Sources of Greenhouse Gas Emissions | US EPA (2024)

Overview

Greenhouse gases trap heat and make the planet warmer. Human activities are responsible for almost all of the increase in greenhouse gases in the atmosphere over the last 150 years.¹ The largest source of greenhouse gas emissions from human activities in the United States is from burning fossil fuels for electricity, heat, and transportation.

EPA tracks total U.S. emissions by publishing the Inventory of U.S. Greenhouse Gas Emissions and Sinks. This annual report estimates the total national greenhouse gas emissions and removals associated with human activities across the United States by source, gas, and economic sector.

The primary sources of greenhouse gas emissions by economic sector in the United States are:

• Transportation (28% of 2021 greenhouse gas emissions) – The transportation sector generates the largest share of greenhouse gas emissions. Greenhouse gas emissions from transportation primarily come from burning fossil fuel for our cars, trucks, ships, trains, and planes. Over 94% of the fuel used for transportation is petroleum based, which includes primarily gasoline and diesel.2
• Electricity production (25% of 2021 greenhouse gas emissions) – Electric power generates the second largest share of greenhouse gas emissions and includes emissions from electricity production used by other end use sectors (e.g., industry). 79% of our electricity comes from burning fossil fuels, mostly coal and natural gas.3
• Industry (23% of 2021 greenhouse gas emissions) – Greenhouse gas emissions from industry primarily come from burning fossil fuels for energy, as well as greenhouse gas emissions from certain chemical reactions necessary to produce goods from raw materials. If emissions from electricity use are allocated to the industrial end-use sector, industrial activities account for a much larger share (30%) of U.S. greenhouse gas emissions.
• Commercial and Residential (13% of 2021 greenhouse gas emissions) – Greenhouse gas emissions from the commercial and residential sector include fossil fuels burned for heat and the use of gases for refrigeration and cooling in buildings, and non-building specific emissions such as the handling of waste. Greenhouse gas emissions from commercial and residential buildings also increase substantially when emissions from electricity end-use are included, due to the relatively large share of electricity use (e.g., heating, ventilation, and air conditioning; lighting; and appliances) in these sectors When emissions from electricity use are allocated to the commercial and residential end-use sector, commercial and residential activities account for a much larger share (30%) of U.S. greenhouse gas emissions.
• Agriculture(10% of 2021 greenhouse gas emissions) – Greenhouse gas emissions from agriculture come from livestock such as cows, agricultural soils, and rice production.
• Land Use and Forestry(offsets 12% of 2021 greenhouse gas emissions) – While not shown in the figure, land areas can act as a sink (absorbing CO₂ from the atmosphere) or a source of greenhouse gas emissions. In the United States, since 1990, managed forests and other lands are a net sink, i.e., they have absorbed more CO₂ from the atmosphere than they emit.

Trends

Since 1990, gross U.S. greenhouse gas emissions have decreased by just over 2%. From year to year, emissions can rise and fall due to changes in the economy, the price of fuel, and other factors. In 2021, U.S. greenhouse gas emissions increased 5% compared to 2020 levels. In 2020, there was a sharp decline in emissions largely due to the impacts of the coronavirus (COVID-19) pandemic on travel and other economic activity. In 2021, the increase in total greenhouse gas emissions was driven largely by an increase in CO₂ emissions from fossil fuel combustion due to economic activity rebounding after the height of the COVID-19 pandemic. In 2021, CO₂ emissions from fossil fuel combustion increased by 7% relative to the previous year. CO₂ emissions from natural gas consumption increased by less than 1 % relative to 2020. In a shift from recent trends, CO₂ emissions from coal consumption increased by 15% from 2020. The increase in natural gas consumption and emissions in 2021 is observed across all sectors except the Electric Power sector and U.S. Territories, while the coal increase is primarily in the Electric Power sector. Emissions from petroleum use also increased by 9% in 2021, and CO₂ emissions from fossil fuel combustion were 2% below emissions in 1990.

