Half a century of discontinuous permafrost persistence and degradation in western Canada

Long-term field studies of permafrost change are needed to validate predictive models but few are possible because of a paucity of direct observations prior to the late 1970s. To help fill this knowledge gap, we resurveyed a transect of 68 sites, originally investigated in 1962, to evaluate change in the isolated patches and sporadic discontinuous permafrost zones between Keg River, Alberta (57.8 degrees N) and Hay River, Northwest Territories (60.8 degrees N). The goal was to establish the degree of permafrost degradation due to approximately 2 degrees C of regional climate warming over the intervening 55 years, compounded at some sites by forest fire. By 2017-2018, permafrost had degraded at 36% of the 44 sites which exhibited it in 1962, but had persisted at a minimum of 50% with a further 14% potentially retaining permafrost. This is much less degradation than reported for a 1988-1989 survey of the same transect. Permafrost was maintained under thicker organic layers (86% > 40 cm) and at the majority of sites with fine-grained substrates, while degradation occurred preferentially at sites with coarse soils and thinner organic layers. Forest fire did not enhance the degree of permafrost loss, but greater frost table depths were observed at some burned locations. This study demonstrates that while the trajectory of change is towards permafrost loss, thin permafrost in the discontinuous zone can be persistent, even when disturbed. It also underlines the importance of considering the range of landscape types when projecting the rate of future permafrost thaw.