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We already have a narrow window of time to ward off the worst impacts of climate change. That could shrink even more if a previously-stable store of carbon is suddenly unleashed into the atmosphere. And a big store that many climate scientists are worried about is permafrost, the frozen soil that covers about a quarter of the northern hemisphere. In a 2018 Special Report, the Intergovernmental Panel on Climate Change warned that permafrost presents a huge uncertainty for our carbon footprint in the future.

What underlies that uncertainty is the fate of the old carbon stored in permafrost. Much like burning fossil fuels, releasing carbon from frozen soil that has been stored in the earth for thousands of years can warm our planet rapidly. But scientists aren’t sure whether that carbon will be released mostly as carbon dioxide, or as methane—a more powerful greenhouse gas.

Permafrost is soil that’s frozen year-round. It can include sand, rocks, or dark earth that’s rich in organic matter. It’s this organic-rich soil that’s most important from a climate perspective. Over thousands of years—tens of thousands, in some places—plants and animals died, decomposed, and became part of the soil as organic matter. Then, when that compost-like soil froze, all the carbon it contained became locked away from the atmosphere.

That makes permafrost an extensive store of carbon. It stretches across roughly seven million square miles at high latitudes, and contains about 1.5 trillion metric tons of carbon—around double the amount of carbon in the entire atmosphere. And with the Earth’s polar zones warming rapidly, this store is increasingly vulnerable.

When permafrost thaws, microbes are able to break down its organic matter. In moist, aerated, soil, this organic matter is mostly converted to carbon dioxide. In wet, low-oxygen conditions, it’s instead converted into methane, which warms the planet 86 times as much as carbon dioxide across 20 years.

Studies estimate that climate warming could release up to 15 percent of the carbon stored in permafrost this century, so understanding this risk is important. The problem is, figuring out how fast this frozen soil will thaw—and which greenhouse gases it will release—is tricky.

Part of the equation is how quickly the permafrost thaws. Most frozen soil thaws slowly—every summer, permafrost in many places is growing thinner and thinner. But, sometimes, pockets of permafrost melt out all at once in what’s called “abrupt thaw.” This process causes the land to sink, since it is no longer held up by an ice layer, eroding hillsides and forming new wetlands and lakes. The resulting pocketed landscape is called thermokarst.

aerial photo of land dotted with ponds
Thawed land in Alaska Bureau of Land Management

Last month, climate scientists reported in Nature Geoscience that these areas of abrupt melt could vastly increase the amount of carbon dioxide and methane released from the ancient carbon stores in permafrost. Under a high emissions future climate scenario, the total area of abrupt thaw reached 1.6 million square kilometers by 2100, and to 2.5 million square kilometers by 2300 (that’s 0.6 million and 1.0 million square miles, respectively). While less than five percent of permafrost lands experience this abrupt thawing, they produce almost half the emissions as gradually-thawing areas, which cover a much greater extent. Current climate models don’t include this rapid thaw process, but the authors conclude that counting it could double previous estimates of carbon emissions from permafrost thaw.

Understanding the relative amounts of abrupt and gradual thaw may also reveal how much carbon dioxide and methane are being produced. Marilena Geng, a climate scientist at Memorial University of Newfoundland, says that when the soil thaws gradually, waterlogged sediment then sits above a still-frozen layer. With nowhere for water to move, this zone loses oxygen, facilitating the work of methane-generating bacteria. In abrupt thaw, water can drain away from soil, leading to a greater proportion of carbon dioxide formation. Indeed, the Nature Geoscience study found that in abrupt thaw scenarios, greenhouse gases were mostly carbon dioxide and only 20 percent methane (though the gas contributed 50 percent of warming).