References

1. IPCC (2013) Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. [Stocker, T.F., D. Qin, G.K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp. .
2. IPCC (2022): Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [P.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. doi: 10.1017/9781009157926
3. U.S. Energy Information Administration (2022). Electricity Explained - Basics

Electric Power Sector Emissions

The Electricity power sector involves the generation, transmission, and distribution of electricity. Carbon dioxide (CO₂) makes up the vast majority of greenhouse gas emissions from the sector, but smaller amounts of methane (CH₄) and nitrous oxide (N₂O) are also emitted. These gases are released during the combustion of fossil fuels, such as coal, oil, and natural gas, to produce electricity. Less than 1% of greenhouse gas emissions from the sector come from sulfur hexafluoride (SF₆), an insulating chemical used in electricity transmission and distribution equipment.

Greenhouse Gas Emissions in the Electric Power Sector by Fuel Source

Coal combustion is more carbon-intensive than burning natural gas or petroleum for electric power production. Although coal use accounted for 59% of CO₂ emissions from the sector, it represented only 23% of the electricity generated in the United States in 2021. Natural gas use accounted for 37% of electricity generation in 2021, and petroleum use accounted for less than 1%. The remaining generation in 2021 came from non-fossil fuel sources, including nuclear (20%) and renewable energy sources (20%), which include hydroelectricity, biomass, wind, and solar.¹ Most of these non-fossil sources, such as nuclear, hydroelectric, wind, and solar, are non-emitting.

Trends

In 2021, the electric power sector was the second largest source of U.S. greenhouse gas emissions, accounting for 25% of the U.S. total. Electric power sector emissions increased 7% in 2021. Greenhouse gas emissions from electric power production have decreased by about 15% since 1990 due to a shift in generation to lower- and non-emitting sources of electricity generation and an increase in end-use energy efficiency.

Greenhouse Gas Emissions by Electricity End-Use

Electricity is used by other end-use sectors—in homes, businesses, and factories—and the greenhouse gas emissions from electricity generation can be allocated to the sectors that use the electricity. Looking at greenhouse gas emissions by end-use sector can help us understand energy demand across sectors and changes in energy use over time.

When emissions from electric power generation are allocated to the industrial end-use sector, industrial activities account for a much larger share of U.S. greenhouse gas emissions. Greenhouse gas emissions from commercial and residential buildings also increase substantially when emissions from electricity end-use are included, due to the relatively large share of electricity use (e.g., heating, ventilation, and air conditioning; lighting; and appliances) in these sectors. The transportation sector currently has a relatively low percentage of electricity use, but it is growing due to the use of electric and plug-in vehicles.

Reducing Emissions from Electric Power Production

There are a variety of opportunities to reduce greenhouse gas emissions associated with electric power production, transmission, and distribution. The table below categorizes these opportunities and provides examples. For a more comprehensive list, see Chapter 6 (PDF) (88 pp, 3.6MB) of the Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.²

References

1. U.S. Energy Information Administration (2022). Electricity Explained - Basics.
2. IPCC, 2022: Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [P.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. doi: 10.1017/9781009157926

Transportation Sector Emissions

The Transportation sector includes the movement of people and goods by cars, trucks, trains, ships, airplanes, and other vehicles. The majority of greenhouse gas emissions from transportation are carbon dioxide (CO₂) emissions resulting from the combustion of petroleum-based products, like gasoline and diesel fuel, in internal combustion engines. The largest sources of transportation-related greenhouse gas emissions include passenger cars, medium- and heavy-duty trucks, and light-duty trucks, including sport utility vehicles, pickup trucks, and minivans. These sources account for over half of the emissions from the transportation sector. The remaining greenhouse gas emissions from the transportation sector come from other modes of transportation, including commercial aircraft, ships, boats, and trains, as well as pipelines and lubricants.

Relatively small amounts of methane (CH₄) and nitrous oxide (N₂O) are emitted during fuel combustion. In addition, hydrofluorocarbon (HFC) emissions also occur from the Transportation sector. These emissions result from the use of mobile air conditioners and refrigerated transport.

