Arctic soil thawed by global warming will send enough carbon into the atmosphere to accelerate climate change for the rest of the century, researchers say.
However, it won't come in a sudden burst.
A review by experts concludes the harmful CO2 and methane generated by microbes digesting thawed plant and animal material will instead enter the atmosphere gradually.
But it's a carbon source that shouldn't be ignored, says Dave McGuire, senior researcher at the US Geological Survey and professor of ecology at the University of Alaska Fairbanks.
"The estimates that we came up with in this synthesis suggest that throughout the rest of this century, it could be in the order of the magnitude of what tropical deforestation currently affects in the global carbon cycle," he said.
McGuire is co-lead author and one of 17 researchers whose conclusions appear in the journal Nature.
The paper is an outcome of the Permafrost Carbon Network, a group of more than 200 scientists from 88 institutions in 17 countries who for four years have studied changes in the Arctic.
Most global warming is tied to the burning of fossil fuels.
Tropical deforestation - the clearing of dense rainforest to make room for farms, to harvest timber or to create urban areas - is blamed for about one-tenth of the warming brought on by burning fossil fuels.
That amount is expected to be matched by the effects of melting permafrost, or permanently frozen ground, the scientists say.
As microbes decompose organic material they will release carbon dioxide or more potent methane. Those greenhouse gases create additional warming.
In the past 30 years, permafrost in Alaska, Russia and other Arctic regions has warmed more than 6 degrees, climbing from an average of -8 to just over -2.2, according to research cited.
Researchers wanted to find out how much carbon is contained in permafrost, how fast it's likely to be released and in what form it will be released.
They also addressed some suggestions of a huge greenhouse gas release associated with warming.
"People have been sort of proposing that there's a potential for a 'permafrost bomb'," McGuire said, a surge that could quickly cause trillions in damage to roads, buildings, runways and other infrastructure built on frozen ground.
"Our research indicates that's not likely," he said.
A gradual and prolonged release will give Arctic communities time to adapt.
But the substantial amount of carbon released into the atmosphere should be important information for scientists forecasting future warming and policy-makers who set targets for reduction of fossil fuel consumption.