|The flow of US greenhouse gas emissions in 2007, from their sources to their distribution across the US end-use sectors. Click to enlarge. Source: EIA|
Total US greenhouse gas (GHG) emissions were 7,282 million metric tons carbon dioxide equivalent (MMTCO2e) in 2007, an increase of 1.4% from the 2006 level, according to Emissions of Greenhouse Gases in the United States 2007, a report released by the Energy Information Administration (EIA). Since 1990, US GHG emissions have grown at an average annual rate of 0.9%.
The transportation sector has led all US end-use sectors in emissions of carbon dioxide since 1999. However, with higher fuel prices and slower economic growth in 2007, emissions from the transportation sector in 2007 (2,104 MMTCO2) were essentially unchanged from their 2006 level (2,103 MMTCO2).
Transportation sector carbon dioxide emissions in 2007 were 431.8 million metric tons higher than in 1990, an increase that represents 44% of the growth in unadjusted energy-related carbon dioxide emissions from all end-use sectors over the period.
Petroleum combustion is the largest source of carbon dioxide emissions in the transportation sector, as opposed to electricity-related emissions in the other end-use sectors. Transportation sector emissions from gasoline and diesel fuel combustion generally parallel total vehicle miles traveled.
Increases in ethanol fuel consumption in recent years have also mitigated the growth in transportation sector emissions somewhat (emissions from energy inputs to ethanol production plants are counted in the industrial sector), according to the EIA.
US GHG emissions per unit of gross domestic product (GDP), or US GHG intensity, fell from 636 metric tons per million 2000 constant dollars of GDP (MMTCO 2e/million dollars GDP) in 2006 to 632 MMTCO 2e /million dollars GDP in 2007, a decline of 0.6%. Since 1990, the annual average decline in GHG intensity has been 1.9 percent.
The EIA report notes that the steady decrease in carbon intensity (carbon/GDP) has resulted mainly from reductions in energy use per unit of GDP (energy/GDP) rather than increased use of low-carbon fuels.
Total estimated US GHG emissions in 2007 consisted of 6,022 million metric tons of carbon dioxide (82.6% of total emissions); 700 MMTCO 2e of methane (9.6% of total emissions); 384 MMTCO 2e of nitrous oxide (5.3% of total emissions); and 177 MMTCO 2e of hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF 6) (2.4% of total emissions).
Emissions of carbon dioxide from energy consumption and industrial processes, which had risen at an average annual rate of 1.1% per year from 1990 to 2006, increased by 1.3% in 2007. Unfavorable weather patterns, where both heating and cooling degree-days were higher in 2007 than 2006, and an increase in the carbon intensity of electricity generation, driven by decreased availability of hydropower, both contributed to higher energy-related carbon dioxide emissions in 2007.
Methane emissions increased by 1.9%, while nitrous oxide emissions rose by 2.2%. Emissions of HFCs, PFCs, and SF6, a group labeled collectively as “high-GWP gases” because of their high heat-trapping capabilities, increased by 3.3%.