Trends

In 2021, greenhouse gas emissions from transportation accounted for 28% of total U.S. greenhouse gas emissions, making it the largest contributor of U.S. greenhouse gas emissions. From 1990 to 2021, total transportation emissions from fossil fuel combustion increased by 19%. In 2021, emissions increased by 12%, which followed a decline of 13% in 2020 due to reduced travel demand during the height of the COVID-19 pandemic. The largest sources of transportation greenhouse gas emissions in 2021 were light-duty trucks, which include sport utility vehicles, pickup trucks, and minivans (37%); medium- and heavy-duty trucks (23%); passenger cars (21%); commercial aircraft (7%); other aircraft (2%); pipelines (4%); ships and boats (3%); and rail (2%). In terms of the overall trend, from 1990 to 2021, total transportation emissions have increased due, in large part, to increased demand for travel. The number of vehicle miles traveled (VMT) by light-duty motor vehicles (passenger cars and light-duty trucks) increased by 45% from 1990 to 2021, as a result of a confluence of factors including population growth, economic growth, urban sprawl, and periods of low fuel prices. Between 1990 and 2004, average fuel economy among new vehicles sold annually declined, as sales of light-duty trucks increased. Starting in 2005, average new vehicle fuel economy began to increase, while light-duty VMT grew only modestly for much of the period. Average new vehicle fuel economy has improved almost every year since 2005, slowing the rate of increase of CO₂ emissions.

Learn more about Greenhouse Gas Emissions from Transportation.

Reducing Emissions from Transportation

There are a variety of opportunities to reduce greenhouse gas emissions associated with transportation. The table shown below categorizes these opportunities and provides examples. For a more comprehensive list, see Chapter 10 of the Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.¹

References

1. IPCC, 2022: Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [P.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. doi: 10.1017/9781009157926

Industry Sector Emissions

The Industry sector produces the goods and raw materials we use every day. The greenhouse gases emitted during industrial production are split into two categories: direct emissions that are produced at the facility, and indirect emissions that occur off site but are associated with the facility's use of electricity.

Direct emissions are produced by burning fuel for power or heat, through chemical reactions, and from leaks from industrial processes or equipment. Most direct emissions come from the consumption of fossil fuels for energy. A smaller amount of direct emissions, roughly one third, come from leaks from natural gas and petroleum systems, the use of fuels in production (e.g., petroleum products used to make plastics), and chemical reactions during the production of chemicals, metals (e.g., iron and steel), and minerals (e.g., cement).

Indirect emissions are produced by burning fossil fuel at a power plant to make electricity, which is then used by an industrial facility to power industrial buildings and machinery.

More information about facility-level emissions from large industrial sources is available through EPA's Greenhouse Gas Reporting Program data publication tool. National-level information about emissions from industry as a whole can be found in the sections on Fossil Fuel Combustion and the Industrial Processes chapter in the Inventory of U.S. Greenhouse Gas Emissions and Sinks.

Trends

In 2021, direct industrial greenhouse gas emissions accounted for 23% of total U.S. greenhouse gas emissions, making it the third largest contributor to U.S. greenhouse gas emissions, after the Transportation and Electric Power sectors. From 2020 to 2021, total energy use in the industrial sector increased by just over 1%. Including both direct emissions and indirect emissions associated with electricity use, industry's share of total U.S. greenhouse gas emissions in 2021 was 30%, making it the largest contributor of greenhouse gas emissions of any sector. Total U.S. greenhouse gas emissions from industry, including electricity, have declined by 14% since 1990.

Reducing Emissions from Industry

There are a wide variety of industrial activities that cause greenhouse gas emissions, and many opportunities to reduce them. The table shown below provides some examples of opportunities for industry to reduce emissions. For a more comprehensive list, see Chapter 11 of the Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. ¹

References

1. IPCC, 2022: Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [P.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. doi: 10.1017/9781009157926

Commercial and Residential Sector Emissions

The Residential and Commercial sectors include all homes and commercial businesses (excluding agricultural and industrial activities). Greenhouse gas emissions from this sector come from direct emissions including fossil fuel combustion for heating and cooking needs, management of waste and wastewater, and leaks from refrigerants in homes and businesses, as well as indirect emissions that occur offsite but are associated with use of electricity by homes and businesses.

