Evidence for warming and thawing of discontinuous permafrost in Alaska

Data show that permafrost temperatures along a north-south transect of Alas ka 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 tra nsect. Estimates of the magnitude of the warming at the permafrost table ra nged from 0.5 degrees C to 1.5 degrees C. Warming rates near the permafrost table were about 0.05 to 0.2 degrees C a(-1). No reliable trends in the de pth of the base of ice-bearing permafrost or in the depth of the 0 degrees C isotherm could be detected. Thermal offset allowed mean annual temperatur es at the permafrost table to remain below 0 degrees C with ground surface temperatures up to 2.5 degrees C for a period of 8 years. The observed warm ing 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 b ase are an order of magnitude smaller. Calibrated numerical models indicate that the permafrost warmed in the late 1960s and early 1970s in response t o changes in air temperatures and snow covers. Additional warming in the la te 1970s was caused by an increase in air temperatures beginning in 1977. P ermafrost 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 re gion of the transect and at Healy. Copyright (C) 1999 John Wiley & Sons, Lt d.