Yifeng Wang, Robert Way, Antoni Gerard Lewkowicz , Rosamond Tutton, Jordan Beer, Victoria Colyn, Anika Forget

Ground temperatures have been monitored since 2014 in four shallow boreholes (up to 5.7 m deep) drilled in palsas along the southeastern Labrador Sea coastline. This borehole network is critical for monitoring the effects of climate change on the terrestrial cryosphere and includes some of the southernmost coastal permafrost in the Northern Hemisphere. In this region, there are very few published measurements of active layer thickness, permafrost thickness, and permafrost temperatures. Mean annual ground temperatures of -1.7 to -0.7°C at 1 m depth were surprisingly low at the beginning of the study period, given the relatively thin bodies of permafrost present (<3 m thick). Statistically significant increases in ground temperatures were observed from 2015 to 2022 at some but not all depths in the four boreholes, despite decreases in permafrost thickness observed at all sites. Permafrost thaw resulted from both increased thaw penetration and thaw from the base of permafrost. Thaw penetration relative to the original ground surface increased by 24 to 92% due to a combination of active layer thickening and ground subsidence. Permafrost thaw at these sensitive locations may be driven by changes in mean annual air temperature, vegetation, snow dynamics, hydrology, and human disturbance. These data provide novel insights into the sensitivity of permafrost in this understudied region and can be used to validate predictive thermal modelling under future climate scenarios.