Direct emissions are produced from residential and commercial activities in a variety of ways:

• Combustion of natural gas and petroleum products for heating and cooking emits carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Emissions from natural gas consumption represent 80 % of the direct fossil fuel CO₂ emissions from the residential and commercial sector in 2021. Coal consumption is a minor component of energy use in both of these sectors.
• Organic waste sent to landfills emits CH₄.
• Wastewater treatment plants emit CH₄ and N₂O.
• Anaerobic digestion at biogas facilities emits CH₄.
• Fluorinated gases (mainly hydrofluorocarbons, or HFCs) used in air conditioning and refrigeration systems can be released during servicing or from leaking equipment.

Indirect emissions are produced by burning fossil fuel at a power plant to make electricity, which is then used in residential and commercial activities, such as lighting and for appliances.

Note: Residential and commercial sector emissions presented here do not necessarily represent the full suite of emissions related to buildings and the broader built environment. The commercial and residential sectors numbers don’t include any emissions/sinks from production of construction materials (e.g., upstream emissions from production of cement, emissions and sinks from land use changes, etc). Those would be reflected in other sectors. Also, as shown above, emissions from these sectors include other emissions that do not occur at the building site (i.e., landfills, etc.). Furthermore, energy use in these sectors may also include energy used for equipment (such as motor gasoline used for non-highway vehicles or lawn and garden equipment), exterior lighting, or construction. In addition, energy use for some large buildings, such as energy-intensive office buildings and factories with large onsite combustion, are typically included in industrial sector energy use.

More national-level information about emissions from the residential and commercial sector can be found in the U.S. Inventory's Trends in Greenhouse Gas Emissions and Energy chapters (Chapters 2 and 4 respectively).

Trends

In 2021, direct greenhouse gas emissions from homes and businesses accounted for 13% of total U.S. greenhouse gas emissions. Greenhouse gas emissions from homes and businesses vary from year to year, often correlated with annual fluctuations in energy use caused primarily by weather conditions. Total residential and commercial greenhouse gas emissions, including direct and indirect emissions, in 2021 have decreased by 2% since 1990. Greenhouse gas emissions from on-site direct emissions in homes and businesses have increased by 2% since 1990. Additionally, indirect emissions from electricity use by homes and businesses increased from 1990 to 2007 but have decreased since then to approximately 4% below 1990 levels in 2021.

Reducing Emissions from Homes and Businesses

The table shown below provides examples of opportunities to reduce emissions from homes and businesses. For a more comprehensive list of options and a detailed assessment of how each option affects different gases, see Chapter 9 and Chapter 12 of the Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.

Agriculture Sector Emissions

Agricultural activities — crop and livestock production — contribute to emissions in a variety of ways:

• Various management practices on agricultural soils can lead to increased availability of nitrogen in the soil and result in emissions of nitrous oxide (N₂O). Specific activities that contribute to N₂O emissions from agricultural lands include the application of synthetic and organic fertilizers, the growth of nitrogen-fixing crops, the drainage of organic soils, and irrigation practices. Management of agricultural soils accounts for just over half of the greenhouse gas emissions from the Agriculture sector.*
• Livestock, especially ruminants such as cattle, produce methane (CH₄) as part of their normal digestive processes. This process is called enteric fermentation, and it represents over a quarter of the greenhouse gas emissions from the Agriculture sector.
• The way in which manure from livestock is managed also contributes to CH₄ and N₂O emissions. Different manure treatment and storage methods affect how much of these greenhouse gases are produced. Manure management accounts for about 11% of the total greenhouse gas emissions from the Agriculture sector in the United States.
• Smaller sources of agricultural emissions include CO₂ from liming and urea application, CH₄ from rice cultivation, and burning crop residues, which produces CH₄ and N₂O.

More information about emissions from agriculture can be found in the agriculture chapter in the Inventory of U.S. Greenhouse Gas Emissions and Sinks.

