Active Layer Thickness in the Northern Hemisphere: Changes From 2000 to 2018 and Future Simulations

Active layer dynamics are basic information for understanding permafrost degradation, but there are gaps in our knowledge of the active layer thickness (ALT) variations during the past two decades and their future changes; this is especially true for data with high spatial resolution. Here, based on permafrost monitoring data and ERA5 (the 5th generation reanalysis project from the European Centre for Medium-Range Weather Forecasts)-Land temperature data, we simulated spatial changes in ALT in the Northern Hemisphere during 2000-2018 at a spatial resolution of 1 km(2) using the Stefan model. We also simulated future changes in the active layer based on the CMIP6 temperature data. The results showed that ALTs less than 50 cm were mainly distributed in northeastern Siberia in Russia, Alaska and Greenland. The areas with ALTs greater than 600 cm were mainly in southern Norway, and the Mongolian Plateau. The mean ALT of permafrost in the Northern Hemisphere from 2000 to 2018 increased from 127.19 to 145.37 cm at a linear rate of 0.65 cm year(-1) (R-2 = 0.37, p < 0.05). During 2000-2018, the ALT increased in 71.17% of the total permafrost area, and only 27.23% of the permafrost area showed a decreasing trend. Under different shared socioeconomic pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5), the ALT will increase considerably by 2,100. There is a greater increase in ALT in low- and mid-latitude permafrost areas such as the Mongolian Plateau and Qinghai-Tibet Plateau, with the ALT increasing from 50 to 500 cm. Our results present an overall picture of contemporary distribution as well as future changes of ALT in the Northern Hemisphere.