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Human-Related Sources and Sinks of Carbon Dioxide

Fossil Fuel Combustion | Industrial Process and Products Uses | Carbon Sequestration | Geologic Sequestration

A variety of human activities lead to the emission (sources) and removal (sinks) of carbon dioxide (CO2):

The figure below displays a breakdown of sources of CO2 emissions in the U.S. in 2006. By far the largest source is fossil fuel combustion:

Figure 1: 2006 Sources of CO2 Emissions. This figure illustrates sources of CO2 emissions for 2006. Fossil fuel combustion is by far the largest source of CO2 emissions, with 5,637.9 Tg CO2 Eq. The next largest source is non energy use of fuels, which accounts for 138 Tg CO2 Eq. The smallest source is Silicon Carbide Production and Consumption, which accounts for 0.2 Tg CO2 Eq. There is also a pie chart indicating that CO2 represents the majority, 84.8%, of all greenhouse gas emissions in 2006.
Source: U.S. Greenhouse Gas Emissions Inventory (y-axis units are teragrams of CO2 equivalent)

For additional information on CO2 emission sources and trends in the U.S., see the U.S. Greenhouse Gas Emissions Inventory.

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Fossil Fuel Combustion

When fossil fuels are burned to produce energy the carbon stored in them is emitted almost entirely as CO2. The main fossil fuels burned by humans are petroleum (oil), natural gas and coal. CO2 is emitted by the burning of fossil fuels for electricity generation, industrial uses, transportation, as well as in homes and commercial buildings. In 2006, petroleum supplied the largest share of domestic energy demands, accounting for an average of 47 percent of total fossil-fuel-based energy consumption in 2006. Coal and natural gas followed in order of importance, accounting for 27 and 26 percent of total fossil fuel consumption, respectively. The figure below displays emissions for each of these sectors, by fuel type in 2006.

Figure 2: 2006 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type. This figure illustrates 2006 CO2 Emissions from the Fossil Fuel Combustion by Sector and Fuel Type using the data presented in Table 3-3. It is apparent that electricity generation, composed mostly of emissions from coal, and transportation, composed mostly of emissions from petroleum, are the largest contributors to CO2 emissions from fossil fuel combustion. In addition, there is a pie chart that indicates that petroleum accounted for 43%, coal accounted for 37%, and natural gas accounted for 20% of emissions from fossil fuel combustion.
Source: U.S. Greenhouse Gas Emissions Inventory (y-axis units are teragrams of CO2 equivalent)

Electricity Generation: The process of generating electricity is the single largest source of CO2 emissions in the United States, representing 41 percent of all CO2 emissions. The electric power industry includes all power producers - both regulated utilities and other entities (e.g., independent power producers, cogenerators, etc.). Total national emissions in the U.S. depend upon the amount of electricity generated and the mix of fuels used to produce the electricity. For example, increases and decreases in the share of electricity generated by burning coal can affect total national emissions. Emissions from electricity generation can be reduced by:

Additional information on emissions from electricity generation in the U.S. is available in the U.S. Greenhouse Gas inventory. For information on emissions of multiple gases and pollutants from power plants, please see the following web sites:

Industry: The industrial sector engages in activities such as manufacturing, construction and mining. Within manufacturing, six industries – petroleum refining, chemical production, primary metal production, paper, food, and mineral production – represent the majority of energy use. Industry consumes significant amounts of electricity, but in the national inventory, only direct onsite CO2 emissions are allocated to this sector. Since 1990, industrial output in the United States has grown significantly, but CO2 emissions experienced only a modest increase.

Residential and Commercial: The residential and commercial sectors are heavily reliant on electricity for meeting their energy needs, particularly for lighting, heating, air conditioning and appliances. The main source of direct CO2 emissions is the burning of natural gas and oil for heating and cooling of buildings. For more information on how to improve energy efficiency in the residential and commercial sectors, please see EPA’s ENERGY STAR program.

Transportation: The transportation sector is the second largest source of CO2 emissions in the U.S. Almost all of the energy consumed in the transportation sector is petroleum based, including gasoline, diesel and jet fuel. Automobiles and light-duty trucks account for almost two-thirds of emissions from the transportation sector and emissions have steadily grown since 1990. Other sources of transportation emissions are freight trucks, aircraft, trains and boats.

Emissions from transportation depend on the number of trips or miles traveled by each type of vehicle each year, which are in turn influenced by larger economic trends and consumer behavior. Over the long term, changes in the fuel efficiency of vehicles (e.g., mileage), and in the type of fuel used can also influence the level of emissions.

EPA has developed a number of programs designed to lower the impact of automobile emissions on the environment:

More information on these and other transportation programs is available at EPA’s Transportation and Air Quality site.

The U.S. Department of Energy Hydrogen Program works in partnership with industry, academia, national laboratories, federal and international agencies to overcome technical barriers, address safety concerns and demonstrate fuel cell technologies in various applications, including the transportation sector.

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Industrial Process and Products Uses

Carbon dioxide emissions are produced as a by-product of various non-energy related industrial activities and product uses. For example, during the production of cement, raw materials such as naturally occurring calcium carbonate are chemically transformed, producing CO2 as a byproduct. There are also a large number of ways petroleum based products are used for purposes other than energy production that can lead to CO2 emissions. Petroleum products are used in plastics, solvents, and lubricants that may evaporate, dissolve, or wear out over time. There are four main types of industrial process CO2 emissions:

Industrial processes can also lead to emissions of methane, nitrous oxide, and fluorinated gases. More information on all emissions from industrial processes and product uses is available in the U.S. Greenhouse Gas Inventory.

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Carbon Sequestration

Carbon sequestration is the process through which plant life removes CO2 from the atmosphere and stores it in biomass. Over the course of a year, plants remove and release CO2 and net sequestration results if the rate of removal is higher than the rate of release. Young, fast-growing trees in particular will remove more carbon dioxide from the atmosphere than they will release. Agricultural and forestry practices can enhance the rate of carbon sequestration, or cause net emissions, depending on the overall balance. The term “sink” is a broader term used to describe agricultural and forestry lands or other processes that absorb or sequester CO2, and other chemical processes that remove other greenhouse gases from the atmosphere (e.g., methane).

All land areas such as farms, grasslands and forests can be sources or sinks of CO2, depending on the particular agricultural and forestry practices on these lands. In the U.S., forests and other types of lands have been significant sinks since 1990, due in large part to forest and soil management practices. Nationally, carbon sequestration offset or removed 13 percent of total greenhouse gas emissions in 2006. The largest share came from forest growth, increasing forest area and an increase in the amount of carbon stored in durable wood products. The rate of carbon sequestration has decreased since 1990, particularly in forests. The following links provide more information on carbon sequestration:

Deforestation

Permanent removal of standing forests ("deforestation") Exit EPA Disclaimer leads to CO2 emissions because the carbon sequestered in trees is emitted to the atmosphere and not counter-balanced by re-growth of new trees. Typically, CO2 is either emitted quickly through burning or slowly through decomposition over time. Deforestation is a significant source of carbon dioxide emissions globally, but a minor source in the U.S.

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Geologic Sequestration

Geologic sequestration refers to a chain of activities that result in collection and transport of concentrated CO2 gas from large emission sources, such as power plants, and subsequent injection into deep underground reservoirs. Currently, carbon storage takes place mainly at oil and gas production facilities, but storage in other types of reservoirs may increase in the future as technologies continue to develop. For a more detailed description of carbon capture and storage, please visit the geologic sequestration page, and the related links below:

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