permafrost and climate change

permafrost and climate change Areas of permafrost are usually defined as those regions of the world in which ground temperatures below freezing have existed for a long time (from two years to tens of thousands of years). The thickest accumulations of permafrost in the world presently occur in north-eastern Siberia, where its thickness normally exceeds 800 m (locally reaching up to 1400 m). The growth of permafrost depends upon the radiative heat loss from the ground during winter being greater than the supply of heat to the ground surface during the summer months. Conversely, permafrost degradation may occur when summer heating is greater than winter heat loss. During summer, melting of the permafrost surface, usually to a depth of no more than 1–2 m, results in the development of an active layer, the base of which is known as the permafrost table. In the context of climate dynamics, areas of permafrost during winter are characteristically associated with the development of high pressure which occurs as a result of the cooling of the lower atmosphere by the ground surface. In contrast, areas of permafrost during summer are usually associated with the development of low air pressure caused by widespread melting of ice, moisture evaporation, and the convergence of air in the lower troposphere.

At present the most extensive areas of permafrost occur in Arctic Canada and in Russia. During ice ages the growth and thickening of permafrost principally took place in arctic and alpine areas that remained unglaciated. The large ice sheets that formed over Canada and Russia during the last ice age served to insulate the underlying ground surface and may even have promoted decay of permafrost beneath the ice sheet. Not surprisingly, therefore, the areas of the world where permafrost is now at its thickest coincide with those areas that are not only cold at present but also were not covered by ice during the last (and previous) glaciations.

A major feature of atmospheric circulation in Europe and North America during glacial periods was an enhanced zonal wind flow associated with the anchoring of a mid-latitude jet stream south of the major ice sheets. The easterly winds that were generated south of the ice-sheet margins, acting in conjunction with katabatic winds and low air temperatures, led to a negative heat budget at the ground surface and the consequent aggradation of permafrost.

Studies of permafrost history in Russia have provided valuable information on patterns of environmental change between the culmination of the last interglacial (125 000 years ago) and the last glacial maximum (18 000 years ago). Two distinct periods of permafrost growth have been recognized: the first cold period appears to have occurred more than 70 000 years ago; a second phase of permafrost growth culminated between 60 000 and 50 000 years ago. During these periods the southern limit of continuous permafrost may have been displaced 6 to 8° of latitude south of its present position reaching the centre of the Russian Plain, where mean annual ground temperatures may have been as low as −3 °C. The greatest expansion of permafrost, however, took place during the last glacial maximum between 24 000 and 18 000 years ago when permafrost may have been as much as 200 m in thickness in European Russia. The most severe climatic conditions and the thickest permafrost at this time was in north-eastern Siberia, where average annual ground temperatures may have fallen below −15 °C.

The evolution of permafrost in response to climate warming and cooling is relatively slow. For example, during the Late Quaternary, changes in permafrost thickness took place at a much slower rate than the growth and decay of ice sheets and sea-level changes. Thus, extensive areas of relict permafrost occur beneath sea level on the sea floor of the Arctic continental shelf as a result of the rise in sea level that has taken place during the last 15 000 years. The existence of certain permafrost regions out of phase with the Earth's present climate makes it difficult to assess the response of permafrost to the present global warming. Changes in permafrost thickness arising from global warming reflect changes in the annual heat balance for particular areas. Surprisingly, there have been relatively few studies of the influence of global warming on permafrost. One might expect, for example, that permafrost thinning as a result of increased air temperatures could result in a transfer of melt water to the world's oceans and a consequent rise in sea level. Yet such analysis has not so far been undertaken and is a notable omission from the Intergovernmental Report on Climate Change. Similarly, permafrost degradation, despite having been described as presently taking place, may also play an important role in the modification of regional climates as a result of changes in rates of air convergence in the lower troposphere during the summer months. It remains to be discovered by the scientific community whether or not the rapid changes in air temperatures experienced over the past two decades have had a profound effect on the stability of permafrost and of the climate over these particular areas.

Alastair G. Dawson

Bibliography

French, H. M. (1976) The periglacial environment. Longman, London.
Washburn, A. L. (1979) Geocryology. Edward Arnold, London.
Péwé, T. L. (1983) The periglacial environment in North America during Late Quaternary time. In S. C. Porter (ed.) Late Quaternary environments of the United States, Vol. 1. The Late Pleistocene, pp. 157–89. Longman, Harlow.