Fewer emissions from U.S. power plants in 2009 were the result of cheaper prices of natural gas, reducing the industry's reliance on coal, researchers say.

As the United States tumbled into economic recession in that year, CO2 greenhouse gas emissions also fell by 6.59 percent relative to 2008, but the recession was not the main cause, researchers at Harvard University said.

Instead, the primary factor in the reduction in emissions from power generation was a decrease in the price of natural gas, which allowed the industry to reduce its traditional reliance on coal, they said.

The economic model used to determine this also suggests emissions could be cut further by the introduction of a carbon tax, with negligible impact on the price of electricity for consumers, the researches reported in the journal Environmental Science and Technology.

"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," environmental studies professor Michael B. McElroy said.

"That's what we saw in 2009," he said, "and we may well see it again."

Researchers said their model identifies the relationship between the cost of electricity generation from coal and gas.

"When the natural gas prices are high, as they were four years ago, if the gas prices come down a little bit, it doesn't make any difference," study lead author Xi Lu said. "But there's a critical price level where the gas systems become more cost-effective than the oldest coal-fired systems."