U.S. Greenhouse Gas Emissions Down in 2008

The decline in total emissions - from 7,209.8 million metric tons carbon dioxide equivalent in 2007 to 7,052.6 MMTCO2e in 2008 - was largely the result of a 177.8 MMTCO2e drop in carbon dioxide (CO2) emissions, the technical report states.

There were small percentage increases in emissions of other greenhouse gases, but the absolute contributions to the change in total emissions were relatively small for methane (CH4), nitrous oxide (N2O), and the three manufactured gases with high global warming potentials - hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).

As a result, the increases in emissions of these gases were more than offset by the drop in carbon dioxide emissions.

The decrease in U.S. carbon dioxide emissions in 2008 resulted primarily from three factors:

• higher energy prices - especially during the summer driving season - that led to a drop in petroleum consumption
• economic contraction in three out of four quarters of the year that resulted in lower energy demand for the year as a whole in all sectors except the commercial sector
• lower demand for electricity along with lower carbon intensity of electricity supply

This chart, published with the EIA report, shows the percentage of each greenhouse gas emitted in the United States.

Most of the increase in methane emissions in 2008 came from coal mining and from natural gas production and processing, according to the report.

Emissions from petroleum systems decreased. Emissions from stationary combustion - primarily from wood combustion for residential heating - increased.

Emissions of nitrous oxide increased by just 0.1 percent. Nitrous oxide is a byproduct of fuel combustion in mobile and stationary sources. More than three-quarters of U.S. nitrous oxide emissions from energy use can be traced to motor vehicles, primarily passenger cars and light trucks. Emissions from energy generation made up about 21 percent of total U.S. nitrous oxide emissions in 2008.

Based on a partial estimate, U.S. emissions of the gases with high global warming potentials totaled 175.6 million metric tons carbon dioxide equivalent in 2008 - 5.4 MMTCO2e above the 2007 level.

The increase resulted mainly from higher emissions levels for hydrofluorocarbons, the report finds. The HFCs are being used as substitutes for other gases that destroy the ozone layer.

Carbon sequestration, the capture and storage of carbon dioxide, the chief greenhouse gas, is on the rise, resulting in less CO2 being released into the atmosphere, the EIA report shows.

In 2007, land use, land-use change, and forests accounted for net carbon sequestration of 1,062.6 MMTCO2e, representing 17.7 percent of total U.S. CO2 emissions.

Net carbon sequestration from land use, land-use change, and forestry activities in 2007 was about 26 percent greater than in 1990. The increase resulted primarily from a higher average annual rate of net carbon accumulation in forest carbon stocks.

Sequestration from land use, land-use change, and forestry peaked in 2004, according to the report.

The largest percentage changes from 1990 to 2007 were seen for urban trees, which increased by 61 percent, and for grasslands, which decreased by 91 percent.

Among the categories of estimated greenhouse gas emissions, land-use change has a relatively high degree of uncertainty. The estimated values are highly dependent on the estimation methods used. The countries that submit their emissions estimates to the UN Framework Convention on Climate Change generally provide two estimates - without land-use change, called gross emissions, and with land-use change, called net emissions.

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