Permafrost Loss On Tibetan Plateau Under Future Warming

Permafrost refers to rocks and soil that contain ice and are at or below zero degrees Celsius.Depending on the duration of their frozen state,permafrost can be categorized into short-term(lasting less than two weeks),seasonal(lasting several months),and continuous(lasting two years or more)permafrost.Permafrost is an important component of the cryosphere,and its distribution and changes have significant impacts on hydrological cycles,ecosystems,and human activities.Currently,permafrost is mainly distributed in high latitudes such as the Arctic and high-altitude regions such as the Qinghai-Tibet Plateau.In recent decades,rapid warming has led to extensive permafrost thawing,which not only exacerbates the threat to infrastructure in permafrost regions but also releases a large amount of organic carbon,potentially reversing the carbon sequestration function of ecosystems.Therefore,assessing the rate and spatial-temporal characteristics of permafrost degradation under future climate change scenarios is crucial to understanding the ecological and environmental impacts of permafrost degradation.Many models have been developed to predict permafrost changes in the future.However,despite increasing efforts to consider permafrost processes,most models are still incomplete and have not fully incorporated permafrost carbon cycling,such as vertical carbon transfer and soil organic carbon insulation.This leads to significant uncertainties in simulating soil temperature and permafrost changes.Accurately predicting future permafrost changes still requires model constraints.To this end,this study proposes two independent approaches for constraining future model predictions.One approach is to constrain time changes through spatial relationships,while the other is to use differences in soil thermal diffusivity between models to constrain model results.The specific research results are as follows:(1)Spatial constraint method.Based on the current distribution data of permafrost and surface temperature data on the Qinghai-Tibet Plateau,a spatial relationship between the distribution of permafrost and the mean annual air temperature(MAAT)is established.Combined with the air temperature data of the fifth and sixth phases of the Coupled Model Intercomparison Project(CMIP5 and CMIP6),it is used to constrain the future response of permafrost in the Qinghai-Tibet Plateau to climate change.The results show that under the moderate emission scenario(RCP45 and SSP245),the estimated area of permafrost on the Qinghai-Tibet Plateau by the spatial constraint method at the end of this century(2080-2099)is 610,000 to 650,000 square kilometers,and the temperature sensitivity of permafrost is 150,000 to 340,000 square kilometers per Celsius degree.The reconstructed area of permafrost on the Qinghai-Tibet Plateau during the current period(2003-2012)is 1.024 million square kilometers.(2)The model constraint based on differences in soil thermal diffusivity.Since CMIP5 models only output historical data up to 2005,we selected the analysis time periods of the historical stage(1986-2005)and the end of this century(2080-2099).Zhu Dan et al.found that differences in soil thermal diffusivity among models are the dominant factor causing differences in the simulation of permafrost range among models.Therefore,we can use differences in soil thermal diffusivity among models to constrain the simulation results of permafrost area.Here,we define permafrost temperature sensitivity as the change in permafrost area with an increase of 1 degree Celsius in air temperature.Based on CMIPs model data and robust regression analysis,we found a significant correlation between soil thermal diffusivity and permafrost temperature sensitivity.We used random forest inversion of observational data from the Qinghai-Tibetan Plateau region to estimate soil thermal diffusivity and combined it with the robust regression function to predict the changes in permafrost area in the QinghaiTibetan Plateau under moderate emissions scenarios at the end of this century.The results show that the permafrost area in the Qinghai-Tibetan Plateau at the end of this century is estimated to be 695,000 to 703,000 square kilometers,with a permafrost temperature sensitivity of 158,000 to 302,000 square kilometers per degree Celsius.(3)The permafrost on the Qinghai-Tibet Plateau is facing a huge degradation risk.In the context of global warming,in order to compare with the Arctic region,we convert the permafrost area in the permafrost temperature sensitivity into the percentage of the historical period of 1986-2005,which means that the spatial constraint method estimates that permafrost degradation on the Qinghai-Tibet Plateau will occur by 19.2%for every 1 degree Celsius increase in temperature,and the soil thermal diffusivity constrained estimate is 18.1%.A study of the northern hemisphere permafrost during the same analysis period of 1986-2005 showed that the permafrost area was 14.9 million square kilometers,and the temperature sensitivity was 1.47 million square kilometers/degree Celsius by the end of this century,which means that the permafrost will degrade by 9.9% for every 1 degree Celsius increase in temperature.Comparing the permafrost sensitivity of the Qinghai-Tibet Plateau and the northern hemisphere permafrost,we found that the permafrost on the Qinghai-Tibet Plateau is disappearing at a faster rate than that in the Arctic region,facing greater risks of degradation.