Cross one thing off the long list of possible apocalyptic climate-change scenarios: A new scientific analysis concludes that a "carbon bomb" of greenhouse gases emitted from thawing terrestrial permafrost is not likely.
Permafrost on land around the circumpolar north will likely continue to thaw and release carbon dioxide and methane, but at a gradual rate through the end of the century and beyond, said the study, published last week in the journal Nature.
The study came out of a four-year project involving many of the world's permafrost experts who evaluated available data about the Arctic and the sub-Arctic's frozen soil and what is happening to it.
The fear had been that carbon trapped in permafrost would, upon thaw, be suddenly released in large amounts, creating a dramatic increase in gases that would cause a global greenhouse effect and trigger a strong climate feedback loop that would accelerate global change.
The scary scenario has been posited for several years, said study co-author David McGuire, a senior U.S. Geological Survey scientist and climate expert at the University of Alaska Fairbanks' Institute of Arctic Biology.
"Twenty years ago, this idea of a permafrost carbon bomb was put forward," McGuire said. There wasn't much research then on the subject, but now "there's a lot more information," he said. He and others in the international Permafrost Carbon Network, a group of scientists formed in 2011, distilled the new information to come up with the conclusions made in the study.
"We came down clearly on the side that the carbon bomb is not likely, but the reality of permafrost carbon is still an issue of concern," said McGuire, who is the Permafrost Carbon Network's co-principal investigator.
Rather than being released in a few decades as a sudden bomb, which "could cause climate change that would incur catastrophic costs to society," the study says, it is much more likely that permafrost carbon will be released in a more gradual way over decades and centuries.
Northern permafrost holds an estimated 1,330 to 1,580 petagrams -- or billion metric tons -- of sequestered carbon, according to the study. If warming trends continue at current rates, thaw is expected to release 5 percent to 15 percent of that carbon into the atmosphere through the end of the century, the study says.
Though not a sudden pulse, it is a significant amount, with an impact on the global carbon cycle that is about equivalent to that of global deforestation, McGuire said.
It is also the equivalent of about one-tenth of the effect of fossil-fuel burning, said McGuire and another co-author, Vladimir Romanovsky, a geophysicist and permafrost expert at UAF's Geophysical Institute.
"The rate is kind of slower than a lot of people feared, but it's persistent," Romanovsky said.
Unlike the case with fossil fuel combustion, which is an activity under the control of humans, there is not much people can do to stop changes in permafrost once they start, he said. "If the permafrost thaws out, it's very difficult to regulate," he said.
Permafrost warming and thaw have been widespread across the circumpolar north over the past three decades, but the rate has not been even and can be "patchy" according to location, Romanovsky said.
The warming, at least in Alaska, was very rapid in the 1980s and 1990s, he said. It slowed considerably in the 2000s, he said. Now the rate is picking up again; on the North Slope, permafrost appears to be warming at about the same rate as in the 1980s and 1990s, he said.
At Prudhoe Bay, for example, permafrost temperature was 28.4 degrees Fahrenheit last year, Romanovsky said. In the 1980s, it was 17.6 degrees, he said.
"The rate (of warming) is amazing," he said.
As for the conclusion that terrestrial permafrost will not likely set off a carbon bomb, do not celebrate too soon, Romanovsky cautions.
The study focuses on permafrost found on land, he notes.
"There is a big wild card with subsea permafrost and methane," he said. There are still many unanswered questions about permafrost at the bottom of places like the East Siberian Arctic Shelf, where UAF scientists are studying methane releases, and other underwater areas that -- for obvious reasons -- are much more difficult and expensive to examine than is terrestrial permafrost.
If gases are released from underwater permafrost, "it could be very important. It could be very abrupt," Romanovsky said. "But there are uncertainties."
The Permafrost Carbon Network is now turning its attention to the questions and uncertainties surrounding the frozen soil beneath bodies of water, he said.