Global Desertification Vulnerability Impacted By Human Activity And Climate Change

According to the United Nations Convention to Combat Desertification(UNCCD),desertification is the impoverishment of arid,semi-arid,and some sub-humid ecosystems due to the combined impact of human activities and climate change.Desertification not only threats the global ecological security,but also affects the economic and social development.Study the growth status of vegetation and the impact of climate change to the vegetation dynamics,which is in favor of taking action to protect and promote restoration of the ecosystems and making policy to combat climate change and desertification.The assessment of global scale desertification vulnerability to climate change and human activities is an important step to help decision makers to formulate the best strategies for land rehabilitation and the fight against global desertification in sensitive areas.However,there is no global desertification vulnerability map that considers both climate change and human activities.In this study,we first analyzed the spatial-temporal character of global leaf area index(LAI)and the impact of temperature and precipitation to vegetation dynamics.Farther,we assessed global desertification vulnerability using a global desertification vulnerability index(GDVI),which was defined by human activities and climate change indicators,to obtain a global overview of desertification vulnerability.Finally,we projected future global desertification vulnerability by the end of this century under different scenarios,the conclusion as follows:1.At global scale,the LAI increased from 1982 to 2014.The increasing trend of LAI in the spring and autumn is further lager than that in the summer,which indicated that the extension of growth length may be the dominant factor in the increasing annual LAI in the northern hemisphere.The regions with the most significant contributions to the global LAI change trend are South Asia,East Asia,Europe and South America.Although humid region dominated the increasing tendency of global LAI,the increase of relative LAI is significant in the drylands.Precipitation is the key factor influencing vegetation growth in the drylands,however,the human activities is an important contribution to global greening.2.The contribution of seasonal trend to annual LAI change shown obviously spatial heterogeneity.The fraction of spring LAI to annual LAI increased in the Central Asia,Europe,East China,Australia and Southern South America.The fraction of Summer LAI to annual LAI decreased in the East China,India,Central Asia,Africa.The fraction of Autumn LAI to annual LAI increased in the Central Africa,America,India and Northeast China.In the humid region,the vegetation growth was constrained by temperature,radiation,rather than precipitation.The sensitive of LAI to precipitation decreased in the recent decades in the drylands,and enhanced in the humid area,that indicated vegetation differently responded to global warming at regional scale.However,The sensitive of LAI to air temperature decreased in the recent decades in the drylands,and enhanced in the humid area.3.The global desertification vulnerability index were constructed by combining climate change and human activities and were used to provide another perspective on desertification vulnerability on a global scale.Using the probability density function of the GDVI,we classified desertification vulnerability into four classes: very high,high,medium,and low.Comparing with the previous studies,GDVI is reasonable and reliable.The results of the analysis indicated that areas around deserts and barren land have a higher risk of desertification.The high level of desertification risk is mainly in the western part of the United States,the Sahel,Central Asia,and northern China,where vegetation is sparse and the climate is dry.The desertification risk is moderate to severe in the Indian subcontinent and North China Plain due to intense human activity.The area coverage of high desertification risk in developing countries is larger than that in developed countries,which may have an effect on local sustainable development.Generally speaking,the desertification is high in the Central Asia and Northern Africa,so we need to take positive action to suppress the expansion of desertification.Although risk of desertification in East Asia is moderate,the area coverage of high and very high desertification risk level is high,we should be alert to the further deterioration of its ecological environment.4.The global mean AI values will decrease in the future projection,while the temperature will increase under both the RCP4.5 and RCP8.5.Under the RCP4.5,the HAI will reach a maximum around 2040 and then decline.The risk of desertification according to the GDVI will increase over Europe,western Asia,northern China,the edge of the Sahel,and Mexico.However,the global mean HAI increases monotonously under the RCP8.5,indicating that human activity would increase in the future.RCP8.5 projected that the area of moderate to very high desertification risk will increase by 23% by the end of this century.Under the RCP8.5,HAI increases significantly,the climate becomes drier,and the surface air temperature increases,resulting in a dramatic increase in desertification risk over northern China,India,Mexico,and Africa.In Australia,climate change is the main contributor to the increased risk of desertification.As the area coverage of high risk desertification expands in the future,poor people in drylands areas will experience more natural disasters,and a global plan of action is therefore needed to reduce the risk of desertification.