Land subsidence (thermokarst) and thermal erosion can also be caused by forest and tundra fires. These fires destroy the insulating layers of humus, peat, grass and roots that protect the permafrost, and thus effectively accelerate its thawing over the long term. In northern Alaska, for example, after the large Anaktuvuk River fire of July 2007, the land surface subsided more than one metre in some places within a period of seven years due to the formation of thermokarst.
The fire in the tundra was ignited by lightning during the unusually warm and dry summer of 2007, and by the time of the first snowfall in October it had devoured an area of 1039 square kilometres. Investigations following the fire determined that it was the largest wildfire in Alaska’s tundra in 5000 years.
Over the past several millennia, thunderstorms have been rare in the North Slope region, as the tundra landscape of northern Alaska is called. For most of the time, the polar air over the region has simply been too cold for the formation of thunderstorm clouds. And when fires did occasionally occur, they were limited to comparatively small areas. But due to the heat and dryness of the summer of 2007, the Anaktuvuk River fire destroyed more tundra in one fell swoop than all of the North Slope fires in the previous five decades combined.
fig. 3.28 > Smoke clouds over the Yukon Delta National Wildlife Refuge in south-western Alaska. In June 2015 two tundra fires raged at the same time in the region.
Researchers consider the cause, scope, and above all the duration of the fire to be clear signals of a transformation in the tundra that they attribute primarily to climatic change. The rising air temperatures in the Arctic increase the danger of thunderstorms and the probability of lightning strikes that can trigger fires. At the time of the fire there had already been a recognizable increase in lightning frequency in the official United States lightning statistics. Furthermore, the winter snow cover in Alaska is now melting much earlier. For that reason, Alaska’s fire officials had already moved the beginning of the annual forest fire season forward from 1 May to 1 April in the year before the big tundra fire.
Forest and tundra fires ignited by lightning strikes have the potential to fundamentally change the landscape of the high Arctic, and to trigger a climatic chain reaction. Once the tundra is burning, huge amounts of the greenhouse gas carbon dioxide are released. More than two million tonnes of carbon dioxide were released into the atmosphere during the burning at the Anaktuvuk River. This is approximately equal to one month of CO2 emissions from a city the size of Las Vegas. In addition, the fire left behind a dark, burnt earth that had between 50 and 71 per cent less reflective capacity than an undisturbed tundra, and that absorbed so much solar radiation in the subsequent years that the underlying permafrost thawed to deeper levels, and the landscape subsided over about one-third of the burnt surface.
Similar consequences for frozen soils have been observed by experts following fires in forests with near-surface permafrost, which are abundant in Alaska. In these areas, the ground temperatures generally rose so rapidly and substantially after a fire that permafrost was no longer detectable three to five years later.