Permafrost warming has the potential to amplify global climate change, because when frozen sediments thaw it unlocks soil organic carbon. Yet to date, no globally consistent assessment of permafrost temperature change has been compiled. Here we use a global data set of permafrost temperature time series from the Global Terrestrial Network for Permafrost to evaluate temperature change across permafrost regions for the period since the International Polar Year (2007–2009). During the reference decade between 2007 and 2016, ground temperature near the depth of zero annual amplitude in the continuous permafrost zone increased by 0.39 ± 0.15 °C. Over the same period, discontinuous permafrost warmed by 0.20 ± 0.10 °C. Permafrost in mountains warmed by 0.19 ± 0.05 °C and in Antarctica by 0.37 ± 0.10 °C. Globally, permafrost temperature increased by 0.29 ± 0.12 °C. The observed trend follows the Arctic amplification of air temperature increase in the Northern Hemisphere. In the discontinuous zone, however, ground warming occurred due to increased snow thickness while air temperature remained statistically unchanged.

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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.

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The formation and evolution of permafrost in China during the last 20 ka were reconstructed on the basis of large amount of paleo-permafrost remains and paleo-periglacial evidence, as well as paleoglacial landforms, paleo-flora and paleofauna records. The results indicate that, during the local Last Glacial Maximum (LLGM) or local Last Permafrost Maximum (LLPMax), the extent of permafrost of China reached 5.310 6 km 2 5.410 6 km 2 , or thrice that of today, but permafrost shrank to only 0.8010 6 km 2 0.8510 6 km 2 , or 50% that of present, during the local Holocene Megathermal Period (LHMP), or the local Last Permafrost Minimum (LLPMin). On the basis of the dating of periglacial remains and their distributive features, the extent of permafrost in China was delineated for the two periods of LLGM (LLPMax) and LHMP (LLPMin), and the evolution of permafrost in China was divided into seven periods as follows: 1) LLGM in Late Pleistocene (ca. 20,000  13,000-10,800 a BP) with extensive evidence for the presence of intensive permafrost expansion for outlining its LLPMax extent; 2) A period of dramatically changing climate during the early Holocene (10,800  8,500-7,000 a BP) when permafrost remained relatively stable but with a general trend of shrinking areal extent; 3) The LHMP in the Mid-Holocene (8,500-7,000  4,000-3,000 a BP) when permafrost degraded intensively and extensively, and shrank to the LLPMin; 4) Neoglaciation during the late Holocene (4,000-3,000  1,000 a BP, when permafrost again expanded; 5) Medieval Warming Period (MWP) in the late Holocene (1,000  500 a BP) when permafrost was in a relative decline; 6) Little Ice Age (LIA) in the late Holocene (500  100 a BP), when permafrost relatively expanded, and; 7) Recent warming (during the 20 th century), when permafrost continuously degraded and still is degrading. The paleo-climate, geography and paleopermafrost extents and other features were reconstructed for each of these seven periods.

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To investigate and monitor permafrost in the Bayan Har Mountains (BHM), north‐eastern Qinghai–Tibet Plateau, southwest China, 19 boreholes ranging from 20 to 100 m in depth were drilled along an elevational transect (4,221–4,833 m a.s.l.) from July to September 2010. Measurements from these boreholes demonstrate that ground temperatures at the depth of zero annual amplitude (TZAA) are generally higher than −2.0°C. The lapse rates of TZAA are 4 and 6 °C km−1, and the lower limits of permafrost with TZAA < −1°C are approximately 4,650 and 4,750 m a.s.l. on the northern (near Yeniugou) and southern (near Qingshui'he) slopes, respectively. TZAA changes abruptly within short distances from −0.2 to +1.2°C near the northern lower limits of permafrost and from about +0.5 to +1.5°C near the southern lower limits of permafrost. Thawing and freezing on the ground surface at Qingshui'he (4,413 m a. s. l.) are 13.3 d earlier and 26 d later than that at Chalaping (4,724 m a. s. l.), respectively. The temperature gradient at Qingshui'he is clearly larger than that at Chalaping. The changes of permafrost TZAA ranged from 0.03°C to 0.2°C from 2010 to 2017. A 3.5‐m‐thick permafrost near Qingshui'he was observed to disappear in summer 2013. There is no significant correlation between elevation and permafrost temperature changes in the study area, whereas the changes of very warm (close to 0°C) permafrost seem to be slow in the intermontane basins.

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