The signs of change:
Satellite imagery provides many indications that the permafrost soils of the Arctic are now thawing

How is climate change affecting the soils of the Arctic? This question has preoccupied climate researchers for the past several years. There are huge expanses of the High North where only the uppermost few centimetres of soil thaw in the summer; the rest of the soil, down to a depth of several hundred metres, remains frozen the whole year round. In the Northern Hemisphere, these permafrost regions can be found below nearly a quarter of the total surface area.

But it’s not likely to stay that way. According to climate models, rising temperatures could thaw between 50 and 90 percent of upper permafrost layers by the year 2100. Experts fear that, if this happens, not only will the faces of entire landscapes be changed; in a warmer climate, microorganisms could also release large quantities of the greenhouse gases carbon dioxide and methane from the soil, which would in turn accelerate climate change.

In fact, there are already signs that the permafrost has begun degenerating – for example, when we see hills starting to slump, or lakes whose size changes substantially. “With regard to several well-researched regions like the Lena Delta or northern Alaska, we already know there are such effects,” says Ingmar Nitze. What was missing until now was a large-scale overview of where these processes are underway.

Nitze and colleagues from Potsdam and Alaska have now provided exactly that, using the US Landsat satellites to do so. In four selected bands covering a total area of 2.3 million square kilometres in Alaska, Canada and Siberia, they have mapped where signs of degeneration manifested between 1999 and 2015, down to a scale of 30 metres.

In one region in eastern Siberia, for example, local lakes have grown in size by an average of 50 percent. Once these lakes form in a warm summer, they continue to store heat, causing them to swell. As such, expanding lakes can be a sign of degenerating permafrost. However, there is also just the opposite effect: when a lake is crossed by a river, or thaws all of the permafrost below it, it drains; in these cases, shrinking lakes indicate thawing soils. The researchers have especially observed this effect on the edge of Alaska’s permafrost regions. The combined area of the hectare-sized lakes in the surveyed areas of the Arctic and Subarctic has declined by 1.45 percent (1,737 km²).

“These insights are important, for example, if you want to estimate the greenhouse-gas emissions from the soil,” explains Ingmar Nitze. Expanding lakes intensify carbon dioxide and methane emissions; in contrast, when lakes drain, new peat can form, allowing carbon dioxide to once again be stored in the frozen soil.

The researchers not only used the satellite imagery to track lake development; the trails of major forest fires can also offer valuable clues to the status quo of the permafrost. These fires burn away the thick, insulating layer of moss and other vegetation that normally protects the icy soil from high summer temperatures. “This can produce massive thawing processes, even years after the actual fire,” says Ingmar Nitze. So far the researchers have chiefly found these scorched areas in taiga regions; in the treeless tundra, fires remain scarce. But that could change in the future, if climate change leads to warmer summers in the tundra – and a higher probability of storms.

The third major indicator that the researchers investigated: slumping. “This is mostly found in hilly regions where the soil contains glacial ice,” says Ingmar Nitze. If the subterranean blocks of ice left over from the last glacial period thaw, the hills become instable. When that happens, large quantities of sediment begin to move, releasing carbon and eventually landing in lakes, rivers and the ocean. “Granted, these slumps are much smaller factors than lakes or forest fires,” the Potsdam-based researcher concedes, “but on a local scale, they can produce major impacts.”

With their mapping efforts, Nitze and his colleagues hope to ensure that these rapid and small-scale, yet extremely important, effects receive more attention in future climate models. According to Nitze: “We also hope to identify those permafrost regions in which we can expect to see major changes in the near future. They’re the ones we have to keep a close eye on.”

This study was supported by the German Federal Ministry of Education and Research (KoPf project).