Fertile soils in U.S. forests could release vast amounts of carbon dioxide due to global warming, creating a "vicious cycle" of heat increases, scientists say.

Research led by the University of California, Irvine, found heating of soil in northeastern U.S. forests by 10 to 20 degrees increased the release of carbon dioxide by as much as eight times.

"We found that decades-old carbon in surface soils is released to the atmosphere faster when temperatures become warmer," Francesca Hopkins of UCI's Earth system science department said. "This suggests that soils could accelerate global warming through a vicious cycle in which man-made warming releases carbon from soils to the atmosphere, which, in turn, would warm the planet more."

Soil, containing large amounts of carbon in decaying leaves and roots, stores more than twice as much of the element as does the atmosphere, scientists said.

Northeastern woodlands that were once farm fields are currently one of Earth's largest carbon sinks, holding nearly 26 billion tons, but climate scientists are concerned that with ongoing warming trees and soils could become sources, rather than repositories, of greenhouse gas emissions.

"These are carbon dioxide sources that, in effect, we can't control," Hopkins said. "We could control how much gasoline we burn, how much coal we burn, but we don't have control over how much carbon the soil will release once this gets going."