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The midnight sun shines across sea ice along the Northwest Passage in the Canadian Arctic Archipelago, on July 22, 2017.David Goldman/The Associated Press

The prospect of runaway greenhouse emissions emerging from the organic matter in thawing permafrost has long been among the more frightening climate change scenarios, stoking fears that warming temperatures will release catastrophic new quantities of carbon into the atmosphere.

But the scholarship on permafrost is changing. A sophisticated analysis of data from satellites and field sensors has found no sign – at least, not yet – of a dramatic rise in carbon output. Instead, scientists say the vast permafrost areas that span the northern polar regions, including the Arctic rim of Canada, have to date actually absorbed more carbon than they have released. What’s more, the Earth’s permafrost may never produce a runaway outpouring of greenhouse gases.

The research suggests a complicated future for the far north, where warming temperatures are remaking the landscape, rewiring its hydrology and, critically, hastening the advent of spring. For carbon emissions, that matters a great deal.

Spring in the high latitudes is a moment of spectacular change, with snow giving way to a verdant flourish. It happens nearly overnight, as plants hurry back to life to make the most of the short growing season.

A warming Arctic has hastened the beginning of that annual reawakening, and brought what University of Montana climate and remote sensing scholar John Kimball calls “an earlier green wave” that, in tundra areas, is producing more woody vegetation cover than before. And those plants, as they surge to life, are storing enough carbon to alter the high-latitude balance.

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“Since around 1980, the Arctic tundra has actually become a much stronger carbon dioxide sink overall,” said Prof. Kimball, one of the researchers responsible for the recent findings on the unexpected role permafrost regions are playing in climate change. He is also director of the Numerical Terradynamic Simulation Group, a NASA partner that has been a pioneer in using satellite observations to monitor climate-related changes in vegetation.

A shift took place around the year 2000, he said. Before that, the tundra was relatively carbon neutral. Now, it’s showing “much stronger carbon uptake.”

The tundra, in other words, is – for now – sucking down more carbon than it is emitting.

“Our study challenges the notion that these high northern latitude permafrost regions are becoming a net source of carbon dioxide to the atmosphere with global warming,” Prof. Kimball said.

But this is likely to be a temporary reprieve. The outlook for decades to come suggests a much different role for permafrost.

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The Geological Survey of Canada studies permafrost thaw and the eroding coastline in the Beaufort Sea, N.W.T.Handout

The High Arctic is warming at a rate three times the global average, and it’s not clear how long the tundra will continue to serve as a carbon sink. Climate change has already brought striking changes that include the rapid draining of far northern lakes, greater rainfall and expanding formations of talik, or unfrozen areas in the permafrost.

A large percentage of permafrost underlies regions of boreal forest, which for now also continue to extract more carbon from the atmosphere than they release. But that effect is weakening. Not only is spring coming earlier, but autumn is lingering longer. The fall is a time when forested areas in particular emit more carbon back into the atmosphere than at other points in the year.

As the climate continues to warm in years to come, those late-season emissions call into question “the continued stability of northern forests as a carbon sink,” Prof. Kimball warned.

Scientists estimate that permafrost contains two to three times the carbon in the atmosphere. As a frozen mass, permafrost locks in place carbon in organic matter. When it thaws, that material can begin to decompose, releasing carbon dioxide and methane, both greenhouse gases.

Research by other scholars shows that the current climate trajectory is likely to result in release of carbon emissions from permafrost equivalent to between five and 10 per cent of current human-caused emissions, said David Olefeldt, a University of Alberta scholar who studies carbon cycles in boreal and Arctic landscapes. Estimates suggest the emissions could exceed that of a large industrialized country such as Germany.

“It’s large enough that we would need to take it into account when negotiating emissions ceilings,” Prof. Olefeldt said.

But contrary to some concerns from years past, the release of great volumes of carbon from permafrost is no longer expected to be a sudden event, resulting in what some had termed “runaway climate change.”

Much about permafrost remains poorly understood.

Researchers studying old soil carbon in northern Siberia found that it decomposed rapidly, meaning it could create high carbon dioxide and methane emissions if thawed. Peatland in northern Canada studied by Prof. Olefeldt and other researchers, by contrast, “seems to be very hard to decompose after it thaws, and thus is unlikely to lead to large carbon dioxide emissions,” he said.

Another study has found that, as the climate warms, northern lakes are becoming greater sinks for carbon dioxide, but greatly increased emitters of methane, a more potent greenhouse gas.

In forested areas, meanwhile, precipitation – and in particular late-season droughts – may also prove important. A drying landscape is more vulnerable to wildfires that release large amounts of carbon and, by burning away foliage, can speed up the thaw of the permafrost below. Drought conditions have become more severe in recent years, Prof. Kimball said.

But the fire risk is not uniform: North America’s black spruce forests tend to burn hotter and more readily than the larch regions of Eurasia.

Even the nature of carbon release from permafrost is uncertain. Will decomposing organic matter release methane into the atmosphere? Or will carbon dioxide be entrained in groundwater and perhaps linger underground or in lakes?

“We’re still trying to understand that,” Prof. Kimball said.

Those who live in the high latitudes have noted unexpected changes. University of Alaska soil ecologist Torre Jorgenson lives in Fairbanks, Ala., where the seasonal shifts have been so great that he now schedules an annual spring snowmobile trip two to three weeks earlier than in the past. “The earlier green-up is a big deal,” he said, marvelling at the speed with which it happens.

“We do this really fast transition from snow to green, with only a very short period of brown. And it’s pretty dramatic in the lives of all us northerners.”

But the climate-related changes he has seen are complex. In some places, thawing permafrost is collapsing the ground, which can sink into a soggy bog or fen. “Those are very good peat-accumulating environments,” he said. And peat deposits can be potent carbon sinks.

It’s all enough that he says computer models forecasting the carbon future of the Arctic remain a rudimentary tool at best. Different scientific models predict dramatically different futures for the permafrost region, some suggesting an increase in its carbon storage by the end of this century, others forecasting a major new release of greenhouse gases.

“We’re constantly being surprised by outcomes of things we didn’t expect,” Mr. Jorgenson said.

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