Sources of Greenhouse Gas Emissions
Electricity Sector Emissions
Total U.S. Greenhouse Gas Emissions by Economic Sector in 2013
The Electricity 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 Electricity Sector by Fuel Source
Coal combustion is generally more carbon intensive than burning natural gas or petroleum for electricity. Although coal accounts for about 77% of CO2 emissions from the sector, it represents about 39% of the electricity generated in the United States. About 27% of electricity generated in 2013 was generated using natural gas, although this percentage decreased relative to 2012. Petroleum accounts for less than 1% of electricity generation. The remaining generation comes from nuclear (about 19%) and renewable sources (about 13%), which includes hydroelectricity, biomass, wind, and solar. These other sources usually release fewer greenhouse gas emissions than fossil fuel combustion, if any emissions at all.
Emissions and Trends
In 2013, the electricity sector was the largest source of U.S. greenhouse gas emissions, accounting for about 31% of the U.S. total. Greenhouse gas emissions from electricity have increased by about 11% since 1990 as electricity demand has grown and fossil fuels have remained the dominant source for generation.
To learn about projected greenhouse gas emissions to 2020, visit the U.S. Climate Action Report 2014 (PDF) (310 pp., 23.1 MB).
Greenhouse Gas Emissions from Electricity
Total U.S. Greenhouse Gas Emissions by Sector with Electricity Distributed
Greenhouse Gas Emissions by Electricity End-Use
Electricity is consumed by other sectors--in homes, businesses, and factories. Therefore, it is possible to attribute the greenhouse gas emissions from electricity production 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 electricity are allocated to the end-use sector, industrial activities account for a much larger share of U.S. greenhouse gas emissions. Emissions from commercial and residential buildings also increase substantially when emissions from electricity are included, due to their relatively large share of electricity consumption (e.g., lighting and appliances).
Reducing Emissions from Electricity
There are a variety of opportunities to reduce greenhouse gas emissions associated with electricity generation, transmission, and distribution. The table shown below categorizes these opportunities and provides examples. For a more comprehensive list, see Chapter 7 of the Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (PDF) (139 pp, 7.5MB).
|Type||How Emissions are Reduced||Examples|
|Increased Efficiency of Power Plants and Fuel Switching||Increasing efficiency of existing power plants by using advanced technologies or substituting fuels that combust more efficiently.||
|Renewable Energy||Using renewable energy sources rather than fossil fuel to generate electricity.||Increasing the share of total electricity generated from wind, solar, hydro, and geothermal sources and from certain biofuel sources.|
|Increased Energy Efficiency (end-use)||Reducing energy demand by increasing efficiency and conservation in homes, businesses, and industry.||EPA's ENERGY STAR® partners removed over 300 million metric tons of greenhouse gases in 2014 alone, and saved consumers and businesses over $34 billion on their utility bills.|
|Nuclear Energy||Generating electricity from nuclear processes rather than the combustion of fossil fuels.||Building nuclear power plants as fossil fuel power plants are retired.|
|Carbon Capture Sequestration and Storage (CCS)||Capturing CO2 as a by-product of fossil fuel combustion before it enters the atmosphere and then transferring the CO2 to a long-term storage area, such as an underground geologic formation.||Capturing CO2 from the stacks of a coal-fired power plant, and then transferring the CO2 via pipeline to a nearby abandoned oil field where the CO2 is injected underground. Learn more about CCS.|
6,822 million metric tons of CO2 equivalent--what does that mean?
An Explanation of Units
A million metric tons is equal to about 2.2 billion pounds, or 1 trillion grams. For comparison, a small car is likely to weigh a little more than 1 metric ton. Thus, a million metric tons is roughly the same mass as 1 million small cars!
The U.S. Inventory uses metric units for consistency and comparability with other countries. For reference, a metric ton is a little bit larger (about 10%) than a U.S. "short" ton.
GHG emissions are often measured in carbon dioxide (CO2) equivalent. To convert emissions of a gas into CO2 equivalent, its emissions are multiplied by the gas's Global Warming Potential (GWP). The GWP takes into account the fact that many gases are more effective at warming Earth than CO2, per unit mass.
The GWP values appearing in the Emissions webpages reflect the values used in the U.S. Inventory, which are drawn from the IPCC's Second Assessment Report (SAR). For further discussion of GWPs and an estimate of GHG emissions using updated GWPs, see Annex 6 of the U.S. Inventory and the IPCC's discussion on GWPs.