WASHINGTON — New research is raising concerns that global warming may be triggering a self-perpetuating climate time bomb trapped in once-frozen permafrost.
As the Earth warms, greenhouse gases once stuck in the long-frozen soil are bubbling into the atmosphere in much larger amounts than previously anticipated, according to a study in Thursday's journal Nature.

Methane trapped in a special type of permafrost is bubbling up at a rate five times faster than originally measured, the journal said.

Scientists are fretting about a global warming vicious cycle that had not been part of their already gloomy climate forecasts: Warming already underway thaws permafrost, soil that had been continuously frozen for thousands of years.

Thawed permafrost releases methane and carbon dioxide. Those gases reach the atmosphere and help trap heat on Earth in the greenhouse effect. The trapped heat thaws more permafrost, and so on.

"The higher the temperature gets, the more permafrost we melt, the more tendency it is to become a more vicious cycle," said Chris Field, director of global ecology at the Carnegie Institution of Washington. "That's the thing that is scary about this whole thing. There are lots of mechanisms that tend to be self-perpetuating and relatively few that tends to shut it off."

The effect reported in Nature is seen mostly in Siberia, but also elsewhere, in a type of carbon-rich permafrost, flash frozen about 40,000 years ago. A new more accurate measuring technique was used on the bubbling methane, which is 23 times more powerful a greenhouse gas than the more prevalent carbon dioxide.

"The effects can be huge," said lead author Katey Walter of the University of Alaska at Fairbanks. "It's coming out a lot and there's a lot more to come out."

Another study earlier this summer in the journal Science found that the amount of carbon trapped in this type of permafrost — called yedoma — is much more prevalent than originally thought and may be 100 times the amount of carbon released into the air each year by the burning of fossil fuels.

It won't all come out at once or even over several decades, but the methane and carbon dioxide will escape the soil if temperatures increase, scientists say.

The issue of methane and carbon dioxide released from permafrost has caused concern this summer among climate scientists and geologists. Specialists in Arctic climate are coming up with research plans to study the effect, which is not well understood or observed, said Robert Corell, chairman of the Arctic Climate Impact Assessment, a group of 300 scientists.

"It's kind of like a slow-motion time bomb," said Ted Schuur, a professor of ecosystem ecology at the University of Florida and co-author of the Science study. "There's these big surprises out there that we don't even know about."

Most of this yedoma is in north and eastern Siberia, areas that until recently had not been studied at length by scientists.

What makes this permafrost special is that during a rapid onset ice age, carbon-rich plants were trapped in the permafrost. As the permafrost thaws, the carbon is released as methane if it's underwater in lakes, like much of the parts of Siberia that Walter studied. If it's dry, it's released into the air as carbon dioxide.

Scientists aren't quite sure which is worse. Methane is far more powerful in trapping heat, but only lasts about a decade before it dissipates into carbon dioxide and other chemicals. Carbon dioxide traps heat for about a century.

"The bottom line is it's better if it stays frozen in the ground," Schuur said. "But we're getting to the point where it's going more and more into the atmosphere."

Vladimir Romanovsky, geophysics professor at the University of Alaska at Fairbanks, said he thinks the big methane or carbon dioxide release hasn't started yet, but it's coming. It's closer in Alaska and Canada, which only has a few hundred square miles of yedoma, he said.

In Siberia, the many lakes of melted water make matters worse because the water, although cold, helps warm and thaw the permafrost, Walter said.

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