Carbon dioxide emissions from power plants in the U.S. dropped in 2009 as the country pumped up natural gas use and shoveled aside coal. A drop in natural gas prices caused the shift.
"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," explained Michael B. McElroy, and environmental studies professor at Harvard University, in a press release.
McElroy led a recent study of U.S. power plants' carbon emissions.
During the depths of the recession in 2009, U.S. greenhouse gas emissions fell by 6.59 percent relative to 2008. But emissions from power plants fell by 8.76 percent over the same time.
The Harvard study, published in Environmental Science and Technology, found that different regions' emissions dropped according to their dependence on coal and ability to switch over to natural gas. Both natural gas and coal plants had to exist in an area for the drop in natural gas prices to have an effect. Emissions dropped most dramatically east of the Mississippi.
When natural gas power plants and coal went head to head, cheaper natural gas out-competed older less efficient coal power plants.
"If the gas price continues to drop, you'll continue to go down this curve so that you'll knock out not just the really ancient coal-fired power plants, but maybe some of the more recent coal-fired plants," said Xi Lu, lead author of the study, in a press release.
The study also suggested that a carbon tax would further lower emissions, while not significantly raising electricity prices.
“With a relatively modest carbon tax—about $5 per ton of CO2—you could save 31 million tons of CO2 in the United States, and that would change the price of electricity by a barely noticeable amount,” said McElroy.
A natural gas power plant, the East River Generating Station, Manhattan, New York (Jim Henderson, Wikimedia Commons)