The thawing of vast areas of frozen soils and the decay of peatlands under higher global temperatures could release massive volumes of carbon dioxide and methane into the atmosphere - potentially doubling the amount of atmospheric greenhouse gases.

Frozen soils (cryosols) and peatlands in the northern hemisphere are estimated to store up to 50 per cent of the world's organic soil carbon. The upper part of cryosols, which are most at risk of thawing, and peatlands store between 1400 billion to 1850 billion tonnes of carbon. There is currently about 700 billion tonnes of carbon in the earth's atmosphere.

University of Wisconsin-Madison soil scientist Dr James Bockheim says global warming threatens to thaw these soils, some of which have been frozen for thousands of years.

"Atmospheric temperatures have increased by 3°C over the past decades in the Arctic and Antarctic regions and this continued warming may cause carbon stored in the surface permafrost to be released to the atmosphere as carbon dioxide," he says.

"Other greenhouse gases such as methane and nitrous oxide could also be released as cryosols thaw."

Dr Bockheim said another risk of melting cryosols is the release of radioactive fallout, toxic metals and other pollutants that have accumulated in frozen soils. Once released, these pollutants could enter terrestrial, aquatic, and marine ecosystems.

The potential release of greenhouse gases from cryosols is not currently taken into account in the Intergovernmental Panel on Climate Change (IPCC) calculations as it is not known exactly how thawing cryosols will impact on greenhouse gas emissions.

Rising global temperatures will also increase decomposition of old carbon stored in peatlands, many of which also are underlain by permafrost.

Dr Merritt Turetsky, from the University of Guelph in Canada, is studying peatlands and describes them as a "time-bomb ticking under our feet".

"Peatlands represent only 1-3 per cent of the world's land surface, but play a major role in the global carbon cycle. Peatlands have served as a long-term sink of carbon dioxide but today also represent one of the largest natural sources of methane. In general, warmer temperatures are expected to accelerate greenhouse gas emissions from peat."

Dr. Turetsky says that, under climate change, northern regions are also becoming more vulnerable to large scale disturbances such as fire. Fires in remote northern regions can burn hundreds of thousands of hectares over several months, and Dr. Turetsky's research has shown that when fires burn into peatlands, emissions of carbon and toxic metals such as mercury can surpass industrial levels.