Permafrost: Not Quite So Permanent

05 Jul 2005 - Articles, Land & Geology, Ice & Snow, Arctic, Antarctic

Cross-section view of permafrost

Permafrost is soil (and/or rock) that remains below zero degrees Celsius year-round for at least two consecutive years. It is mostly found in the polar regions and in mountain ranges at high altitude. In recent decades, with global warming, permafrost has decreased in the Arctic and across the world's mountain ranges. This has had a significant impact on natural features and man-made structures.

Despite its name, permafrost is characterized to a certain extent by instability. It is covered by a layer that freezes and thaws with the seasons (the "active layer") and although it can be thousands of years old, it often exists close to melting point. Also, depending on its location and landscape, permafrost can be made up of anything from virtually no ice to much more than 50% ice. It can be covered by snow (to several metres) or without snow at all. Its thickness and distribution varies depending on atmospheric temperature at ground level and on other factors such as vegetation type and density, snow cover, drainage and soil type.

Permafrost represents up to a quarter of the northern hemisphere's land surface

In the Arctic, permafrost occurs on land and on offshore continental shelves. It ranges in thickness from less than 1 metre to more than 1000 metres. The thicker the permafrost, the more stable it is as seasonal fluctuations in temperature are smaller.

Depending on calculation methods, permafrost represents between an eighth and a quarter of the land surface of the northern hemisphere. By contrast, ground which freezes for only part of the year (ie subject to seasonal frost) represents some 55 percent of the northern hemisphere's land surface.

Permafrost occurs mostly at latitudes higher than 60°N, but can be found high up in the mountains in lower latitudes. It even occurs in equatorial regions, in the Andes or on Kilimanjaro, although only at heights over around 5000 metres.

In contrast with the Arctic, information about the permafrost of Antarctica and the sub-Antarctic islands is somewhat limited. It is clear though that several aspects of Antarctic permafrost are different from the northern hemisphere. In some places Antarctic permafrost is much older and there is extensive "dry" permafrost (ie little ice is contained in the soil).

Full credit for this picture/map to UNEP/GRID-Arendal, Philippe Rekacewicz.

Permafrost and global warming

Permafrost is being significantly affected by global warming and the impact on permafrost has the potential to accelerate further the warming phenomenon.

With average temperatures rising, permafrost is currently on the retreat in the Arctic regions, from northern Canada to Siberia. This is destabilising both natural features and man-made structures as the previously frozen-solid ground gradually becomes soft, damp or waterlogged. Permafrost warming is also increasing the incidence of rockfall (and rockslide) activity in mountain ranges the world over as the cementing effect of ice within soil and rock structures decreases with increasing ice temperature.

According to the recent Arctic Climate Impact Assessment (ACIA) and the Global Terrestrial Network for Permafrost (GTNP), thawing permafrost is also contributing to an increasing rate of erosion in coastal areas in the Arctic region, with the result that some coastal communities are already being forced to relocate.

Perhaps most importantly, Arctic permafrost is estimated to contain up to some 14% of the Earth's stored soil carbon. The annual thawing of the "active layer" allows vegetation to grow on otherwise frozen ground but residual dead vegetation does not fully decompose as is the case in other climes. This vegetation is absorbed by the active layer and frozen, slowing decay considerably. Over time significant quantities of vegetation are in fact frozen solid and their stored carbon suspended in the permafrost. Thawing would mean considerably greater decomposition of the stored vegetation and consequent release of significant quantities of carbon dioxide and particularly methane (a more potent greenhouse gas).

There is a complex equation to be considered, however: the release of carbon would also stimulate additional plant growth. The concern of many scientists is that although some of the carbon released would be absorbed by greater plant growth there would, nonetheless, be a net carbon increase, contributing to a dangerous vicious circle of further temperature rise and consequent carbon release.

By: Jean de Pomereu