Scientists say this Arctic “permafrost” may contain nearly twice as much carbon as there is in the atmosphere today.

Some of this carbon may have been trapped underground, harmless, for thousands of years. But scientists worry it might not stay there forever. As Arctic temperatures continue to rise, large swaths of permafrost are beginning to thaw — and as they do, they may release their colossal carbon stores in the form of climate-warming carbon dioxide and methane.

Just how much of that carbon is reaching the atmosphere — and in what form — is a source of ongoing scientific debate. And it’s one with huge implications for climate science.

Permafrost emissions have the potential to speed up the pace of global climate change if they’re large enough. Some experts worry that the whole process could turn into a kind of vicious climate feedback — permafrost emissions cause more warming, which causes more thawing, which leads to more emissions, and so on.

Determining the seriousness of the problem depends heavily on accurate predictions about permafrost emissions — both carbon dioxide and methane. And there’s still a lot of uncertainty on that front.

One recent paper claims to have found a small bit of good news: Not much of that ancient carbon is likely to reach the atmosphere in the form of methane — a far more potent greenhouse gas than carbon dioxide.

The research, published in February in the journal Science, looks into the ancient past to find clues about the future. It focuses on a period of time about 10,000 years ago, known as the last deglaciation, when the Earth was experiencing a period of relatively rapid warming and large-scale ice melt.

The researchers, led by Michael Dyonisius of the University of Rochester, wanted to know how much long-trapped carbon bubbled out into the atmosphere in the form of methane during this period.

There are a variety of ways this could happen, scientists have theorized.

Thawing permafrost, releasing its long-trapped carbon, is one. Stores of frozen methane, known as methane hydrates, are another.

Scientists know there are ancient methane hydrates trapped both at the bottom of the sea and underneath the world’s ice sheets. Some experts fear that as the world warms, these hydrates may become unstable and allow the trapped methane to escape.

If huge quantities of methane from any of these sources were to suddenly bubble into the atmosphere, it could be a devastating blow to the climate system.

To find out if this happened during the Earth’s last major warming episode, the researchers analyzed samples of ancient Antarctic ice, which had been frozen since the last deglaciation. These ice cores contained frozen bubbles of methane gas, trapped inside for thousands of years.

The researchers were able to conduct a form of carbon dating on the methane samples to determine how old they were.

They found that most of the carbon was about the same age as the ice. That means the methane was made up of relatively new carbon stores — that is, carbon stores that were new at the time the last deglaciation was taking place. This suggests that relatively little ancient methane made it into the atmosphere during this period.

The study doesn’t explain exactly what was preventing the old carbon from leaking out. But the authors have some ideas.

With undersea methane hydrates, for instance, recent research suggests that much of the leaking methane gets converted into other gases — like carbon dioxide — on its way up through the water column. Not much of it actually gets all the way to the surface.

And with permafrost, the authors suggest a similar mechanism. In permafrost, the ancient carbon is typically stored in the form of frozen organic matter, including dead plants and animals. As permafrost thaws, microbes in the soil become active and begin converting the organic matter into methane and carbon dioxide.

Methane-producing microbes often are active in deeper layers of the soil. The methane they produce must travel up through shallower layers before it gets to the surface. And on its way up, large portions of it may be seized by other microbes, which convert it into carbon dioxide before it hits the atmosphere.

If the same processes hold true today, then much of the ancient carbon stored in permafrost would likely be released as carbon dioxide, rather than methane.

Methane is a shorter-lived, but more potent, greenhouse gas than carbon dioxide. So in one sense, the theory could be taken as good news — or, at least, the lesser of two evils.

On the other hand, carbon dioxide emissions still contribute substantially to climate change.

“If it’s emitted as CO2, it’s still bad,” noted lead author Dyonisius, a Ph.D. candidate who studies the methane budget. “So we’re explicitly not saying that permafrost is not a problem.”

Still more questions than answers

In recent years, the possibility of climate feedbacks — self-reinforcing natural processes that could worsen the pace of global warming — has emerged as one of the greatest uncertainties about future climate change. And many experts point to permafrost as one of the biggest question marks.

The ancient past may hold some valuable clues about the future, as the new study points out. But it’s not necessarily the final word on the subject.

For one thing, the past isn’t a perfect analogue for the future.

