Research On The Attribution, Projection And Economic And Social Impact Of Global Aridification Based On The Results Of The CMIP6 Model

Widespread aridification of the land surface exerts pervasive influences on terrestrial ecosystems and causes substantial challenges to human society.However,the mechanisms underlying increased aridity still relatively underexplored and uncertainties remain in quantitatively analyzing the contribution of the natural and anthropogenic forcings to increased aridification.Besides,aridity projections and their possible societal influences are also subject to uncertainties.Based on the abovementioned problems,global aridity changes were detected and attributed using multisource observational,reanalysis and Coupled Model Intercomparison Project phase 6(CMIP6)model data.The contributions from anthropogenic and natural forcings were quantified and the impacts of meteorological variables on aridity changes were also evaluated.Moreover,we further improved the projections of aridity and their possible socioeconomic consequences using the emergent constraint method.The future changes of aridity as well as their socioeconomic consequences in China were also evaluated.The main conclusions are as follows:(1)The anthropogenic forcings caused the increased aridification of the globe and each hemisphere over 1965-2014,especially as a result of massive greenhouse gases.ALL forcing,anthropogenic forcing,greenhouse gas forcing,and aerosol forcing can be detected and attributed over globe and each hemisphere,while the natural forcings were rarely detected.The greenhouse gas forcing contributed to drying trends,whereas the aerosol emissions led to wetting tendencies.The dominants of aridity changes were identified by using decomposition method.Results show that precipitation and temperate are responsible for aridity changes over the most parts of land surface.Precipitation dominates the aridity changes over mid-low latitudes of northern hemisphere and most parts of the southern hemisphere,while temperature mainly affects high latitudes of northern hemisphere.The net radiation changes resulted from aerosol emissions should take responsibility for aridity changes over central Europe,central Africa and East Asia.(2)The use of emergent constraint shown to reduce the uncertainties of global aridity projections of CMIP6 models.In the future,drying trends prevail over most land surface(including central North America,central South America,southern Europe,southern Africa and East Asia).The constrained results show a slightly lower mean drying rate over global and smaller drying regions.The CMIP6 models underestimate the dryland coverage over North America,Africa,and Asia,while they overestimate the area of dryland over Europe.North America,South America,Europe and Oceania are projected to experience increasing dryland coverage in the future,while Africa and Asia show decreasing trends.The constrained projections suggest intensified increasing(decreasing)rates of drylands over North America and Oceania(Africa and Asia),but relieved increasing rates of dryland over South America and Europe.For the subtypes of dryland,CMIP6 models underestimate the areas of arid,semiarid,and dry subhumid regions,but they overestimate the area of hyper-arid regions.The original and constrained projections suggest an increasing coverage of arid,semiarid,and dry subhumid regions,whereas a slight reduction in hyper-arid region.(3)CMIP6 models overestimate the population and agricultural area exposures,but they underestimate the GDP exposure over global.The original and constrained results both suggest the population and agricultural area exposures will peak around 2060 and then decrease throughout the rest of the century,and GDP exposure will continuously increase.The changes of global socioeconomic exposures mainly determined by socioeconomic effects with limited contribution from climate change effects.At the continental scale,the causes of population and agricultural area exposures variations are different from each other.The climate change effect accounts for the increasing agricultural area exposures over North America and Europe,as well as the decreasing population and agricultural area exposures over Asia.The variations of GDP exposure mainly driven by the GDP increase over all six continents.At the 2? warming level,the constrained projections show that most countries may experience higher socioeconomic risks than the original projections.(4)Precipitation and temperature are the dominant factors of aridity changes over most regions over China(ALL,GHG and AER forcings).Under ALL and AER forcings,the dominant role of net radiation mainly concentrates over southern China.The CMIP6 models overestimated the AI magnitudes(wet biases)and underestimated the dryland area over China.In the future,the north of China shows wetting tendencies,while the southern China may experience drying trends.The constrained projections show lower mean wetting rate but higher rate of dryland decrease over China.The projected reductions in population and agriculture risks over China mainly result from the interaction and climate change effects,respectively,while the increase of GDP exposure mainly dominated by the GDP effect.