The reduction in greenhouse gas emissions from U.S. power plants in 2009 can be explained by a fall in the price of natural gas, according to an article published last month by researchers at the Harvard School of Engineering and Applied Sciences.

The power generation sector, which typically accounts for 40 percent of U.S. carbon dioxide emissions, reduced its greenhouse gas output by 8.76 percent in 2009 relative to 2008, according to the research.

The study found that most of this reduction could be attributed to a shift from coal to natural gas—a change driven by a decrease in gas prices.

“Generating one 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,” said Michael B. McElroy, the study’s lead researcher, in a SEAS press release.

McElroy, an environmental studies professor, published the article with postdoctoral researcher Xi Lu and applied math alumnus Jackson S. Salovaara ’11 in the most recent issue of Environmental Science and Technology.

The researchers developed an econometric model that divided the nation into nine different regions to account for differences in the price and patterns of power generation and usage.

Lu said that the study’s results suggest important implications for U.S. environmental and energy policy.

For example, Lu said that even a modest tax on carbon emissions could push power plants to switch to gas, which would further reduce carbon dioxide emissions. Although a carbon tax would affect power plants that use both coal and gas, the higher carbon dioxide output of coal usage would give plants a financial incentive to switch.

Lu said, however, that the article did not fully address the methods used to extract natural gas.

“The main driver of the price decrease in 2009 was from shale gas,” said Lu, referring to natural gas trapped in shale deposits and extracted through the controversial technique of hydraulic fracturing, or “fracking.”

“We are not in a position to defend shale gas as cleaner,” Lu said about lifetime-cycle greenhouse gas emissions. “It’s a very controversial topic. Some papers argue that shale gas is cleaner than coal, but other people have the opposite opinion.”

In addition to the desirable effects of decreasing carbon dioxide emissions, the article cited several other advantages of using natural gas, regardless of extraction technique. According to the research, those benefits include lower emission rates of other pollutants such as mercury and higher overall efficiency in the power sector.

“The advantage of an increase in the supply of natural gas and an associated decrease in the price of the gas relative to coal is clear,” the study said.