry, agriculture, and waste management activities, described below.

Energy and Industry. Natural gas and petroleum systems are the largest source of CH₄ emissions in the United States. Methane is the primary component of natural gas. Methane is emitted to the atmosphere during the production, processing, storage, transmission, and distribution of natural gas. Because gas is often found alongside petroleum, the production, refinement, transportation, and storage of crude oil is also a source of CH₄ emissions. For more information, see the Inventory of U.S. Greenhouse Gas Emissions and Sinks sections on Natural Gas Systems and Petroleum Systems.

Agriculture. Domestic livestock such as cattle, buffalo, sheep, and goats produce large amounts of CH₄ as part of their normal digestive process. Also, when animals' manure is stored or managed in lagoons or holding tanks, CH₄ is produced. Because humans raise these animals for food and other products, the emissions are considered human-related. When livestock and manure emissions are combined, the Agriculture sector is the primary source of CH₄ emissions. For more information, see the Inventory of U.S. Greenhouse Gas Emissions and Sinks Agriculture chapter.

Waste from Homes and Businesses. Methane is generated in landfills as waste decomposes and in the treatment of wastewater. Landfills are the third largest source of CH₄ emissions in the United States. For more information, see the Inventory of U.S. Greenhouse Gas Emissions and Sinks Waste chapter.

Methane is also emitted from a number of natural sources. Natural wetlands are the largest source, emitting CH ₄ from bacteria that decompose organic materials in the absence of oxygen. Smaller sources include termites, oceans, sediments, volcanoes, and wildfires.

To find out more about the role of CH ₄ in warming the atmosphere and its sources, visit the Climate Change Indicators page.

Emissions and Trends

Methane emissions in the United States decreased by 16 percent between 1990 and 2016. During this time period, emissions increased from sources associated with agricultural activities, while emissions decreased from sources associated with landfills, coal mining, and the exploration through distribution of natural gas and petroleum products.

Reducing Methane Emissions

There are a number of ways to reduce CH4 emissions. Some examples are discussed below. EPA has a series of voluntary programs for reducing CH ₄ emissions, in addition to regulatory initiatives. EPA also supports the Global Methane Initiative Exit, an international partnership encouraging global methane reduction strategies.

References

IPCC (2007) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press. Cambridge, United Kingdom 996 pp.

² EPA (2010). Methane and Nitrous Oxide Emissions from Natural Sources. U.S. Environmental Protection Agency, Washington, DC, USA.

Nitrous Oxide Emissions

In 2016, nitrous oxide (N ₂O) accounted for about 6 percent of all U.S. greenhouse gas emissions from human activities. Human activities such as agriculture, fuel combustion, wastewater management, and industrial processes are increasing the amount of N ₂O in the atmosphere. Nitrous oxide is also naturally present in the atmosphere as part of the Earth's nitrogen cycle, and has a variety of natural sources. Nitrous oxide molecules stay in the atmosphere for an average of 114 years before being removed by a sink or destroyed through chemical reactions. The impact of 1 pound of N ₂ O on warming the atmosphere is almost 300 times that of 1 pound of carbon dioxide.

Globally, about 40 percent of total N ₂ O emissions come from human activities. Nitrous oxide is emitted from agriculture, transportation, and industry activities, described below.

Agriculture. Nitrous oxide can result from various agricultural soil management activities, such as synthe t ic and organic fertilizer application and other cropping practices, the management of manure, or burning of agricultural residues. Agricultural soil management is the largest source of N₂O emissions in the United States, accounting for about 77 percent of total U.S. N₂O emissions in 2016.

Fuel Combustion. Nitrous oxide is emitted when fuels are burned. The amount of N₂O emitted from burning fuels depends on the type of fuel and combustion technology, maintenance, and operating practices.

Industry. Nitrous oxide is generated as a byproduct during the production of nitric acid, which is used to make synthetic commercial fertilizer, and in the production of adipic acid, which is used to make fibers, like nylon, and other synthetic products.

Nitrous oxide emissions occur naturally through many sources associated with the nitrogen cycle, which is the natural circulation of nitrogen among the atmosphere, plants, animals, and microorganisms that live in soil and water. Nitrogen takes on a variety of chemical forms throughout the nitrogen cycle, including N ₂O. Natural emissions of N ₂ O are mainly from bacteria breaking down nitrogen in soils and the oceans. Nitrous oxide is removed from the atmosphere when it is absorbed by certain types of bacteria or destroyed by ultraviolet radiation or chemical reactions.

To find out more about the sources of N ₂O and its role in warming the atmosphere, visit the Climate Change Indicators page.

