What are greenhouse gases and how much are emitted by the United States?

Greenhouse gases trap heat from the sun and warm the planet's surface. Of U.S. greenhouse gas emissions, the majority are related to energy consumption, and most of those are carbon dioxide. From 1990 to 2011, energy-related carbon dioxide emissions in the United States increased by about 0.4% per year. The United States produced about 18% of the world's total energy-related carbon dioxide in 2010 — the last year for which comparable data are available.

Because greenhouse gases trap radiation (heat) from the sun and warm the planet's surface, a certain amount of these gases is beneficial (see "Did You Know?"). But as concentrations of these gases increase due to human activity, more warming occurs than would happen naturally. According to the U.S. Environmental Protection Agency, about 6.8 billion metric tons carbon dioxide equivalent (CO2e) of greenhouse gases were emitted by the United States in 2010 (the last year the full inventory is available).1 Other countries with significant emissions include China, the countries of Europe, Russia, and Japan.

The major greenhouse gases the United States emits as a result of human activity and that are included in U.S. and international emissions estimates are:

Carbon dioxide (CO2) Methane (CH4)Nitrous oxide (N2O) High-GWP gases, which are: Hydrofluorocarbons (HFCs) Perfluorocarbons (PFCs) Sulfur hexafluoride (SF6)

There are other greenhouse gases that are not counted in U.S. or international greenhouse gas inventories:

Water vapor is the most abundant greenhouse gas, but most scientists believe that water vapor produced directly by human activity contributes very little to the amount of water vapor in the atmosphere, and therefore EIA does not estimate emissions of water vapor. Research by NASA suggests a stronger impact from the indirect human effects on water vapor concentrations.Ozone is technically a greenhouse gas because it has an effect on global temperature. However, at higher elevations in the atmosphere (stratosphere), where it occurs naturally, it is needed to block harmful UV light. At lower elevations of the atmosphere (troposphere) it is harmful to human health and is a pollutant regulated independently of its warming effects.

Of the total amount of U.S. greenhouse gases emitted in 2010, about 87% were energy-related and 91% of those energy-related gases were carbon dioxide from the combustion of fossil fuels.

Petroleum is the largest fuel source of carbon dioxide emissions from energy consumption in the United States. Other important fossil fuel sources of carbon dioxide emissions include:

Petroleum — accounting for 2.3 billion metric tons (42%) in 2011 Coal — accounting for 1.9 billion metric tons (34%) in 2011 Natural gas — accounting for 1.3 billion metric tons (24%) in 2011

Of the non-CO2 gases that contribute to energy-related greenhouse gas emissions, methane contributes the most (6%) — mainly from emissions that leak out of natural gas systems, coal mines, and petroleum exploration and production facilities. Nitrous oxide contributes another 1% — from mobile and stationary combustion of fuels and waste.

Electric power generation and transportation are the biggest sources of energy-related CO2 emissions in our nation, with respective shares of 40% and 34% of our total energy-related CO2 emissions in 2011. Taken together, emissions in power generation and transportation increased at an average annual rate of 0.8% between 1990 and 2011. The rest of our CO2 emissions result from direct use of fossil fuels in homes, commercial buildings, and industry. These emissions declined on average by 0.4% per year since 1990.

Since electric power is ultimately used in homes, commercial buildings, and industry, emissions associated with power generation can be allocated to each end-use sector based on their electricity consumption to obtain another perspective. Using this approach, the transportation sector is currently the largest emitter. Our cars, trucks, planes, trains, ships, and barges produced 1.8 billion metric tons CO2 in 2011. Emissions of CO2 from this sector have grown at an average rate of 0.7% since 1990.

The industrial sector — which consists of activities such as manufacturing, construction, mining, and agriculture, is the next biggest source of energy-related CO2 to the transportation sector — a total of 1.5 billion metric tons in 2011. Its emissions have been declining since 1990 due primarily to the loss of energy-intensive industries such as steel.

The commercial sector — which includes such sources as schools, office buildings, and shopping malls — accounts for a total of 1.0 billion metric tons of energy-related CO2 emissions, with about 77% of it coming from the power plants providing the electricity used in the buildings. Its CO2 emissions have grown the fastest since 1990, at an average annual rate of 1.1%.

The residential sector — the homes we live in — accounts for 1.2 billion metric tons of energy-related CO2, 71% of which is produced at power plants providing homes electricity. Residential sector emissions have grown at an average annual rate of about 1.0% since 1990.

1Values expressed as carbon dioxide equivalents (CO2e) are calculated based on their global warming potential (GWP). GWP is the ratio of the warming that would result from the emission of one kilogram of a greenhouse gas to that from the emission of one kilogram of carbon dioxide over a fixed period of time such as 100 years.

If it were not for naturally occurring greenhouse gases, the Earth would be too cold to support life as we know it. Without the greenhouse effect, the average temperature of the Earth would be about -2 degrees Fahrenheit rather than the +57 degrees Fahrenheit we currently experience.

Petroleum is the fossil fuel that accounts for the most carbon dioxide emissions.

Electricity generation and transportation are the biggest sources of energy-related greenhouse gases.

The electric power industry currently emits the most energy-related greenhouse gas.

 

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