Without immediate, rigorous efforts to eliminate global greenhouse gas emissions, the Earth is likely to warm beyond the temperatures that occurred during the last deglaciation. It’s still unclear how this extra warming would affect the response of permafrost or the behavior of the microbial communities inside the soil.

There was also likely less old carbon stored on the landscape during the last deglaciation than there is today, according to permafrost expert David Olefeldt, a scientist at the University of Alberta who commented on the new research for E&E News.

Before the last deglaciation, much of the Northern Hemisphere had been covered in sheets of ice for thousands of years. There wasn’t much vegetation on the landscape, meaning not much organic matter was getting locked away in the soil. But since the last ice age ended, the Earth has had about 10,000 years for vegetation to grow, die and get locked away in the ground.

“We’ve been adding lots of plant material to the permafrost soils over the last 10,000 years,” Olefeldt pointed out.

As a result, there’s the potential that the way carbon was released from the landscape during the last deglaciation “might not be comparable to what we are seeing today,” permafrost expert Merritt Turetsky, director of the Institute of Arctic and Alpine Research at the University of Colorado, Boulder, said in an email to E&E News.

It’s unclear exactly how the combination of these factors — stronger warming plus more organic matter — would affect the release of carbon on the landscape, Katey Walter Anthony, a biogeochemist and permafrost expert at the University of Alaska, Fairbanks, said in an email to E&E News.

For instance, higher temperatures could produce a stronger, faster gas release than occurred in the past.

On the other hand, when ancient carbon does make it to the atmosphere in the form of methane, it usually happens in a specific type of environment known as a thermokarst lake — this is a place where permafrost has thawed rapidly and formed a pool of water on top of the ground. These lakes have been known to belch bubbles of old-carbon methane into the atmosphere.

Because the modern landscape contains more carbon and thicker layers of peat, which helps promote better drainage through the soil, “formation of large thermokarst lakes will be limited compared to the past,” Anthony pointed out.

Even if the new study’s theories about old carbon are correct, thawing permafrost may still release methane in other ways.

“The methane emissions that we are expecting due to permafrost thaw doesn’t have to be old in nature,” Olefeldt noted.

As permafrost thaws, the landscape becomes wetter and more vegetation starts to grow in. These plants contribute new organic matter to the soil, some of which is quickly converted into methane.

“My expectations for future methane emissions from boreal and Arctic lakes is that they’re going to be increasing quite substantially, but most of that increase is in the form of modern methane,” Olefeldt said.

Scientists are working to improve their observations of permafrost emissions today so they can make better predictions. It’s not easy work. The Arctic tundra is a vast, remote stretch of land, and monitoring it consistently is difficult.

But, Turetsky noted, “there are increasingly improved technologies that allow us to map hot spots of methane release.”

For the time being, this data seems to suggest that methane emissions from permafrost aren’t really increasing, she said. Still, that doesn’t mean they won’t in the future.

“The big question is, as permafrost thaw increases in area and affects deeper layers, will we see more permafrost carbon release to the atmosphere in future decades and centuries?” she added. “All of our empirical observations — plus the latest modeling — suggests that this will occur, likely with the largest increases in emissions after 2100.”

The same is probably true for carbon dioxide emissions, particularly as deeper permafrost layers begin to thaw and soil microbes start chowing down on that ancient carbon.

Altogether, the new study isn’t meant to suggest that thawing permafrost isn’t a climate threat — far from it, Dyonisius said. It suggests that one potential fear about thawing permafrost may not be so likely after all. But it isn’t intended to gloss over the potential impact of “newer” methane emissions, or ancient carbon released as CO2.

“We’re showing that the old carbon that is locked in permafrost are not emitted to the atmosphere as methane — so we are avoiding the very, very worst-case scenario of permafrost thaw,” Dyonisius said. “But it’s still emitted to the atmosphere as CO2 on a longer time scale.”

Olefeldt doesn’t believe thawing permafrost is likely to cause the kind of catastrophic, runaway climate change that some worst-case climate scenarios warn of. Ongoing human-caused emissions of greenhouse gases — including methane — will continue to be far more significant drivers of climate change between now and the end of the century, he said.

But permafrost isn’t a factor that should be ignored, either, he added.

“We are not saying methane emissions from permafrost thaw is anywhere near being characterized as catastrophic in nature,” he said. “But it might be large enough that we need to take it into consideration.”