Permafrost - An introduction
For decades, huge expanses of land at the higher latitudes, which sit atop frozen soil – referred to as permafrost – have been thawing. When the subterranean organic matter, which accumulated over thousands of years in frozen soils, thaws and decomposes eventually, massive quantities of greenhouse gases can be released into the atmosphere, intensifying global warming. As such, the thawing of permafrost regions not only has local effects, but impacts the climate as a whole and people around the globe. However, the numerous interactions between permafrost soil and its surroundings make it difficult to accurately assess the risk – which is why researchers from the Alfred Wegener Institute embark on expeditions to the polar regions every year in order to better understand the complex processes at work in the permafrost landscapes, and to assess how likely it is to decay in the future.
What is permafrost?
Researchers use the term permafrost, or permanently frozen ground, when the temperature of the ground remains under zero degrees Celsius for at least two consecutive years. The material can consist of rock, sediment or soil, and can contain varying quantities of ice. In some regions of the Arctic, the makeup is 70 percent ice. Especially Northeast Siberia experienced extremely long and cold winters during the last ice ages, lasting from about 100,000 to 10,000 years ago. At the same time, the ground there was not protected by an ice sheet, and cold air deeply penetrated into the ground. As a result, the permafrost in this region reaches deep into the Earth – extending as far as 1.7 kilometres down. Most permafrost landscapes can be recognised by the typical patterning of their surface, for example polygons, formed by repeated deep freezing in winter. The very cold Arctic winter temperatures cause the frozen soils to contract across the land surface, resulting in a regular pattern of cracks much like drying cracks. Subsequently, the centimetre-wide and meter-deep cracks are then filled with snow melt water during the spring thaw. Thanks to the soil’s intense cold, the water then refreezes, creating vertical veins of ice that grow over decades to millennia into ice wedges.
What is the structure of permafrost soil?
Permafrost typically is overlain by an active layer, circa 15 - 100 centimetres of which thaw every summer. In this thin layer, the majority of biological and biochemical activity happens in Arctic soils. Researchers regularly measure the temperatures of the active layer and the frozen layers beneath and track the boundary – the thaw depth - between them, saving their findings in the database of the Global Terrestrial Network for Permafrost (gtnp.org).. While the thaw depth offers insights into short-term climate fluctuations, the temperatures of the permafrost’s lower layers reflect longer-term climate changes, making it possible to gauge the effects of global warming on the polar and mountainous regions.
Where can permafrost be found?
Permafrost is much more common than generally assumed – and underlies roughly a quarter of the land area in the Northern Hemisphere. Though the majority is in the Polar Regions, it can also be found in high mountain ranges. As such, there is even alpine permafrost in Germany, namely on the Zugspitze. Depending on the areal extent, a distinction is made between continuous permafrost in colder regions, where at least 90 per cent of the land is underlain by permafrost; and discontinuous and sporadic permafrost in warmer regions, where the number lies between 10 and 90 per cent. There are also isolated “permafrost islands”, which often exist underneath boreal peatlands.
What happens when permafrost thaws?
Just like a gigantic freezer, permafrost stores tremendous amounts of organic matter. Unlike in tropical or moderate climate zones, this organic material can’t be broken down by microbes, since these bacteria only become active when the permafrost thaws. But if our climate continues to grow warmer, the door of that gigantic freezer is left open and the organic matter starts to decompose - carbon will be broken down and be released into the atmosphere as greenhouse gas, which will in turn accelerate the climate-warming process. The permafrost carbon feedback would affect the global climate system as a whole. Further, melting of ground ice present in permafrost can have drastic consequences for arctic landscapes and settled areas, because the land surface settles unevenly where ice turns to water and streets, railroad tracks, runways, buildings, and oil and gas pipelines become damaged.
Unfortunately, predicting the physical and biochemical development of permafrost is an extremely complex undertaking. Many surface characteristics change simultaneously, like snow cover and vegetation, which produces varied and often opposing effects. Further, human beings are increasingly interfering in landscape development. As such, predictions still involve a great deal of uncertainty. However, studies of past climate change such as during the period immediately after the last ice age when the Arctic warmed very rapidly have shown that permafrost was dramatically affected by warming, suggesting that permafrost may not be so permanent at last.
How vulnerable is the permafrost?
Just like a gigantic freezer, permafrost stores tremendous amounts of organic matter. Unlike in tropical or moderate climate zones, this organic material can’t be broken down by microbes, since these bacteria only become active when the permafrost thaws. But if our climate continues to grow warmer, the door of that gigantic freezer is left open and the organic matter starts to decompose - carbon will be broken down and be released into the atmosphere as greenhouse gas, which will in turn accelerate the climate-warming process. The permafrost carbon feedback would affect the global climate system as a whole. Further, melting of ground ice present in permafrost can have drastic consequences for arctic landscapes and settled areas, because the land surface settles unevenly where ice turns to water and streets, railroad tracks, runways, buildings, and oil and gas pipelines become damaged.
Unfortunately, predicting the physical and biochemical development of permafrost is an extremely complex undertaking. Many surface characteristics change simultaneously, like snow cover and vegetation, which produces varied and often opposing effects. Further, human beings are increasingly interfering in landscape development. As such, predictions still involve a great deal of uncertainty. However, studies of past climate change such as during the period immediately after the last ice age when the Arctic warmed very rapidly have shown that permafrost was dramatically affected by warming, suggesting that permafrost may not be so permanent at last.
(Boris Biskaborn)
