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Climate warming in regions of ice-rich permafrost can result in widespread thermokarst development, which reconfigures the landscape and damages infrastructure. We present multisite time series observations which couple ground temperature measurements with thermokarst development in a region of very cold permafrost. In the Canadian High Arctic between 2003 and 2016, a series of anomalously warm summers caused mean thawing indices to be 150-240% above the 1979-2000 normal resulting in up to 90 cm of subsidence over the 12-year observation period. Our data illustrate that despite low mean annual ground temperatures, very cold permafrost (<-10 degrees C) with massive ground ice close to the surface is highly vulnerable to rapid permafrost degradation and thermokarst development. We suggest that this is due to little thermal buffering from soil organic layers and near-surface vegetation, and the presence of near-surface ground ice. Observed maximum thaw depths at our sites are already exceeding those projected to occur by 2090 under representative concentration pathway version 4.5.

Plain Language Summary Permafrost is ground that remains at or below 0 degrees C for two years or longer and it underlies much of the Arctic. Permafrost in Arctic lowland regions is frequently characterized by large volumes of ground ice which, when it melts, causes the ground surface to collapse. As the Arctic warms, ice-rich permafrost degradation is expected to be widespread. Our data illustrate that very cold permafrost, which has a mean annual ground temperature of -10 degrees C or lower, is experiencing a rapid increase in active layer thickness at annual time scales. At three permafrost monitoring sites in the Canadian Arctic we have observed that warmer than average summer air temperatures have caused the active layer to deepen, near-surface ground ice to melt, and the overlying ground surface to subside, in some cases leading to the formation of small thaw ponds. Our results show that very cold permafrost terrain is responding rapidly to ongoing warming.