Carbon dioxide (CO2)

Carbon dioxide (CO₂) is the primary greenhouse gas emitted through human activities. In 2011, 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. 

U.S. Carbon Dioxide Emissions, By Source

Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011.

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 CO₂ 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. The combustion of fossil fuels to generate electricity is the largest single source of CO₂ emissions in the nation, accounting for about 38% of total U.S. CO₂ emissions and 32% of total U.S. greenhouse gas emissions in 2011. 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 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 2011. 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. Fossil fuel combustion from various industrial processes accounted for about 14% of total U.S. CO₂ emissions and 12% of total U.S. greenhouse gas emissions in 2011. 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 14% of total emissions in 2011 and is discussed in more detail in the Land

Emissions and Trends

Carbon dioxide (CO₂) emissions in the United States increased by about 10% between 1990 and 2011. 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 long-term and short-term factors, including population growth, economic growth, changing energy prices, new technologies, changing behavior, and seasonal temperatures. Between 1990 and 2011, the increase in CO₂ emissions corresponded with increased energy use by an expanding economy and population, and an overall growth in emissions from electricity generation. Transportation emissions also contributed to the 10% increase, largely due to an increase in miles traveled by motor vehicles.

U.S. Carbon Dioxide Gas Emissions, 1990-2011

Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011.

Going forward, CO₂ emissions in the United States are projected to grow by about 1.5% between 2005 and 2020. 

Reducing Carbon Dioxide Emissions

The most effective way to reduce carbon dioxide (CO₂) emissions is to reduce fossil fuel consumption. Many strategies for reducing CO₂ emissions from energy are cross-cutting and apply to homes, businesses, industry, and transportation.

Examples of Reduction Opportunities for Carbon Dioxide

Strategy	Examples of How Emissions Can be Reduced
Energy Efficiency	Improving the insulation of buildings, traveling in more fuel-efficient vehicles, and using more efficient electrical appliances are all ways to reduce energy consumption, and thus CO2 emissions.
Energy Conservation	Reducing personal energy use by turning off lights and electronics when not in use reduces electricity demand. Reducing distance traveled in vehicles reduces petroleum consumption. Both are ways to reduce energy CO2 emissions through conservation. Learn more about What You Can Do at Home, at School, in the Office, and on the Road to save energy and reduce your carbon footprint.
Fuel Switching	Producing more energy from renewable sources and using fuels with lower carbon contents are ways to reduce carbon emissions.
Carbon Capture and Sequestration	Carbon dioxide capture and sequestration is a set of technologies that can potentially greatly reduce CO2 emissions from new and existing coal- and gas-fired power plants, industrial processes, and other stationary sources of CO2. Learn more.