The evolution of Arctic permafrost over the last three centuries

Understanding the future evolution of permafrost requires a better understanding of its climatological past. This requires permafrost models to efficiently simulate the thermal dynamics of permafrost over the past centuries to millennia, taking into account highly uncertain soil and snow properties. In this study, we present a computationally efficient numerical permafrost model which satisfactorily reproduces the current thermal state of permafrost in the Arctic and its recent trend over the last decade. Also, the active layer dynamics and its trend is realistically captured. The performed simulations provide 5 insights into the evolution of permafrost since the 18th century and show that permafrost on the North American continent is subject to early degradation, while permafrost on the Eurasian continent is relatively stable over the investigated 300-year period. Permafrost warming since industrialization has occurred primarily in three “hotspot” regions in northeastern Canada, northern Alaska, and, to a lesser extent, western Siberia. The extent of near-surface permafrost has changed substantially since the 18th century. In particular, loss of continuous permafrost has accelerated from low ( − 0 . 10 × 10 5 km 2 dec − 1 ) to moderate 10 ( − 0 . 77 × 10 5 km 2 dec − 1 ) rates for the 18th and 19th centuries, respectively. In the 20th century, the loss rate nearly doubled ( − 1 . 36 × 10 5 km 2 dec − 1 ), with the highest near-surface permafrost losses occurring in the last 50 years. Our simulations further indicate that climate disturbances due to large volcanic eruptions in the Northern Hemisphere, can only counteract near-surface permafrost loss for a relatively short period of a few decades. Despite some limitations, the presented model shows great potential for further investigation of the climatological past of permafrost, especially in conjunction with paleoclimate 15 modeling. its current warming rate. Taking into account possible biases caused by neglecting subgrid variations in topography due to coarse spatial resolution of the climate data used, even higher accuracies are obtained (RMSE= 1 . 74 K ). The model is also able to provide a realistic estimate of Active Layer Thickness (ALT), especially in the 430 zone of continuous permafrost. Large uncertainties in ALT simulations are found in the zones of discontinuous to isolated permafrost attributed to uncertain groundwater and ground ice contents. The simulations performed show spatially heterogeneous warming of the permafrost during the last 250 years. Three “hotspot” regions characterized by particularly strong warming (> 1 K ) since industrialization (1900) were identified. Changes in ALT are simulated to occur mainly along the boundary between continuous and discontinuous permafrost. In particular, permafrost on the North American continent has been affected by a substantial increase in ALT (>100%) since industrialization whereas much smaller changes in ALT are simulated for the Eurasian Arctic permafrost. Generally, the North American continent is characterized by intense permafrost thaw while Eurasian permafrost appears to have been less affected over the past 250 years.