Sunlight in the arctic is delivering a double climate blow, researchers say, by both melting ice and speeding up the release of greenhouse gases.

Dead vegetation preserved in far northern permafrost under ice is estimated to contain twice as much carbon dioxide as is held by the atmosphere, and global warming could allow this plant matter to decompose, releasing both that CO2 and methane, they say.

Rose Cory at the University of North Carolina at Chapel Hill and her colleagues, studying melting permafrost sites in the arctic, found the amount of CO2 released was 40 percent higher when the melt water was exposed to ultraviolet light than when kept dark, buried in the permafrost.

The increase is because ultraviolet light, a component of sunlight, puts more energy into soil bacteria and fungi and accelerates the rate at which they break down organic matter and release CO2.

Major thawing in the arctic could be a major source of positive feedback that could accelerate global warming, the researchers said.

"Our task now is to quantify how fast this previously frozen carbon may be converted to CO2 so that models can include the process," Cory told NewScientist.com.