Recent Permafrost Retreat Is Likely Unprecedented Over the Common Era

Permafrost, a defining feature of high-latitude landscapes, has undergone rapid degradation in recent decades as a result of ongoing global warming. One of the clearest indicators of this process is the loss of permafrost area. While recent retreat is well documented, few studies have placed changes in permafrost extent within the context of pre-industrial natural variability. Here, we show that recent permafrost retreat is very likely unprecedented over the last 2,000 years, the Common Era (CE). To do so, we derive permafrost area estimates using a probabilistic model that relates mean annual air temperatures to the likelihood of permafrost occurrence (PROB). We apply PROB to all available MAAT gridded data sources spanning all or part of the Common Era, including simulations from the Paleoclimate Model Intercomparison Project Phases 3 and 4 (PMIP3 and PMIP4), reanalyses, and reconstruction products. Across datasets, we estimate a permafrost area decrease of 0.7 to 2.5 million km², with the largest losses occurring along the southern margins of discontinuous permafrost.

A direct consequence of permafrost retreat is an increase in the amount of soil organic carbon that is available for microbial decomposition. This work shows that recent permafrost retreat would account for 43 and 138 PgC of newly vulnerable soil organic carbon. The resulting carbon emissions may have already contributed up to 0.2 °C of additional global warming, with important implications for the pan-Arctic environment and the stability of the global climate system. Our results highlight the value of Earth system model simulations for assessing permafrost variability over the Common Era, particularly when interpreted alongside proxy-based paleoclimate reconstructions.