Permafrost controls the displacement rates of large unstable rock-slopes in subarctic environments

Determining the link between permafrost and the displacement rates of large unstable rock-slopes (LURSs) is fundamental for understanding future hazard scenarios and establishing appropriate management strategies. From an inventory of >500 LURSs in Norway, we investigate the controls of those with available information on their displacement rates (299 LURSs), presenting the first statistical evidence of permafrost as the main driver of displacement rates of LURSs. The probability for a LURS to displace if permafrost is present now or was during the Little Ice Age (LIA) is around 2.1 times higher than if permafrost was already absent during the LIA. This probability is 1.5 times higher for LURSs with current permafrost than for LURSs with permafrost during the LIA that has since melted. Therefore, our findings enrich the classical conception that warming of permafrost increases displacement rates of LURSs, by showing that the complete thawing of permafrost can result in a decrease in displacement rate or even a complete halt of displacement.