Thawing Siberian bogs are releasing more of the greenhouse gas methane than previously believed, according to new scientific research.
Methane bubbles can be trapped in ice during the autumn freeze
Scientists from Russia and the US measured methane bubbling from a number of thawing lakes.
Writing in the journal Nature, they suggest the methane release is hastened by warmer temperatures, positively feeding back into global warming.
Methane's contribution to present-day global warming is second only to CO2.
The Intergovernmental Panel on Climate Change (IPCC) estimates that atmospheric concentrations are about two and a half times those seen in pre-industrial times.
"Thaw lakes in north Siberia are known to emit methane, but the magnitude of these emissions remains uncertain," the scientists write.
"We show that methane flux from thaw lakes in our study region may be five times greater than previously estimated."
The lakes are produced in summers when land which is usually permanently frozen - permafrost - melts.
The study depended on the systematic deployment of bubble traps on two lakes in the Cherskii region of Siberia, supplemented by ground-based and aerial observations of a further 95 lakes.
Katey Walker from the University of Alaska at Fairbanks and her colleagues calculate that across the region, thaw lakes lakes emit 3.8 teragrams (Tg, million million grams) per year.
The contribution of these lakes is small compared to the IPCC estimate of total global methane production, 600 Tg per year.
More than half of this total comes from human activities, notably farming.
The importance of the Siberian release may lie in the relationship between warming and methane production.
If a high release rate of a greenhouse gas is being triggered by rising temperatures, that will in turn stimulate still higher temperatures - a positive feedback mechanism.
Extra context comes from the age of the emerging gas. Using radiocarbon techniques, the researchers showed that some of the escaping methane molecules had been formed more than 40,000 years ago.
The area of the planet covered by permafrost is projected to shrink as the surface warms.
Boreholes in permafrost in Svalbard, Norway, indicate that ground temperatures rose 0.4C over the past decade, four times faster than they did in the previous century.