The period between 2008 and 2009 saw the largest decrease in U.S. greenhouse gas emissions in 20 years, according to U.S. EPA's Greenhouse Gas Inventory Report.
While the 6 percent decrease correlates with the constriction of the global economy that followed the financial crisis in 2008, it was not the only reason for the drop, the report says. Advances in technology, efficiency and reuse of scrap industrial material also decreased energy use in key sectors. In addition, an increase in coal prices and a decrease in natural gas prices led to a switch from one source to the other.
However, over the past 20 years, greenhouse gas emissions in the United States have increased steadily by about 0.4 percent annually.
"In a year with increased consumption of goods and services, low fuel prices, severe summer and winter weather conditions, nuclear plant closures, and lower precipitation feeding hydroelectric dams, there would likely be proportionally greater fossil fuel consumption than a year with poor economic performance, high fuel prices, mild temperatures and increased output from nuclear and hydroelectric plants," states the report.
Industries that lowered their emissions include iron and steel manufacturing and metallurgical coke, whose emissions decreased by 35.5 percent from 2008 to 2009 and by more than half over 20 years, thanks to increased efficiency in scrap use, technological improvements and industrial restructuring.
Emissions from cement making also went down by 28 percent from 2008. The decrease has been a trend over the previous three years, after a steady increase throughout the 1990s and early 2000s.
Output of carbon dioxide, the most abundant greenhouse gas, dropped 7 percent from 2008 to 2009 and rose almost 8 percent over 20 years. Transportation accounts for 33 percent of CO2 emissions. Over 20 years, emissions rose by 16 percent in this sector, due to an increase in demand for flying and vehicle usage -- 38 percent for cars and light trucks.
Residential and commercial emissions have increased 25 percent in the past 20 years.
The report also calculated carbon sequestration through "sinks," carbon-trapping bodies in the form of forests, cropland, grassland and even landfilled yard trimmings and food scraps. Land-use changes to sinks resulted in an 18 percent offset in carbon emissions and a 15 percent offset in total greenhouse gas emissions in 2009.
Slight increase of methane, drop in nitrous oxide
Methane emissions between 2008 and 2009 increased minimally by 1 percent, and rose 1.7 percent since 1990, sourced primarily from the digestive mechanics of livestock, natural gas systems and decaying organic matter in landfills.
A gas 20 times more effective in trapping heat than carbon, methane is a byproduct of natural gas combustion. These systems have increased their methane output by 16 percent over 20 years. A flurry of natural gas well construction in 2008 and 2009 pushed methane emissions up 4 percent.
However, methane has the advantage of being captured, stored and combusted as a fuel -- a method that has been put to use in many landfills. As a result, methane emissions from landfills have decreased a whopping 20 percent over 20 years.
"[This] has more than offset the additional CH4 emissions resulting from an increase in the amount of municipal solid waste landfilled," states the report.
Even coal, the scapegoat of carbon-heavy industry, has lowered its emissions of methane by 15.5 percent. Mining companies are extracting less gassy coal from underground mines.
Nitrous oxide, with a warming effect more than 300 times that of carbon dioxide, dropped by 3.6 percent in 2009. The 1990-2009 period saw fluctuations in nitrous emissions from soils, but overall, emissions in 2009 were 3.4 percent higher than in 1990. Emissions are caused by a reaction between soil and water and vary according to the amount of nitrogen fertilizer that is applied on crop beds.