Decadal expansion and contraction of permafrost in the Three-River Source Region,Qinghaie-Tibet Plateau(1901-2020)

Permafrost and its spatiotemporal variation considerably influence the surface and sub-surface hydrological processes,biogeochemical cycles,fauna and flora growth and cold region engineering projects in the Three-River Source Region(TRSR),Qinghai–Tibet Plateau.However,the dynamics of permafrost over a relatively long term duration(e.g.>100 years)in the TRSR is not well quantified.Thus,the spatial and temporal variations of the temperature at the top of the perennially frozen/unfrozen ground(TTOP),active layer thickness(ALT)in permafrost regions and the maximum depth of frost penetration(MDFP)in the seasonally frozen ground of the TRSR during 1901–2020 were simulated using the TTOP model and Stefan equation driven by the widely used reanalysis Climatic Research Unit 4.05 dataset.Results revealed that the permafrost in the TRSR over the past 120 years did not degrade monotonically but experienced considerable fluctuations in area with the decadal oscillations of climate warming and cooling:shrinking from 263.9×10^(3)km^(2)in the 1900s to 233.3×10^(3)km^(2)in the 1930s,expanding from 232.3×10^(3)km^(2)in the 1940s to 260.9×10^(3)km^(2)in the 1970s and shrinking again from 254.1×10^(3)km^(2)in the 1980s to 228.9×10^(3)km^(2)in the 2010s.The regional average TTOP increased from−1.34±2.74℃in the 1910s to−0.48±2.69℃in the 2010s,demonstrating the most noticeable change for the extremely stable permafrost(TTOP<−5℃)from 8%to 1%.The regional average ALT increased from 2.68±0.52 m to 2.87±0.46 m,with the area proportion of ALT>3.0 m by 12%from 1901 to 2020.Notably,minor changes were observed for the regional average MDFP,probably due to the increase in the area proportion of MDFP<3.0 m(caused by climate warming)and MDFP>3.5 m(owing to the transformation of permafrost to seasonally frozen ground)by 7.39%and 4.77%,respectively.These findings can facilitate an in-depth understanding of permafrost dynamics and thus provide a scientific reference for eco-environment protection and sustainable development under climate change in the TRSR.

Changes in the ground surface temperature in permafrost regions along the Qinghaie-Tibet engineering corridor from 1900 to 2014:A modified assessment of CMIP6

Numerous studies were published in the last two decades to evaluate and project the permafrost changes in its thermal state,mainly based on the soil temperature datasets from the Coupled Model Intercomparison Project(CMIP),and discuss the impacts of permafrost changes on regional hydrological,ecological and climatic systems and even carbon cycles.However,limited monitored soil temperature data are available to validate the CMIP outputs,resulting in the over-projection of future permafrost changes in CMIP3 and CMIP5.Moreover,future permafrost changes in CMIP6,particularly over the QinghaieTibet Plateau(QTP),where permafrost covers more than 40%of its territory,are still unknown.To address this gap,we evaluated and calibrated the monthly ground surface temperature(GST;5 cm below the ground surface),which was often used as the upper boundary to simulate and project permafrost changes derived from 19 CMIP6 Earth System Models(ESMs)against in situ measurements over the QTP.We generated the monthly GST series from 1900 to 2014 for five sites based on the linear calibration models and validated them through the three other sites using the same calibration methods.Results showed that all of the ESMs could capture the dynamics of in situ GST with high correlations(r>0.90).However,large errors were detected with a broad range of centred root-mean-square errors(1.14-4.98℃).The Top 5 model ensembles(MME5)performed better than most individual ESMs and averaged multi-model ensembles(MME19).The calibrated GST performed better than the GST obtained from MME5.Both annual and seasonal GSTs exhibited warming trends with an average annual rate of 0.04℃ per decade in the annual GST.The average seasonal warming rate was highest in winter and spring and lowest in summer.This reconstructed GST data series could be used to simulate the long-term permafrost temperature over the QTP.