| The frozen ground region of northeast China(FGRN China)is the second largest permafrost region and the main permafrost region in high latitude.Against the background of global warming,as the temperature rises,the permafrost is degrading significantly in the FGRN China.The hydrological,environmental and ecological problems caused by frozen ground degradation have become the focus of scientific community.Therefore,it is necessary to study the distribution change of frozen soil in Northeast China.As one of the main soil carbon pools,frozen ground degradation will lead to the release of a large number of greenhouse gases,which will greatly increase the uncertainty of regional and global climate system change.Therefore,understanding the distribution of permafrost and seasonal frozen ground in FGRN China is of great significance for the construction of Northeast China permafrost and the formulation of ecological environment policies.In addition,the understanding of the change law of frozen ground in FGRN China can provide basic data for the study of hydrology,climate and ecological change under the background of frozen ground degradation.At present,the research on frozen ground in FGRN China is mainly based on meteorological stations data,and the research period is short.In this study,the temporal and spatial variations of air freezing/thawing indices(AFI/ATI)and ground surface freezing/thawing indices(GFI/GTI)in the Northeast permafrost region from 1900 to 2017were analyzed using the monthly air temperature reanalysis data set of the University of Delaware.Based on GFI/GTI and surface frost number model,this paper simulates the distribution of frozen ground in FGRN China and predicts the changes of frozen ground in the whole 21st century.The main conclusions are as follows:(1)The annual mean air temperature in FGRN China from 1900 to 2017 was increased significantly,with a rate of 0.01℃/a,and abrupt change occurred in 1985.Spatially,the annual mean air temperature in the FGRN China presents a trend of low temperature in the north and high temperature in the south,with a range of-7.86～11.30℃from north to south,showing an obvious latitudinal zonality.The freezing N-factor values of land cover types in the FGRN China vary greatly,and the freezing N-factor values of land cover types in different regions range from 0.06 to 0.85,while the thawing N-factor values are vary little,mainly ranging from 1.14 to 1.34.The land cover types with the maximum and minimum freezing N-factors were grassland and woodland,respectively.The land cover types with the maximum and minimum thawing N-factors are the same as those with freezing N-factors.(2)The change trend of AFI/GFI from 1900 to 2017 was similar,and both showed a significant decreasing trend,with decreasing rates of-3.5℃d/a and-1.2℃d/a,respectively,and abrupt changes occurred in 1973 and 1977,respectively.The AFI decreased by 709.2℃d in 118 years,while the GFI decreased by 252.4℃d.In addition,the spatial distribution of AFI/GFI is similar,showing a trend of high in the north and low in the south,with a significant gap between the north and the south.The maximum,minimum and average values of AFI were 4158.4℃d,137.3℃d and 2462.4℃d,respectively.The maximum,minimum and average values of GFI were 1663.4℃d,237.7℃d and 1028.7℃d,respectively.The gap with the AFI is obvious.(3)The change trend of ATI/GTI from 1900 to 2017 was opposite to that of the freezing index,and the increase rate was 1.9℃d/a and 2.0℃d/a,respectively,with abrupt changes in 1993 and 1996,respectively.The GTI increased by 451℃d in 118years,while the ATI increased by 364.2℃d.Spatially,the ATI/GTI are lower in the north and higher in the south.The maximum,minimum and average values of ATI were 3764.9℃d 1279.2℃d and 2659.5℃d,respectively.The maximum,minimum and average values of GTI were 4517.9℃d,1339.9℃d and 3193.6℃d,respectively.(4)When E=0.57,the frost number model has the highest accuracy for the distribution of frozen ground in the FGRN China in the 1980s and the early 21st century.The overall accuracy is 0.95 and 0.93,and the Kappa coefficient is 0.89 and 0.81,respectively.The permafrost area in FGRN China decreased by 22.66×10~4km~2 from 1900to 2017.With the continuous change of climate,the permafrost island in the higher elevation area is gradually disappearing,and the area of the permafrost is reduced from2.57×10~4km~2 to 0.07×10~4km~2.From the prediction results of frozen ground distribution in the FGRN China in the 21st century,it can be seen that the permafrost boundary is gradually approaching to the Greater Khingan Mountains,and the permafrost near the Lesser Khingan Mountains is obviously degraded. |
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