Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. In 2010, CO2 accounted for about 84% 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 CO2 to the atmosphere and by influencing the ability of natural sinks, like forests, to remove CO2 from the atmosphere. While CO2 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. [1]

U.S. Carbon Dioxide Emissions, By Source
Pie chart that shows emissions by use. 39 percent is electricity, 32 percent is transportation, 13 percent is industry, 10 percent is residential and commercial, and 6 percent is other (non-fossil fuel combustion).

Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2010.

The main human activity that emits CO2 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 CO2. The main sources of CO2emissions 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. The combustion of fossil fuels to generate electricity is the largest single source of CO2 emissions in the nation, accounting for about 40% of total U.S. CO2emissions and 33% of total U.S. greenhouse gas emissions in 2009. 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 CO2than oil or natural gas.
  • Transportation. The combustion of fossil fuels. such as gasoline and diesel to transport people and goods is the second largest source of CO2 emissions, accounting for about 31% of total U.S. CO2 emissions and 26% of total U.S. greenhouse gas emissions in 2010. This category includes transportation sources such as highway vehicles, air travel, marine transportation, and rail.
  • Industry. 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. Various industrial processes accounted for about 14% of total U.S. CO2 emissions and 20% of total U.S. greenhouse gas emissions in 2010. 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 CO2 by these natural processes tend to balance. Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding CO2 and other heat-trapping gases to the atmosphere.

In the United States, since 1990, the management of forests and non-agricultural land has acted as a net sink of CO2, which means that more CO2 is removed from the atmosphere, and stored in plants and trees, than is emitted. This sink offset about 15% of total emissions in 2010 and is discussed in more detail in the Land Use and Forestry section.

To find out more about the role of CO2 warming the atmosphere and its sources, visit the Causes of Climate Changepage and the Greenhouse Gas Indicators page in the Science section.

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Carbon dioxide (CO2) emissions in the United States increased by about 12% between 1990 and 2010. 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 CO2 emissions from fossil fuel combustion are influenced by many factors, including population growth, economic growth, changing energy prices, new technologies, changing behavior, and seasonal temperatures. Between 1990 and 2010, the increase in CO2 emissions corresponded with increased energy use by an expanding economy and population, although the economic downturn starting in 2008 influenced the decrease in emissions in 2009. Transportation emissions also contributed to the 12% increase, largely due to an increase in miles traveled by motor vehicles.

U.S. Carbon Dioxide Gas Emissions, 1990-2010
Line graph that shows the U.S. carbon dioxide emissions from 1990 to 2009. In 1990 carbon dioxide emissions started around 5,000 million metric tons. The emissions rose to about 6,000 million metric tons in 2000 where it remained until about 2006 when it began to decline. By 2009, the carbon dioxide emissions were at about 5,500 million metric tons.

Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2010.