| Summary: | Active‐layer studies have been maintained for more than a decade throughout the 1200 km length of the Mackenzie Valley. An important goal of these programmes is to monitor processes linking climate, permafrost and the active layer. Thirteen sites have been selected from these studies for the Circumpolar Active Layer Monitoring programme of the International Permafrost Association. Annual maximum thaw penetration and ground surface movements are measured relative to thaw tubes and soil temperature probes anchored in the permafrost. Active layer thickness, calculated from thaw penetration and surface movement or measured directly, varies more with local soil properties, vegetation and microclimate than with regional atmospheric climate. While depth of thaw penetration has increased at most sites for much of the record, this increase is not always reflected by a proportional increase in active layer thickness because of variable thaw settlement. Air thawing degree‐days (DDT) in the Subarctic and Boreal regions are more than double those in the Arctic regions. These increases in DDT are not reflected in active‐layer depths, suggesting the insulating effect of the surface organic layer, vegetation and snow cover on active‐layer development. Copyright © 2004 John Wiley & Sons, Ltd.
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