Reversal Of Desertification In Qinghai-Xizang Plateau And Its Response To Abrupt Climate Change

Desertification is one of the most serious ecological environment and social development problems in the world,and China is one of the countries most seriously affected by desertification.The Qinghai-Tibet Plateau is known as the"third pole"of the earth.Most of the areas have a cold and arid climate,and the ecological environment is fragile,which has the environmental conditions and potential factors for the occurrence and development of land desertification.The development of desertification in the plateau is closely related to climate change and vegetation change.Based on the temperature,precipitation and wind speed data of 81 stations in the Qinghai-Tibet Plateau from 1971 to 2013,the observation data of snow days on the Qinghai-Tibet Plateau from 1971 to 2016,the lower boundary of the first frozen layer,Landsat remote sensing image data of 1990,2000,2010 and 2015,normalized vegetation index(NDVI)and NCEP/NCAR reanalysis data from 1982 to 2015,this paper focuses on the change characteristics of vegetation(NDVI)and desertification land distribution on the Qinghai-Tibet Plateau.The climatic characteristics of climatic factors(air temperature,precipitation,wind speed)and underlying surface factors(snow,permafrost)and their relationship with vegetation change before and after the abrupt warming of the Qinghai-Tibet Plateau are studied.the relationship between the South China Sea summer monsoon and the plateau monsoon is analyzed,and the possible influence mechanism of the abnormal ending time of the South China Sea summer monsoon on the plateau winter climate is discussed,which has important scientific value for further scientific assessment of the impact of climate change.It has important practical significance for the construction of national ecological security barrier,the rational development and utilization of resources and the sustainable development of social economy.The main conclusions of this paper are as follows:(1)The study on the distribution of desertified land on the Qinghai-Tibet Plateau shows that from1990 to 2015,the desertified land on the Qinghai-Tibet Plateau showed an overall decreasing trend,and the area of desertified land decreased by 3 826 km²,equivalent to 0.96%of the desertified land area in 1990,with an average annual decrease of 153 km².In particular,the desertification on the Qinghai-Tibet Plateau has been reversed continuously since 2000.(2)Under the background of global warming,the climate of the Qinghai-Tibet Plateau has changed significantly,from the cold and dry climate in the 1970s to the warm and humid climate in the middle and late 1990s.The macroscopic performance of the main meteorological elements from 1971 to 2013 is as follows:(1)Temperature.The plateau shows a consistent warming,with a warming range of 0.38?/(10 a),which is higher than the global warming rate in the same period,especially in autumn and winter.The warming of the plateau shows a warming trend of strong in the west and weak in the east and a reverse change in the north and south.The climate warming in the edge of the plateau is more obvious than that in the hinterland of the plateau,and the warming in the north of the plateau is greater than that in the south.The temperature of the plateau abruptly occurred in 1997/1998,and the warming after the abrupt change was more obvious in the high altitude area.The maximum temperature and minimum temperature increased asymmetrically,and the increasing rate of the minimum temperature(0.46?/(10 a))was higher than that of the maximum temperature(0.37?/(10 a)).(2)Precipitation.The precipitation in the plateau increases at the rate of 8.5 mm/(10 a).The precipitation increases obviously in the winter half year and decreases weakly in the summer half year.(3)Wind speed.The annual and seasonal average wind speed of the plateau shows a decreasing trend,especially in spring,reaching-0.25(m·s^-1)/(10 a).The linear tendency rate of plateau wind speed changed from negative to positive after 2000,showing a significant increasing trend,which was dominated by the increase of average wind speed in summer and winter.(4)Days of snow cover.The snow cover days in the plateau decreased at an average rate of 3.5 d/(10 a),and reached 5.1 d/(10 a),with the Interdecadal variation of"less-more-less"after the sudden warming of the plateau temperature.(5)Permafrost.The average climatic inclination rate of the lower boundary of the first frozen layer in the Qinghai-Xizang Plateau is-3.7 cm/(10 a),and abruptly occurred in 1987/1988.(3)The vegetation change(desertification)in the Qinghai-Xizang Plateau has a significant response to the climate change on the Qinghai-Xizang Plateau.From 1982 to 2015,the maximum value of plateau NDVI showed an increasing trend,with a linear growth trend of 0.002/(10 a),with an annual change rate of0.0291%;the linear growth trend of the maximum value of NDVI in the growing season(June-September)was 0.003/(10 a),with an annual change rate of 0.0349%.In terms of spatial distribution,the maximum value of NDVI in the plateau shows the characteristics of"overall improvement and regional degradation".The maximum value of NDVI,which characterizes the change of desertified land,begins to decline at the beginning of the 21st century.The area of vegetation improvement area is larger than that of degraded area,and the area of desertified land is decreasing.The change of the maximum value of NDVI in the plateau shows a significant adaptive adjustment process under the background of plateau warming,which has a good correlation with the changes of climate factors such as temperature and precipitation,and has obvious regional differences.Under the background of plateau warming,from 1982 to 1997,temperature change was the dominant factor of NDVI change,followed by precipitation change,while from 1998 to 2015,precipitation change became the dominant factor of NDVI change,followed by temperature change.The primary factor affecting the growth of vegetation in the alpine area of the Qinghai-Tibet Plateau is heat.When the heat condition is satisfied,the evaporation increases,and the water condition shows its importance.After the abrupt warming of the plateau,the change trends of air temperature,precipitation and wind speed all reached a significant level,and the area of land desertification in the Qinghai-Tibet Plateau decreased.The change of climatic factors in this stage is the main factor affecting the reduction(reversal)of land desertification area.(4)The change of vegetation index((NDVI))indicates the desertification of the Qinghai-Tibet Plateau,which is closely related to the abrupt climate change of the plateau,and the climate change of the plateau is affected by the monsoon.The end date of the South China Sea summer monsoon is inversely related to the establishment date of the plateau winter monsoon,and is significantly related to the number of winter snow days over the plateau.When the South China Sea summer monsoon ends late,the geopotential height near Baikal Lake is negative and the geopotential height near Ural Mountain is positive at 500 hPa and 600 hPa in winter.Under its influence,the zonal wind in the northeast of the plateau weakens and the zonal wind in the southwest of the plateau strengthens;the temperature in the northeast of the plateau rises abnormally,and the number of winter snow days on the plateau is less.The humidity of water vapor in and around the plateau increases,there is an abnormal updraft in the northeast of the plateau,the westerly jet of 200 hPa moves southward,and the precipitation in the northeast of the plateau increases,and vice versa.When the South China Sea summer monsoon ends earlier,the establishment of the plateau winter monsoon is late,the plateau winter monsoon(cold high pressure)weakens,and the plateau is snowy and humid,which is beneficial to the reversal of desertification on the Qinghai-Tibet Plateau.