Evidence for warming and thawing of discontinuous permafrost in Alaska

Data show that permafrost temperatures along a north-south transect of Alaska from Old Man to Gulkana and at Healy generally warmed in the late 1980s to 1996. This trend was not followed at Eagle, about 330 km east of the transect. Estimates of the magnitude of the warming at the permafrost table ranged from 0.5°C to 1.5°C. Warming rates near the permafrost table were about 0.05 to 0.2°C a-1. No reliable trends in the depth of the base of ice-bearing permafrost or in the depth of the 0°C isotherm could be detected. Thermal offset allowed mean annual temperatures at the permafrost table to remain below 0°C with ground surface temperatures up to 2.5°C for a period of 8 years. The observed warming has probably caused discontinuous permafrost in marginal areas to begin thawing. Thawing permafrost and thermokarst have been observed at several sites. Thawing rates at the permafrost table at two sites were about 0.1 m a-1, indicating time scales of the order of a century to thaw the top 10 metres of ice-rich permafrost. Calculated thawing rates at the permafrost base are an order of magnitude smaller. Calibrated numerical models indicate that the permafrost warmed in the late 1960s and early 1970s in response to changes in air temperatures and snow covers. Additional warming in the late 1970s was caused by an increase in air temperatures beginning in 1977. Permafrost temperatures were nearly stable during the 1980s and then warmed again from the late 1980s to 1996, primarily in response to increased snow depths. This interpretation appears to be valid for all the sites in the region of the transect and at Healy.