Increasing extreme heat events in the permafrost region of the Northern Hemisphere

The intensification of extreme heat events is a potent thermal disturbance that can trigger abrupt permafrost degradation. However, a systematic understanding of their spatiotemporal variation across the permafrost region of the Northern Hemisphere (PRONH) is lacking, hindering predictions of regional-scale responses and climate feedback tipping points. In this study, six indices are systematically employed to analyse the historical spatiotemporal variations (1991‒2020) of extreme heat events in the PRONH and to project their future changes (2021‒2100) under Shared Socioeconomic Pathway (SSP) scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5), particularly across four permafrost types. The results indicate that the increasing trends of warm day (TA95p; 2.95 ± 0.56 d per decade), warmer day (TX95p; 3.20 ± 0.59 d per decade), warmest day (TXx; 0.35 ± 0.12 °C per decade), heatwave intensity (2.67 ± 0.96 °C per decade), heatwave frequency (0.23 ± 0.05 events per decade) and heatwave duration (0.54 ± 0.44 d per decade) were significant ( p < 0.05) during 1991‒2020. Under SSP5-8.5, the Arctic and Tibetan Plateau are projected to experience 150–200 d TX95p annually, making such events routine by the late 21st century (2076‒2100). Under the same scenario, continuous permafrost regions are projected to face the most severe exposure, with TX95p reaching 148 ± 24 d annually, whereas the discontinuous (134 ± 23 d), sporadic (130 ± 22 d) and isolated (109 ± 19 d) permafrost regions are expected to experience fewer extreme heat events. However, their fragmented distributions render them particularly vulnerable and heighten the risk of degradation. This study underscores the urgent need to integrate these extreme heat events into permafrost vulnerability assessments and climate adaptation strategies.