| Summary: | The geothermal record for 1977–2014 from a 29 m deep borehole in permafrost on Mont Jacques‐Cartier, in southeastern Canada, shows substantial decadal fluctuations and an overall warming trend. An extremely thin winter snow cover on the wind‐blown summit favours the presence of permafrost. As a consequence, the instability of the thermal regime was found to be a direct response to air temperature variations modelled from data produced by the National Center for Environmental Prediction and National Center for Atmospheric Research. At a depth of 14 m, an increase of 0.4 °C from 1979 to 1984 was followed by a decrease of 0.7 °C over the next decade, and then by a marked, but irregular increase of 1 °C up to 2013. Since 2008, diurnal data, refined by a one‐dimensional, transient heat transfer model, indicate an active layer averaging 8.6 m in depth, but whose thickness is sensitive to fluctuations in annual mean ground surface temperatures. For a permafrost body already close to the thawing point, the continuation of the overall warming trend of the last 37 years would lead to its rapid degradation, and the permafrost would then become relict, thinning progressively both from the base and the surface. Copyright © 2016 John Wiley & Sons, Ltd.
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