Desertification Change And Its Future Projection Over The Northern China Under Climate Change

Desertification is one of the major environmental issues of great concern to the international community,it not only threatens the living environment of mankind,but also affects social stability and economic development.China is one of the countries most seriously affected by desertification,which mainly occurs in Northern China（NC）and is dominated by sandy desertification.With the intensification of global warming,it is of great significance for China’s ecological protection,food supply and sustainable development to clarify the characteristics of desertification in the historical period and project desertification in the future over NC under climate change.The reanalysis and satellite remote sensing data are used to analyze the basic climatic characteristics of NC in recent decades;on this basis,the desertification characteristics of NC in the past decades are explored.According to the products of the Sixth Coupled Model Intercomparison Project（CMIP6）,desertification in NC under different scenarios is projected and the influence of climate change on desertification was analyzed using the dune mobility index.Using numerical simulation experiments,the feedback effect of desertification reversal on climate in China was analyzed.The main conclusions are as follows:（1）From 1961 to 2020,the annual mean temperature over the NC is colder in the northwest but warmer in the southeast.The annual precipitation increases from northwest to southeast.The annual mean near-surface wind speed presents obvious local characteristics,is similar to the annual potential evaporation and its the large value area is located in the desert and sandy land.In general,the annual mean temperature,the annual precipitation and annual potential evaporation have increased significantly,and the annual mean near-surface wind speed has decreased significantly in NC.The vegetation is significantly greening in most areas of NC and barren is significantly reduced,but the local vegetation degraded significantly.The obviously spatial variability in the role of natural and anthropogenic factors on vegetation change.Climate change has a significant impact on vegetation change in NC,where vegetation greening is influenced by warming trends in most areas,while in some arid areas vegetation change responds to internal variability of precipitation.Human activities have had a positive effect on vegetation change in areas such as the middle and lower reaches of the Yellow River Basin,the North China Plain and the Northeast Plain,and a negative effect in local areas.（2）Desertification process in NC has undergone remarkable temporal and spatial changes.A general reversal of desertification occurs in NC from 1961 to 2014,but desertification still occurs and intensifies in localized areas.The potential desert area declined from 1961 to 1990,then increased in the 1990s and declined when entering the 21^st century.With the intensification of global warming,future changes in desertification in NC show clear regional characteristics.Under the SSP2-4.5 scenario,desertification will intensify in northern Xinjiang,and reverse in central Inner Mongolia to northeast China.Under the SSP5-8.5 scenario,desertification will reverse from central Inner Mongolia to the North China Plain,while desertification will intensify in northern Xinjiang,Horqin desert and Nenjiang desert.Compared with the medium emission scenario SSP2-4.5,the extent and degree of desertification intensification will increase under the high emission scenario SSP5-8.5.（3）Climate change is an important driver of desertification.With the increase of temperature,the increase of potential evaporation is the main cause of desertification in northern Xinjiang.The desertification reversal from central Inner Mongolia to North China Plain was mainly affected by increasing precipitation.A reduction in near-surface wind speed would also help reverse desertification across the whole NC,but its impact would be smaller than that of precipitation and potential evaporation.From 2015 to2099,the potential desert area in NC under the SSP2-4.5 scenario generally shows an upward-slowing-downward trend,which is due to the control of greenhouse gas emissions in the mid-to late-21st century under this scenario,and curbs the continued occurrence of aridification.The greater interannual and interdecadal variability of potential desert area under SSP5-8.5 compared to SSP2-4.5 implies that desertification changes would be more volatile under the high emissions scenario.（4）Desertification change has important impacts on regional climate.The results of numerical experiments show that the feedback of desertification reversal in China and its surrounding areas on climate increases the annual mean temperature by 0.38°C and the annual precipitation by 38.8 mm in China.In the regions where desertification reversal occurs,the temperature rises significantly by over 1°C,where the decrease in surface albedo and increase in surface emissivity has led to a significant decrease in upward short-wave radiation and a significant increase in downward long-wave radiation,resulting in a significant increase in net surface radiation and a corresponding increase in both sensible and latent heat fluxes.Annual precipitation increases significantly in NC,while the annual precipitation in southern China had obvious local variation.Changes in annual evapotranspiration largely correspond to changes in annual precipitation.Soil moisture decreases in southern Xinjiang due to a significant increase in evapotranspiration,and the change of soil moisture is consistent with the change of precipitation in the rest of China.The low-level cyclonic circulation anomaly caused by surface diabatic heating contributes to the convergence of water vapor and corresponds to the increase of precipitation,which is coupled with anticyclonic circulation anomalies at high altitudes to provide favorable large-scale conditions for precipitation formation.