* Management of croplands and grasslands can also lead to emissions or sequestration of carbon dioxide (CO₂). These emissions and removals are included under the Land Use, Land-Use Change, and Forestry sector.

Trends

In 2021, greenhouse gas emissions from the agriculture sector accounted for 10% of total U.S. greenhouse gas emissions. Greenhouse gas emissions from agriculture have increased by 7% since 1990. Agricultural soil management activities, such as application of synthetic and organic fertilizers, deposition of livestock manure, and growing nitrogen fixing plants, were the largest contributors to U.S. N₂O emissions in 2021, accounting for 75% of total N₂O emissions. Emissions from other agricultural sources have generally remained flat or changed by a relatively small amount since 1990.

Reducing Emissions from Agriculture

The table shown below provides examples of opportunities to reduce emissions from agriculture. For a more comprehensive list of options and a detailed assessment of how each option affects different gases, see Chapter 7 of the Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.

Land Use, Land-Use Change, and Forestry Sector Emissions and Sequestration

Plants absorb carbon dioxide (CO₂) from the atmosphere as they grow, and they store some of this carbon as perennial aboveground and belowground biomass throughout their lifetime. Soils and dead organic matter/litter can also store some of the carbon from these plants depending on how the soil is managed and other environmental conditions (e.g., climate). This storage of carbon in plants, dead organic matter/litter and soils is called biological carbon sequestration. Because biological sequestration takes CO₂ out of the atmosphere and stores it in these carbon pools, it is also called a carbon "sink."

Emissions or sequestration of CO₂, as well as emissions of CH₄ and N₂O, can occur from management of lands in their current use or as lands are converted to other land uses. Carbon dioxide is exchanged between the atmosphere and the plants and soils on land, for example, as cropland is converted into grassland, as lands are cultivated for crops, or as forests grow. In addition, using biological feedstocks (such as energy crops or wood) for purposes such as electricity generation, as inputs to processes that create liquid fuels, or as building materials can lead to emissions or sequestration.*

In the United States overall, Land Use, Land-Use Change, and Forestry (LULUCF) activities have resulted in more removal of CO₂ from the atmosphere than emissions. Because of this, the LULUCF sector in the United States is considered a net sink, rather than a source, of CO₂. In many areas of the world, the opposite is true, particularly in countries where large areas of forest land are cleared, often for conversion to agricultural purposes or for settlements. In these situations, the LULUCF sector can be a net source of greenhouse gas emissions.

• More national-level information about land use, land-use change, and forestry is available from the Land Use, Land-Use Change, and Forestry chapter in the Inventory of U.S. Greenhouse Gas Emissions and Sinks. For more information on emissions and sequestration from forest land and urban trees in settlement areas, see also the USDA's USFS Resource Bulletin.
• For more information about global emissions from land use and forestry activities, see EPA's Global Greenhouse Gas emissions page and the Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.

* Emissions and sequestration of CO₂ are presented under the Land Use, Land-Use Change, and Forestry sector in the Inventory. Emissions of methane (CH₄) and nitrous oxide (N₂O) also occur as a result of land use and management activities in the LULUCF sector. Other emissions from CH₄, and N₂O are also presented in the Energy sector.

Emissions and Trends

In 2021, the net CO₂ removed from the atmosphere from the LULUCF sector was 12% of total U.S. greenhouse gas emissions. Between 1990 and 2021, total carbon sequestration in the LULUCF sector decreased by 14%, primarily due to a decrease in the rate of net carbon accumulation in forests, as well as an increase in CO₂ emissions from urbanization. Additionally, while episodic in nature, increased CO₂, CH₄ and N₂O emissions from forest fires have also occurred over the time series.

Reducing Emissions and Enhancing Sinks from Land Use, Land-Use Change, and Forestry

In the LULUCF sector, opportunities exist to reduce greenhouse gas emissions and increase the potential to sequester carbon from the atmosphere by enhancing sinks. The table shown below provides some examples of opportunities for both reducing emissions and enhancing sinks. For a more comprehensive list, see Chapter 7 of the Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.