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 largest source of CO2 emissions globally is the combustion of fossil fuels such as coal, oil and gas in power plants, automobiles, industrial facilities and other sources.
- A number of specialized industrial production processes and product uses such as mineral production, metal production and the use of petroleum-based products can also lead to CO2 emissions.
- Carbon sequestration is the process by which growing trees and plants absorb or remove CO2 from the atmosphere and turn it into biomass (e.g., wood, leaves, etc.). Deforestation, conversely, can lead to significant levels of CO2 emissions in some countries.
- Carbon dioxide can be captured from power plants and industrial facilities before it is released into the atmosphere, and then injected deep underground.
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:
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.
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.
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:
- Increasing the share of electricity generated from low carbon fuel or renewable sources. EPA’s Clean Energy Programs are designed to help consumers improve their knowledge about their Clean Energy options by providing objective information, creating networks between the public and private sector and providing technical assistance.
- Lowering total electricity consumption by consumers through improvements in energy efficiency. For more information on how to improve energy efficiency, please see EPA’s ENERGY STAR program.
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:
- EPA’s Acid Rain Program is designed to minimize emissions of sulfur dioxide and nitrogen oxides within parts of the U.S. The Acid Rain program tracks emissions of these gases and CO2, and makes the data publicly available at its Data and Maps Web site.
- EPA’s Emissions and Generation Resource Integrated Database (EGRID) provides comprehensive information on the environmental characteristics of almost all electric power generated in the United States.
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.
- For more information on emissions from industry, please see the U.S. Greenhouse Gas Inventory (PDF, 68 pp., 454 kb, About PDF).
- A number of industrial companies have joined EPA's Climate Leaders program and taken on voluntary commitments to reduce their emissions of greenhouse gases.
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:
- The SmartWay Transport Partnership is a collaborative voluntary program between EPA and the freight industry that will increase the energy efficiency and energy security of our country while significantly reducing air pollution and greenhouse gas emissions.
- The Green Vehicle Guide helps consumers choose the cleanest and most efficient vehicles that meet their needs. The Guide rates cars and trucks according to their emissions and fuel economy performance and provides consumers with information on how to make environmentally-informed choices when purchasing vehicles.
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.
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:
- Production and consumption of mineral products such as cement, lime and soda ash
- Production of metals such as iron and steel, aluminum, zinc and lead
- Chemical production (e.g., ammonia, petrochemicals and titanium dioxide)
- Consumption of petroleum products in feedstocks and other end-uses
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.
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:
- The U.S. Greenhouse Gas Inventory (PDF, 58 pages, 6.2 MB, About PDF) provides additional information on the rates and trends in carbon sequestration since 1990.
- EPA’s Carbon Sequestration Site is a source of answers to frequently asked questions, analyses, and other tools and resources.
- The Intergovernmental Panel on Climate Change (IPCC) Special Report on Land Use, Land-Use Change and Forestry (PDF, 30 pp., 340 KB, About PDF) provides a comprehensive assessment of the scientific and technical state of understanding for carbon sequestration strategies.
- Permanent removal of standing forests ("deforestation") is a significant source of carbon dioxide emissions globally.
Deforestation
Permanent removal of standing forests ("deforestation") 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.
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:
- The Carbon Sequestration Leadership Forum (CSLF) is an international climate change initiative focused on the development of improved cost-effective technologies for the separation and capture of CO2 for its transport and long-term safe storage. The purpose of the CSLF is to make these technologies broadly available internationally; and to identify and address wider issues relating to carbon capture and storage. This could include promoting the appropriate technical, political, and regulatory environments for the development of such technology.
- The Department of Energy’s Fossil Energy program is developing a portfolio of technologies that can capture and permanently store greenhouse gases.
- The Department of Energy’s Regional Partnership program is creating a nationwide network of federal, state and private sector partnerships to determine the most suitable technologies, regulations, and infrastructure for future carbon capture, storage and sequestration in different areas of the country.