Nowadays there are several million thermokarst lakes in the Arctic. Most of these shallow water bodies came into being around 10,000 years ago when the permafrost soil thawed within a few decades after the last glacial period. As a consequence of that, the subsoil collapsed back then and formed depressions in which melt and rain water subsequently collected. The lakes created in this way reinforced the thawing of the permafrost soil down to great depths. This process had direct impacts on the climate in the Arctic: “When the permafrost thaws and such a lake forms, the microbes living on the bottom and in the sediment decompose part of the plant remains that were previously trapped in the frozen soil. In this process they produce the greenhouse gas methane, which up to now led us researchers to assume the lakes constantly reinforced global warming through their emissions,” says Dr. Guido Grosse, permafrost researcher at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research and co-author of the new study.
As the scientists now know, only parts of this assumption are correct, however: “We examined the deposits in the 10,000-year-old thermokarst lakes and found out that these water bodies emitted large amounts of methane only in the first half of this period,” says lead author Katey Walter Anthony from the University of Alaska in Fairbanks. 5,000 years ago the carbon balance of the lakes then reversed – triggered by a natural chain reaction.
“When permafrost thaws, it releases many nutrients, as a result of which an extremely large number of mosses and other plants grow in the lakes and on their shores. They in turn sequester carbon from the air by means of photosynthesis,” states Guido Grosse. When the plants then die, their remains drop to the bottom of the lake and form thick layers of sediment there. Because of the dynamic nature of these constantly growing water bodies, however, many of the lakes drain after a few thousand years. “A phenomenon that frequently occurs in permafrost regions and leads to freezing of the then exposed lake sediments. New permafrost is formed which encloses all plant remains and thus also the fixed carbon in them – and in such large amounts that the carbon store rate of the lakes and depressions is 1.6 times greater than their emission rate and the lakes have a climate-cooling effect in the long run,” says Guido Grosse.
However, this effect takes place only as long as the soil layers remain frozen. An increasingly warmer Arctic could reverse the process within a short time. Guido Grosse: “According to our estimations, the Arctic thermokarst lakes and basins store about 160 petagrams of carbon. This is roughly comparable to the storage capacity of the tropical rainforests, which is given as 212 petagrams. We additionally know that the air and ground temperatures in the Arctic are currently rising and the permafrost in several regions is already thawing. The big question now is: What will happen to these large carbon deposits in the near future?” He wants to find an answer to this with his ERC junior researcher group PETA-CARB, which started its work at the Alfred Wegener Institute in Potsdam in October 2013 and builds on the findings of the current study. “With the help of the new results we can not only gain a better understanding of the role of permafrost in the global carbon cycle, it can also help us to further develop computer models in such a way that they better forecast the feedback effects between permafrost changes and climate change. Particularly thermokarst processes – this refers to the rapid release and now also storage of carbon – have not been taken into account thus far by commonly applied models, such as those used by the IPCC,” explains Guido Grosse.
Source: Alfred-Wegener-Institute, www.awi.de