Emissions and Trends

Nitrous oxide emissions in the United States have increased by about 4 percent between 1990 and 2016. This increase in emissions is due in part to an increase in emissions from mobile combustion resulting from emission control standards for on-road vehicles. Nitrous oxide emissions from agricultural soils have varied during this period and were about 13 percent higher in 2016 than in 1990.

Reducing Nitrous Oxide Emissions

There are a number of ways to reduce emissions of N ₂ O, discussed below.

References

¹ IPCC (2007) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press. Cambridge, United Kingdom 996 pp.
² EPA (2010). Methane and Nitrous Oxide Emissions from Natural Sources. U.S. Environmental Protection Agency, Washington, DC, USA.
³ EPA (2005). Greenhouse Gas Mitigation Potential in U.S. Forestry and Agriculture. U.S. Environmental Protection Agency, Washington, DC, USA.

Emissions of Fluorinated Gases

Unlike many other greenhouse gases, fluorinated gases have no natural sources and only come from human-related activities. They are emitted through a variety of industrial processes such as aluminum and semiconductor manufacturing. Many fluorinated gases have very high global warming potentials (GWPs) relative to other greenhouse gases, so small atmospheric concentrations can have proportionately large effects on global temperatures. They can also have long atmospheric lifetimes-in some cases, lasting thousands of years. Like other long-lived greenhouse gases, most fluorinated gases are well-mixed in the atmosphere, spreading around the world after they are emitted. Many fluorinated gases are removed from the atmosphere only when they are destroyed by sunlight in the far upper atmosphere. In general, fluorinated gases are the most potent and longest lasting type of greenhouse gases emitted by human activities.

There are four main categories of fluorinated gases-hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF ₆), and nitrogen trifluoride (NF ₃). The largest sources of fluorinated gas emissions are described below.

Substitution for Ozone-Depleting Substances. Hydrofluorocarbons are used as refrigerants, aerosol propellants, foam blowing agents, solvents, and fire retardants. The major emissions source of these compounds is their use as refrigerants-for example, in air conditioning systems in both vehicles and buildings. These chemicals were developed as a replacement for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) because they do not deplete the stratospheric ozone layer. Chlorofluorocarbons and HCFCs are being phased out under an international agreement, called the Montreal Protocol. HFCs are potent greenhouse gases with high GWPs, and they are released into the atmosphere during manufacturing processes and through leaks, servicing, and disposal of equipment in which they are used. The Kigali Amendment to the Montreal Protocol calls for a phasedown in the production and consumption of the most potent HFCs. Newly developed hydrofluoroolefins (HFOs) are a subset of HFCs and are characterized by short atmospheric lifetimes and low GWPs. HFOs are currently being introduced as refrigerants, aerosol propellants and foam blowing agents.

Industry. Perfluorocarbons are compounds produced as a byproduct of various industrial processes associated with aluminum production and the manufacturing of semiconductors. PFCs generally have long atmospheric lifetimes and high GWPs. Sulfur hexafluoride is used in magnesium processing and semiconductor manufacturing, as well as a tracer gas for leak detection. HFC-23 is produced as a byproduct of HCFC-22 production.

Transmission and Distribution of Electricity. Sulfur hexafluoride is used in electrical transmission equipment, including circuit breakers. The GWP of SF₆ is 22,800, making it the most potent greenhouse gas that the Intergovernmental Panel on Climate Change has evaluated.

To find out more about the role of fluorinated gases in warming the atmosphere and their sources, visit the Climate Change Indicators page.

Emissions and Trends

Overall, fluorinated gas emissions in the United States have increased by about 74 percent between 1990 and 2016. This increase has been driven by a 249 percent increase in emissions of hydrofluorocarbons (HFCs) since 1990, as they have been widely used as a substitute for ozone-depleting substances. Emissions of perfluorocarbons (PFCs) and sulfur hexafluoride (SF ₆) have actually declined during this time due to emission reduction efforts in the aluminum production industry (PFCs) and the electricity transmission and distribution industry (SF ₆).

Reducing Fluorinated Gas Emissions

Because most fluorinated gases have a very long atmospheric lifetime, it will take many years to see a noticeable decline in current concentrations. However, there are a number of ways to reduce emissions of fluorinated gases, described below.

References

¹ IPCC (2007) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press. Cambridge, United Kingdom 996 pp.

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Gases that trap heat in the atmosphere are called greenhouse gases. This section provides information on emissions and removals of the main greenhouse gases to and from the atmosphere. For more information on the other climate forcers, such as black carbon, please visit the Climate Change Indicators: Climate Forcing page.

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Brainstorm, predict, discuss, collaborate, create...

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What renewable resources can reduce methane emissions?

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What are the main causes of increased methane emissions?

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Explain methane’s role in driving climate change.

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Explain the implications of the melting permafrost region.

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FOCUS QUESTIONS 37-40