Characteristics of the recent warming of permafrost in Alaska

Tentative answers are provided to questions concerning the recent warming of permafrost in Alaska, particularly those regarding timing, duration, magnitude, spatial distribution, seasonality, active layer effects, thawing, thermokarst terrain, and causes. Permafrost warmed at most sites north of the Brooks Range from the Chukchi Sea to the Alaska‐Canada border, south along a transect from Prudhoe Bay to Gulkana and at sites up to 300 km from the transect. The warming was coincident with the statewide warming of air temperatures that began in 1976/1977 and appears to have occurred statewide with some exceptions. Magnitude of the warming was 3 to 4°C for the Arctic Coastal Plain, 1 to 2°C for the Brooks Range including its northern and southern foothills, and 0.3 to 1°C south of the Yukon River. This suggests a total warming of >6°C at Prudhoe Bay during the last century. The warming was seasonal (primarily in winter) with little change in summer conditions. Consequently, active layer thicknesses did not increase and were not correlated with warming permafrost conditions. Natural thawing at the permafrost surface (∼0.1 m/yr) occurred at both a tundra and forest site. Basal thawing at one site was ∼0.04 m/yr until 2000 when it accelerated to ∼0.09 m/yr. New thermokarst terrain has been observed in interior and northern Alaska. Probable causes of the warming include increased air temperatures, snow cover effects, and combinations of these. New investigations are needed to further determine the characteristics, especially the causes, of this recent permafrost warming.