Greenhouse Gas Emissions
There are six main greenhouse gases released by human activity; carbon-dioxide (CO2), methane (NH4), nitrous-oxide (N2O), hydrofluorocarbons (HFC), perfluorocarbons (PFC), and sulfur-hexafluoride (SF6). Each gas is generated from different activities and each has a unique ability to affect climate, called its Global Warming Potential (Table 1). Global Warming Potential (GWP) is an index used to assess a gas’s impact on climate over a specific period of time, usually 100 years. GWP is measured relative to CO2, in “CO2 equivalents” (CO2-e). For instance, one metric ton of methane has an effect on climate equal to 21 tons of carbon-dioxide, giving methane a GWP of 21 CO2-e.
Table 1 – Common greenhouse gases, their human sources, and 100 year global warming potential (GWP). Table adapted from EPA, 20071.
| Greenhouse Gas |
Human Sources |
GWP (100 yr) |
| Carbon Dioxide |
Fossil-fuel combustion, Land-use conversion, Cement Production |
1 |
| Methane |
Fossil fuel production and combustion, Agriculture, Waste decomposition |
21 |
| Nitrous Oxide |
Fertilizer, Industrial processes, Fossil fuel combustion |
310 |
| Hydrofluorocarbons |
Refrigerants |
140 – 11,700 |
| Perfluorocarbons |
Aluminum smelting, Semiconductor manufacturing |
6,500 – 9,200 |
| Sulfur Hexafluoride |
Dielectric fluid used in electrical equipment |
23,900 |
In the United States, the primary greenhouse gas emitted from human activities is CO2, followed by methane, nitrous-oxide, and the fluorinated artificial gases (Figure 1). Electric power represents the largest source of GHGs, generating over 33% of total emissions. Transportation accounts for nearly 28% of annual emissions. The remaining emissions come from agricultural, commercial, and residential sectors, representing 8.2%, 5.9%, and 5.2% respectively (Figure 2) 2.
Figure 1 – Breakdown of U.S. greenhouse gas emissions by gas in 2005.
Figure 2 – U.S. greenhouse gas emissions by economic sector in 2005.
In addition to generating a majority of annual greenhouse gas emissions, electricity generation releases a number of pollutants harmful to humans and the environment. Each year electricity generation produces 70% of sulphur-dioxide (SO2) emissions3, 20% of nitrogen-oxide (NOX) emissions4 , and 40% of mercury (Hg) emissions5 ; pollutants responsible for acid rain, smog, and neurological disorders, respectively. The rate at which these emissions are created is primarily driven by the fuel mix used. The higher the percentage of fossil fuel in the fuel mix, the greater the volume of greenhouse gas produced. The fuel mix currently used to generate electricity in the U.S. is dominated by fossil fuels, particularly coal (Figure 3).
Figure 3 – Fuel mix of the U.S. electricricity generation in 2004.
Because emissions are so closely linked to fuel mix, emission rates (lbs or tons/MWh) vary across the U.S.. For instance, areas with large hydroelectric resources, like the Pacific Northwest, or high nuclear capacity, like the Northeast, have low emission rates. Regions with a greater dependence on coal generation, like the Great Plains and Mid-West, generate more CO2 per Megawatt hour (Figure 4).
Figure 4 – Emission rates of CO2 from electricity generation in the U.S. Emission data and region designations based on eGRID2006 Version 2.1, April 2007.
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1 Global Warming Potential table adapted from the EPA “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2005,” http://www.epa.gov/climatechange/emissions/usinventoryreport.html
2 All 2005 greenhouse gas emissions statistics quoted on this page are from the EPA “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2005,” http://www.epa.gov/climatechange/emissions/usinventoryreport.html
3 SO2 emissions data from EPA “National Emissions Inventory (NEI) Air Pollutant Emissions Trends Data,” http://www.epa.gov/ttn/chief/trends/
4 NOX emissions data from EPA “National Emissions Inventory (NEI) Air Pollutant Emissions Trends Data,” http://www.epa.gov/ttn/chief/trends/
5 Mercury emissions data from EPA, http://www.epa.gov/mercury/