Recent Warming of European Permafrost: Evidence from Borehole Monitoring

Charles Harris
School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff, CF10 3YE UK
Ketil Isaksen
Norwegian Meteorological Institute, P.O.Box 43 Blindern, NO-0313 Oslo, Norway

Abstract

Here we present a review of recent ground thermal data derived largely from the continent-scale network of instrumented boreholes within mountain permafrost established between 1998 and 2001 by the European Union PACE project. More recently, networks of intermediate and shallow boreholes in Switzerland, Norway, and Iceland have been added. A large number of complex variables determine permafrost temperatures, including altitude, topography, net radiation, and snow distribution. Thus, modeling the above-ground climate signal from observations of permafrost temperatures and coupling downscaled climate models to assess future permafrost thermal responses to climate forcing remain major research goals. Boreholes drilled in areas of steep mountain topography may penetrate complex three-dimensional thermal fields, making interpretation of thermal profiles in terms of changes in the upper thermal boundary extremely challenging. However, in the lower relief settings of the Scandinavian and Svalbard PACE boreholes, observed warm-side deviation in thermal profiles strongly suggests a period of sustained surface warming in the latter half of the 20th century and in the early 21st century. The significance of short-term extreme thermal events is illustrated with reference to the record-breaking summer of 2003 in the Alps and the anomalously warm winter-spring-summer period in 2005-2006 in Svalbard. It is concluded that such events may initially be more significant than the longer-term underlying trends in climate. Permafrost thermal responses to climate change occur at markedly different time scales, with changes in active layer thickness being more or less immediate, modification of thermal profiles below the depth of zero amplitude taking decades or centuries, and basal melting associated with progressive permafrost thinning requiring millennial time scales. However, as illustrated here by the example of Icelandic permafrost, where the frozen ground layer is thin and warm and the geothermal heat flux rates are high, permafrost decay and disappearance may be much more rapid.

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