But there’s more to understanding methane, as another recent study points out. In a paper published mid-February in Science, researchers analyzed Antarctic ice cores from between 18,000 and 8,000 years ago—the period in which the Earth was warming back up after the last ice age. Using pockets of air in the ice, they could quantify the gases present in the atmosphere during that period. They looked for methane free of the radioactive carbon-14 isotope, which is naturally present in living and recently decayed material, but is lost as that organic matter ages. Because permafrost contains very old carbon, methane formed from it typically has virtually no carbon-14. If the air bubbles contained this type of methane, that would indicate that the deglaciation period experienced a permafrost-caused methane boom.

But most of the methane in the ice core samples did contain C-14, which means it came from sources other than permafrost and other old carbon stores. From that period of warming, in which the Arctic got to be a little warmer than it is today, the researchers concluded that very little methane from permafrost was released.

Isaac Vimont, a carbon cycle researcher at NOAA’s Earth System Research Laboratory and co-author of the study, says the results suggest that we might not see the global warming-caused methane explosion that other studies have pointed to. “To a certain extent, we can say that these old carbon reservoirs [like permafrost] will stay stable,” says Vimont. “Going forward, we don’t know.” If temperatures keep rising past those observed in the last deglaciation, we might still have methane surprises in store.

Vimont and his colleagues offer one explanation for how their assessment landed at a different conclusion. While some microbes essentially breathe out methane in waterlogged environments, other organisms eat methane and breathe out carbon dioxide. In this way, not all of the methane that gets released actually ends up in the atmosphere. Some studies have documented this process in seeps of methane from the Arctic seafloor. “Old methane release occurs much slower than the pace of modern climate change,” writes Joshua Dean, biogeochemist at the University of Liverpool, in an article accompanying the Science study. “This is because methane is a rich source of energy within ecosystem food webs.”

Certainly, there will be—and currently is—some methane release. But it might not be significant compared to other sources of methane, like the production and burning of natural gas.

Even if we don’t experience a sudden spike in methane, though, that doesn’t mean the Arctic isn’t at risk of becoming a major carbon source. Permafrost is still being lost to climate change. Even if it’s mostly turning into carbon dioxide, that’s still a lot of emissions wafting off these lands. Having comparatively more carbon dioxide isn’t necessarily better, says Geng. Since carbon dioxide is long-lived in the atmosphere, it sticks around and warms the planet longer than methane. “That past warming [during deglaciation] showed that carbon stored in permafrost is released as carbon dioxide rather than methane,” said Geng about the Science study. “Neither carbon nor methane is something we’d be looking forward to be released from permafrost.”

Whether it’s methane or carbon, the effect of extra carbon pollution in the Arctic is—surprise!—also hard to quantify. That’s because, to some extent, more plants can take root as ice is lost and can start storing carbon themselves.

But Geng and Alexander Kholodov, a permafrost researcher with the University of Alaska, Fairbanks, still say we’re facing a major climate feedback effect: carbon emissions from permafrost will warm the planet, leading to more emissions, which will melt more land, and so on. Kholodov studies permafrost in interior Alaska, a region he says is as dry as the Mojave Desert. He says permanent ground ice helps keep soils in the region moist in the summer, fostering plant growth. Without ice, those landscapes might start to look more like the American Southwest.

Part of the problem with projecting the future of permafrost, say Geng and Kholodov, is that permafrost regions underly remote tundra and boreal forests with few human dwellings and accessing this wilderness can be challenging.

But the researchers are doing their part to solve this puzzle. Based on her field studies in Greenland, Geng is developing a correlation between increases in air temperature and corresponding increases in methane generation on the permafrost landscape. This relationship could be incorporated into existing climate models, which she says often lack information on this climate feedback. With the new information, perhaps we could better budget for how much carbon pollution the planet can still stand.

That’s important because, by the time Vimont and researchers at NOAA start noticing the chemical fingerprint of old carbon at their atmosphere monitoring stations, it will be too late. While we can stop our methane emissions by ceasing burning fossil fuels, we can’t stop a powerful natural cycle once we unleash it. “If you have natural sources [releasing methane], there’s no way to put a lid back on it,” says Vimont. “There’s no way to stop it if it starts coming out.”

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