Distribution And Changes In The Extent Of Cold Regions In China And Their Response To Climate Change From 1979 To 2100

As an important part of the earth system,cold regions are extremely sensitive to climate change and store a large number of water resources.The changes in cold regions can affect the balance of land-atmosphere energy,the hydrological cycle,and the stability of the ecosystem,which have a crucial impact on human production and life as well as social and economic development.In China,there are many areas in high altitude and high latitude,cold regions are widely distributed because of the low air temperature environment,and global warming has led to significant changes in cold regions.At present,the research on the extent of cold regions in China is limited.Most of the previous studies used meteorological station data to analyze the cold regions over a historical period,and the study on the spatiotemporal distribution pattern and changes of cold regions and their response to future climate change has been infrequently reported.Meanwhile,the CMIP6 global model data have a large bias in air temperature simulation over China,which needs to be corrected before predicting future air temperature and cold regions extent.Therefore,based on China meteorological grid observation data CN05.1,China meteorological forcing dataset CMFD,reanalysis data ERA5＿Land,and 26 CMIP6 global model data,together with mathematical statistics and model correction methods,this paper investigated the historical(1979-2016)and future(2015-2100)changes of cold regions extent in China and their responses to climate change.The study strategies are implemented as follows:(1)Using CN05.1observation data as a benchmark,we used the bias correction method to correct the air temperature biases of CMIP6 global model data,compare and evaluate the simulation performance of the original and corrected 26 CMIP6 global model data and their multi-model ensemble average results on temporal and spatial scales.(2)Using CN05.1,CMFD,ERA5＿Land,and air temperature of the original and corrected CMIP6 global model data,the spatiotemporal distribution and variation characteristics of annual and seasonal mean air temperature in historical and future periods over China were analyzed.(3)The extent of cold regions in China is defined based on air temperature(determined by three conditions:the average air temperature of the coldest month is less than-3 ~oC,the months of average air temperature higher than 10 ~oC are less than 5 months,and annal mean air temperature is not higher than 5 ~oC),the spatiotemporal distribution and change characteristics of cold regions extent in historical and future periods were analyzed,and the changes of cold regions extent in three typical sub-regions(northwest,southwest,and northeast)in China were explored.Finally,the contribution of the three conditions in the definition to the change of cold regions extent was quantitatively estimated.The main conclusions are as follows:(1)CMIP6 global model data generally underestimate air temperature in China.Based on CN05.1 observation data,the air temperature biases of CMIP6 global model data are reduced by using the dynamic downscaling bias correction method,and air temperature changes of 26 global model data are kept within an acceptable range.Compared to the original results,the corrected annual and seasonal mean air temperature of CMIP6 global model data changed by-0.3 ~oC(summer)to 1.2 ~oC(spring)during 1979-2016,and by-0.3 ~oC(summer)to 1.3 ~oC(spring)during 2015-2100.The air temperature of the original CMIP6 global model data is overestimated in summer and underestimated in other seasons.After correction,the simulation performance of50%of the models is improved for interannual variation,and the spatial distribution of the multi-model ensemble mean is significantly improved.(2)The annual mean air temperature of China calculated by CN05.1 observation data is 6.4 ~oC during 1979-2016,and the warming rate is 0.37 ~oC/10a.Due to the influence of altitude,the Tibetan Plateau,the Greater Khingan Mountains,and the Tianshan Mountains have low air temperatures,while the southeast coastal areas and the Tarim Basin in Xinjiang have high air temperatures.The warming in the Tibetan Plateau,the eastern part of the Inner Mongolia Plateau,and the northern part of Xinjiang are faster,while in the southern part of China is slower,however,obvious seasonal differences exist in the warming rates in China.The three groups of air temperature data products(CN05.1,CMFD,and ERA5＿Land)consistently show the order of warming rate:spring>autumn>winter>summer,while the original CMIP6 global model data show a different situation:summer>winter>autumn>spring.The warming rates of the original CMIP6 global model data range from 0.13 ~oC/10a(SSP126)to 0.70 ~oC/10a(SSP585)during 2015-2100,and decrease from 0.12 ~oC/10a(SSP126)to 0.61 ~oC/10a(SSP585)for the corrected model data.Before and after correction,the warming rates of CMIP6 global model data for the future period are ranked as autumn/winter>summer>spring.(3)The annual mean cold regions extent of China estimated by CN05.1observation data is 3.87×10~6 km~2(accounting for 40.4%of the land area of China)during 1979-2016,and the decreasing rate is-1.54×10~5 km~2/10a.In terms of spatial distribution,cold regions in China are concentrated in the northeast(eastern Inner Mongolia Plateau,Greater and Lesser Khingan Mountains,and Changbai Mountains),northwest(Tianshan Mountains and Altai Mountains),and southwest(Tibetan Plateau).The decreased cold regions extent is mainly located in the periphery of these three regions.The original CMIP6 global model data shows that the decreasing rate of the cold regions extent ranges from-0.48×10~5 km~2/10a(SSP126)to-2.47×10~5 km~2/10a(SSP585)during 2015 to 2100.After correction,it becomes-0.54×10~5 km~2/10a(SSP126)to-2.10×10~5 km~2/10a(SSP585).For the three regions of cold regions,the extent in southwest China is the largest,followed by northeast China and northwest China.However,the decreasing rate in northeast China is the greatest,followed by southwest China and northwest China.The possible reason is that the cold regions in northwest and southwest China are mainly located in the high-altitude regions with low base air temperatures.Although the warming rates are higher,the critical air temperature values used to define the cold regions do not exceed,so their decreasing rates are relatively lower compared to that in northeast China.(4)The correction for CMIP6 global model data has a significant impact on the estimation of the cold regions extent.Compared with the original CMIP6 global model data,the cold regions extent of the corrected model data in each future scenario and time period decrease by 1%to 6%more than that in the baseline period(1995-2014).Compared with the baseline period,the extent of cold regions based on the corrected model data in the near-term future(2041-2060)is reduced by 18.9%(SSP126)to 28.6%(SSP585),by about 20.4%(SSP126)to 40.1%(SSP585)in the mid-term future(2061-2080),and by about 20.7%(SSP126)to 51.6%(SSP585)in the long-term future(2081-2100).The variation of the extent of cold regions is mainly influenced by the condition of annual mean air temperature.Among the three conditions of the cold regions definition,the contribution of the third condition(annual mean air temperature is not higher than 5 ~oC)is the largest,reaching 94.6%for the historical period and 94.9%for the future period.Due to the general underestimation of air temperature by CMIP6 global model data in China,this paper evaluated the applicability of CMIP6 global model data and conducted its bias correction for the study of air temperature distribution and changes in China.Finally,the spatiotemporal distribution and variation characteristics of the cold regions were investigated.The contribution of the three conditions in the definition of cold regions to the changes in the cold regions extent was further analyzed.The study is helpful to explore the characteristics of air temperature change in China,the influencing process and mechanism on the shrinkage of cold regions under the background of climate warming,which would provide scientific support for realizing the national demand for the optimal allocation of water resources and regional ecological security guarantee.