Duration of frozen days show a strong decline in the Northern Hemisphere mainly driven by autumn temperature increase

Thawing permafrost releases methane and carbon dioxide to the atmosphere, contributing to positive feedback loop in global warming. Therefore, accurately monitoring changes in the permafrost freeze–thaw status is imperative. However, the spatiotemporal evolution and potential driving factors remain elusive. Here, we investigated the freeze–thaw status and driving factors by developing novel machine learning models trained on satellite and in situ observations in the Northern Hemisphere. We find that the frozen duration decreased on average by 0.17 days/yr since 1990 with the highest decrease of approximately up to 1.0 days/yr in parts of Belarus and Ukraine, followed by the Yukon region in Canada and Alaska. This decrease is primarily driven by temperatures in boreal autumn and spring and by precipitation and vegetation cover in boreal spring. The frozen duration is projected to decline further with reduction rates doubling until 2050 for the highest and moderate emission scenarios.