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Variability of active layer thickness (ALT) in permafrost regions is critical for assessments of climate change, water resources, and engineering applications. Detailed knowledge of ALT variations is also important for studies on ecosystem, hydrological, and geomorphological processes in cold regions. The primary objective of this study is therefore to provide a comprehensive 1971-2000 climatology of ALT and its changes across the entire Northern Hemisphere from 1850 through 2100. To accomplish this, in situ observations, the Stefan solution based on a thawing index, and the edaphic factor (E factor) are employed to calculate ALT. The thawing index is derived from (i) the multimodel ensemble mean of 16 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) over 1850-2005, (ii) three representative concentration pathways (RCP2.6, RCP4.5, and RCP8.5) for 2006-2100, and (iii) Climatic Research Unit (CRU) gridded observations for 1901-2014. The results show significant spatial variability in in situ ALT that generally ranges from 40 to 320 cm, with some extreme values of 900 cm in the Alps. The differences in the ALT climatology between the three RCPs and the historical experiments ranged from 0 to 200 cm. The biggest increases, of 120-200 cm, are on the Qinghai-Tibetan Plateau, while the smallest increases of less than 20 cm are in Alaska. Averaged over all permafrost regions, mean ALT from CMIP5 increased significantly at 0.57 +/- 0.04 cm decade(-1) during 1850-2005, while 2006-2100 projections show ALT increases of 0.77 +/- 0.08 cm decade(-1) for RCP2.6, 2.56 +/- 0.07 cm decade(-1) for RCP4.5, and 6.51 +/- 0.07 cm decade(-1) for RCP8.5.