Total estimated U.S. GHG emissions in 2008 consisted of 5,839.3 million metric tons of carbon dioxide (82.8 percent of total emissions); 737.4 MMTCO2e of methane (10.5 percent of total emissions); 300.3 MMTCO2e of nitrous oxide (4.3 percent of total emissions); and 175.6 MMTCO2e of hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) (2.5 percent of total emissions).
Emissions of energy-related carbon dioxide decreased by 2.9 percent in 2008, having risen at an average annual rate of 1.0 percent per year from 1990 to 2007. Factors that influenced the emissions decrease included record-high oil prices and a decline in economic activity in three out of four quarters of the year.
Oil-related emissions declined by 5.9 percent in 2008, accounting for the bulk of overall reduction in energy-related carbon dioxide emissions.
The 2.9-percent decline in 2008 energy-related carbon dioxide emissions reflects the combined effects of a 0.4-percent increase in Gross Domestic Product (GDP), a 2.5-percent decrease in energy use per dollar of GDP, and a 0.9-percent decrease in carbon dioxide emissions per unit of energy.
Methane emissions increased by 2.0 percent, while nitrous oxide emissions rose by 0.1 percent in 2008. Based on partial data constituting about 85 percent of the category, combined emissions of HFCs, PFCs and SF6 increased by 3.1 percent.