Forced and Unforced Permafrost Changes in the Northern Hemisphere during 1901-2100

Permafrost regions are very sensitive to rapid changes in climate and environment. In recent decades, there has been growing interest to better understand the permafrost degradation over the Northern Hemisphere in the context of human-induced climate change. Understanding permafrost dynamics is not only important for infrastructure but also for environmental protection in cold regions. In-situ permafrost measurements are important for assessing permafrost conditions. However, direct permafrost observations are sparse and asymmetrical in both spatial and temporal coverage. Active layer thickness (ALT) modeling is another approach that can overcome many of these limitations, but the models have large uncertainty in predicting future permafrost changes.This doctoral research firstly investigated the impacts of climate forcings on active layer thickness changes over Alaska since 1990 based on data derived from the in-situ Circumpolar Active Layer Monitoring (CALM) measurements. The results suggested that changes in ALT over Alaskan permafrost regions were not only controlled by warming in regional temperature but were also influenced by large-scale atmospheric and oceanic forcings, particularly in the North Atlantic and North Pacific. Meanwhile, an obvious gap was found between simulated and observed active layer thickness, indicating potential uncertainty in model simulations. To quantify these uncertainties, detection and attribution analyses were applied based on simulations made with CESM-LENS and CMIP6 models across the permafrost regions in the northern hemisphere. For a single model (CESM), the multiple ensemble simulations average showed that the deepening ALT during the historical period was generally small over most of the Northern Hemisphere permafrost zone, except in the western Siberia, Mongolia, and portions of the Canadian Arctic. The deepening trends in ALT are projected to increase under the most extreme RCP8.5 scenario and would be two to four times greater than the observed historical trend in ALT. The further evaluation suggested that the CESM model underestimates the ALT changes induced by anthropogenic forcing and overestimates the relative contribution from internal climate variability. When multiple CMIP6 models were analyzed, internal variability plays a minor role compared with anthropogenic forcing over the permafrost region as a whole, especially in a future warmer world. Model uncertainty is the dominant contributor during the historical period, and it is still a considerable source of uncertainty in future projections. The scenario uncertainty becomes increasingly important near the end of the 21st century. These results highlight the need to improve permafrost model physics to reduce the uncertainty of future permafrost projections.

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