Climate Change Mechanisms And Future Risk Projections In Drylands

As the home to more than 38%of the world's population,the global drylands have been a major concern for achieving the Sustainable Development Goals,with their economic backwardness,environmental fragility,and social instability.Against the background of global warming in the past century,climate disasters such as enhanced warming,intensified droughts,and increased extreme weather in global drylands have become more and more serious.Based on previous work,this paper explores the characteristics,mechanisms,and future impacts of climate change in drylands from three climate change factors of temperature,precipitation,and oxygen.On the one hand,we analyze the spatio-temporal characteristics of precipitation and temperature and precipitation in drylands and explored the possible mechanisms of precipitation changes in typical drylands of Central Asia and the intensification of warming in global drylands.On the other hand,we use CMIP5 and CESM model simulations to estimate the future responses of temperature,precipitation,evapotranspiration,and area changes of global drylands to the 1.5°C and 2°C global warming targets proposed in the Paris Agreement and assess the impact of both targets on hydrology,drought,agriculture,and health in drylands.Finally,starting from the oxygen cycle in the Earth system,an oxygen index suitable for quantitatively assessing the oxygen balance of cities in global drylands is constructed,and the relationship between the urban oxygen index and the risk of high-temperature heat waves,as well as water shortages and hypoxia,are discussed.Through the research,the main results of this paper are as follows.(1)The radiation balance mechanism of enhanced warming in drylands is proposed and its future changes are estimated.In the context of global warming,the warming trend of drylands in the past century has been about 20%-40%higher than that of humid regions.The main reason for this is that the small evapotranspiration caused by lower soil moisture and vegetation cover in drylands leads to the net radiation it receives mainly increase surface temperature in the form of large sensible heat fluxes.In addition,the heating effect of ice clouds is dominant in drylands,while the cooling effect of water clouds and more anthropogenic aerosol is predominant in the humid area.Using climate models to estimate future temperature changes in drylands,it is found that limiting global warming to less than 1.5°C can not only keep the temperature increase within 3°C in drylands and within 2°C in humid regions,but also greatly reduce the threats to agriculture,hydrology,drought and health in drylands.The necessity for a 1.5°C warming target was emphasized.(2)The spatio-temporal characteristics and influencing mechanisms of precipitation changes in the drylands of Central Asia are revealed.From 1979 to 2019,the summer precipitation in Central Asia was mainly characterized by interannual variation,which is closely related to the change in sea surface temperature(SST)in the Indian Ocean.When the SST of the Indian Ocean is warmer,the decrease in the thermal difference between the land and the sea leads to a weakening of the South Asian summer monsoon,a decrease in water vapor transported to the north Indian continent leads to less precipitation there.As a result,the latent heat of condensation on the North Indian continent decreased,and cyclone anomalies occurred over Central Asia.The temperature cold advection made the temperature in the middle and upper troposphere of Central Asia abnormally cold,weakening the horizontal temperature gradient in the middle and high latitudes,resulting in the southerly location of the subtropical westerly jet stream,the abnormal vertical upward movementover Central Asia,and the decrease in water vapor output on the south side of Central Asia,which eventually leads to more summer rainfall in Central Asia.(3)The differences and causes of climate response in drylands under different definitions of warming targets are compared.Using the future projections under four scenarios in the CESM model,the response of climate change in drylands to transient and stable warming of 1.5°C/2°C is compared.The results show that the warming and area expansion of drylands under the transient scenario are generally higher than those in the stable scenario,mainly due to the radiative forcing of greenhouse gases in the transient warming scenario being greater than that in the stable warming scenario.An additional 0.5°C warming would lead to a decrease in precipitation in North America,South Africa,and North Africa,while an increase in PET would ultimately increase the risk of drought in these drylands.Compared with the 2°C global warming target,advancing carbon neutrality can not only controlling global warming within 1.5°C by the end of the century and reduce the warming in drylands by 0.7°C,but also avoid about 60%increase in drought intensity and 44%area expansion in global drylands,and reduce the degree of drought deterioration in drylands at such as southern North America and northern Africa.(4)The oxygen imbalance and the associated risk of hypoxia,water and heatwaves are assessed.First,we calculate oxygen consumption and oxygen production at global grid points and construct an oxygen index(O_I),that is,the ratio of oxygen consumption to oxygen production.The oxygen balance and associated risks of heatwave and water scarcity are compared between arid and humid cities.The results show that the average O_Iin arid urban areas is as high as?81.0,which is approximate twice the oxygen index(?47.6)in humid urban areas.Cities with more population have higher oxygen indices on average,and arid large cities are much more likely to have high O_Ithan humid large cities.Under global warming,people living in large cities with higher O_Iare more likely to be affected by higher temperatures and more frequent and prolonged heat waves,and will be at risk of severe water shortages and hypoxia.These risks of water shortages and hypoxia are more pronounced in arid cities,which makes the ecological environment in arid urban regions more fragile.In summary,this paper systematically studies climate change mechanism and in drylands its future risk prediction along the framework of"characteristic analysis,mechanism exploration and impact estimation".This paper not only starts from the two basic climate change factors of temperature and precipitation,but also uses the oxygen cycle as a link to explore the threats faced by global drylands from various aspects together with the hydrothermal cycle in drylands.It provides a scientific reference for the realization of sustainable development and especially has a positive effect on developing strategies that are more suitable for solving various issues of drylands around the world.