Study Explains the U.S. Greenhouse Gas Emissions Yo-Yo: Decrease in 2009, Rise in 2010

The Environmental Protection Agency (EPA) has released a draft of the “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2010,” estimating total emissions from greenhouse gasses (GHG’s) to be 6866 million metric tons of CO2 equivalent in 2010. According to the EPA, this 3.3 percent increase over the previous year stems from “an increase in energy consumption across all economic sectors, due to increasing energy demand associated with an expansion in the economy” and “an increase in air conditioning use due to warmer summer weather during 2010.” Overall emissions have increased 11 percent since 1990. The inventory includes CO2, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride, and also takes into account CO2 removed from the atmosphere and stored in carbon “sinks” such as forests and soils. The annual report is written to fulfill U.S. commitments under the United Nations Framework on Climate Change, and demonstrates the challenge of the U.S. pledge to reduce GHG emissions 17 percent below 2005 levels by 2020.

The link between economy and GHG emissions may not be as straightforward as was previously assumed. In 2009, a year of U.S. economic recession, GHG emissions fell 6.59 percent relative to 2008. The EPA attributed this change to “a decrease in fuel and electricity consumption across all U.S. economic sectors.” New research from the Harvard School of Engineering and Applied Sciences has shown that falling natural gas prices were primarily responsible for the 8.76 percent decrease in emissions from the power sector, which accounts for 40% of U.S. emissions. Study leader Michael B. McElroy explains that “generating 1 kilowatt-hour of electricity from coal releases twice as much CO2 to the atmosphere as generating the same amount from natural gas, so a slight shift in the relative prices of coal and natural gas can result in a sharp drop in carbon emissions.” This effect is limited by the capacity of gas-fired power plants in different regions. McElroy comment “While the data from 2011 are not yet available, based on the gas prices, we’re making a confident prediction that there should be a continued shift from coal to natural gas in 2011 as compared to 2008.” The study also determined that a small carbon tax could shift more power generation from coal to natural gas, reducing CO2 emissions while barely affecting electricity prices.