Permafrost is the condition of a soil that has been permanently frozen to a depth of several metres since the last glacial period about 20,000 years ago. The largest permafrost regions are locat- ed in the Arctic areas of Alaska, Canada, Siberia and Scandinavia. In all, permafrost covers almost a quarter of the total land mass of the northern hemisphere. Although these areas are very remote and only sparsely settled, the permafrost is of global importance because, like a giant deep freeze, it conserves massive amounts of dead biomass, especially plant material. A critical current problem is accelerated thawing of perma-frost due to climate change. Previously conserved biomass is now being made available for degradation by microorganisms. The metabolism of the microorganisms, however, produces the greenhouse gases carbon dioxide and methane, whereby the amount produced depends upon various factors. One of these is the form in which the carbon is bound up within the biomass and another is the favourability of environmental conditions for the micro-organisms. Carbon compounds bound up in the biomass are either stable or labile compounds. In wood pulp lignin, for example, the carbon bonds are very stable, so wood degrades very slowly microbiologically. This will remain the case in the future under cold Arctic conditions because at sub-freezing temperatures the microorganisms are very weakly or not at all active. Labile compounds like plant tissue that were frozen during the glacial period could virtually be degraded immediately. How rapidly biomass will be degraded in the coming years due to permafrost thawing has not yet been conclusively determined. The appearance of many ­thawed permafrost areas today is similar to moorlands, with standing water on the surface. Because of the low oxygen content of water in boggy soils, however, biomass is only weakly degraded. This is the reason that historical wooden objects or animal pelts remain well preserved in moors. Thus, the questions of whether and to what extent the thawing permafrost will release greenhouse gases are likewise still open. It is quite obvious today, however, that thawing is causing a reduction of permafrost on the coasts. This is releasing more biomass, which is becoming available to microorganisms. One factor is that summers in the Arctic are becoming longer as a result of global warming. The ground thaws earlier in the year and freezes later. The waves thus have a longer time window to erode the permafrost. Another factor is the shrinking ice cover in the Arctic Ocean, which promotes an increased intensity of the waves attacking the coasts. At some locations the permafrost grounds are breaking off at a rate of 20 metres per year.

fig. 1.21 > The Siberian island of Muostakh exhibits a permafrost coast that is increasingly susceptible to erosion due to global warming.

Permafrost thawing is also a problem for local human populations. In Alaska many Inuit are losing their ancestral homes on the sea. According to reports by the U. S. Government Accountability Office (GAO) many villages are threatened by the accelerated ­melting of permafrost and loss of coastal land. Presumably, these villages will have to be abandoned in the future. At a meeting in August 2016, for instance, the community of Shishmaref decided to relocate to a safer site on the mainland that is yet to be determined. The village of 600 residents is located on an island in the Bering Strait off the coast of Alaska that has long been inhabited by the Inuit. Around 30 metres of shoreline have been lost over the past 20 years due to the thawing of permafrost. 13 houses have had to be dismantled and rebuilt. Although breakwaters were constructed to protect the island, they have not been able to stop the loss of land. Specialists estimate that relocation to the mainland will cost around 180 million US dollars. It has not yet been determined who will bear the cost.