據3月31日美國能源資訊管理局（EIA）公佈的一份報告， 2009年美國溫室氣體排放量下降5.8%，創下自1990年有記錄以來的最大降幅。
EIA從1990年開始記錄總溫室氣體總排放量，EIA的統計和分析部門表示，自1990年以來，美國溫室氣體排放量以年均0.4%的速度增加。不過，到了2009年，美國溫室氣體總排放量相當於65.76億公噸的二氧化碳當量值（即6576百萬噸碳當量，MMTCO2e），比2008年的水平下降了5.8%。該降幅創下美國總溫室氣體排放量自1990年以來最大的下降比。
EIA的管理員理查德紐厄爾（Richard Newell）表示：「2009年排放量大幅下降主因為經濟不景氣，加上低耗能經濟趨勢，和低碳的能源供應來源所致。」
相對於1990至2008年每年年均0.8%的成長率，2009年與能源相關的二氧化碳排放卻減少了7.1%。
影響排放量下降的因素還有國內生產總值（GDP）減少了2.6%。
2009年美國經濟體的能源密集度（每GDP美元所耗費的能源）下降了2.2%。而根據EIA報告指出，同一年度美國的二氧化碳密集度（每供應一個能源單位所產生的的二氧化碳）也是呈現下降趨勢，導因於天然氣價格相對於煤炭呈現下滑，導致使用天然氣發電的比重增加。
此外，在使用風力和水力發電所產生的可再生能源愈來愈普遍的情況帶動下，也有助於降低排量。
2009年美國甲烷排放量增加了0.9%，而一氧化二氮排放量下降了1.7%。
另外，氫氟碳化合物、全氟化碳、六氟化硫（HFCs，PFCs， SF6）等溫室氣體，根據已蒐集到的7成7資料來推估，總排放量增加了4.9%。
2009年美國溫室氣體總排放量估計值包括：
- 二氧化碳排放達54.468億公噸，佔總排放量的82.8%。
- 甲烷排放約7.309億公噸的二氧化碳當量，佔總排放量的11.1%。
- 一氧化二氮排放約2.196億公噸的二氧化碳當量，佔總排放量的3.3%。
- 氫氟碳化合物（HFCs）、全氟化碳（PFCs）和六氟化硫（SF6）排放約為1.782億公噸的二氧化碳當量，佔總排放量的2.7%。
U.S. greenhouse gas emissions dropped a record-setting 5.8 percent in 2009, the largest percentage decline since recordkeeping began in 1990, according to a report released Thursday by the U.S. Energy Information Administration.
Since 1990, U.S. greenhouse gas emissions have grown at an average annual rate of 0.4 percent, said the EIA, the statistical and analytical agency within the U.S. Department of Energy.
But in 2009, total U.S. greenhouse gas emissions were 6,576 million metric tons carbon dioxide equivalent, or MMTCO2e, a drop of 5.8 percent from the 2008 level.
This is the largest percentage decline in total U.S. greenhouse gas emissions since 1990, the starting year for EIA's data on total greenhouse gas emissions.
"The large decline in emissions in 2009 was driven by the economic downturn, combined with an ongoing trend toward a less energy-intensive economy and a decrease in the carbon-intensity of the energy supply," said EIA Administrator Richard Newell.
Emissions of energy-related carbon dioxide decreased by 7.1 percent in 2009, having risen at an average annual rate of 0.8 percent per year from 1990 to 2008.
Among the factors that influenced the emissions decrease was a decline in Gross Domestic Product (GDP) of 2.6 percent.
The energy intensity of the U.S. economy, measured as energy consumed per dollar of GDP, fell by 2.2 percent in 2009.
There was also a decline in the carbon dioxide intensity (CO2 per unit of energy) of U.S. energy supply in 2009, caused primarily by a drop in the price of natural gas relative to coal that led to more natural gas consumed for the generation of electricity, according to the report.
Also contributing was an increase in renewable energy consumption, led by wind and hydropower.
Methane emissions increased by 0.9 percent, while nitrous oxide emissions fell by 1.7 percent in 2009.
Based on partial data constituting about 77 percent of the category, combined emissions of HFCs, PFCs and SF6 increased by 4.9 percent.
Total estimated U.S. greenhouse gas emissions in 2009 consisted of:
- 5,446.8 million metric tons of carbon dioxide - 82.8 percent of total emissions
- 730.9 MMTCO2e of methane - 11.1 percent of total emissions
- 219.6 MMTCO2e of nitrous oxide - 3.3 percent of total emissions
- 178.2 MMTCO2e of hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) - 2.7 percent of total emissions
Click here for the full report, "Emissions of Greenhouse Gases in the United States 2009."
EIA also publishes ongoing monthly estimates of carbon dioxide emissions from the consumption of energy.
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