Enhanced Warming Of Lake Surface Water Temperature In Drylands And Its Effects

As an important part of the water cycle,lakes are key indicators of global and regional climate change,particularly in drylands,where they provide extremely valuable water resources and influence many aspects of ecosystems and human activity.Drylands account for approximately 41% of the total global land area,and it is the region with the most significant regions of global warming in the past century.Due to scarce precipitation and significant warming,sparse vegetation and fragile ecosystems,drylands are extremely sensitive to climate change.Dryland lakes have undergone dramatic changes in recent decades.Lake surface water temperature(LSWT)is one of the important physical properties of lake water,which directly or indirectly affects the physicochemical and biological processes of lakes,and is an important indicator for monitoring the changes in the state of lakes.Therefore,it is necessary to investigate the variability and impacts of LSWT in drylands.This paper uses satellite observations to analyze the variability of dryland LSWT globally and in China,and to quantify the drivers of global dryland LSWT change.The impacts of dryland LSWT changes are also investigated from the perspectives of lake heatwaves and lake drought events.The spatial and temporal variability of dryland lake heatwaves and lake drought events are analyzed,and the causes of lake heatwaves and lake drought changes are explored.The main conclusions are as follows:(1)A significant warming of dryland lakes was found.The warming rate of global lakes is uneven,with significant regional differences.Comparing the warming of LSWT in different climatic zones,it revealed that LSWTs in drylands warm faster than in humid regions.The phenomenon still persists at the regional scale,and the results indicate that LSWTs in Chinese drylands increase faster than in humid regions and that the warming of dryland lakes in China is more significant.Dryland lakes in China are warming significantly.Quantifying the drivers of warmer LSWT in drylands,it was found that air temperature,evaporation,wind speed,cloudiness,net shortwave radiation and net longwave radiation were the main impact factors.Among them,air temperature is the main driver of global and Chinese dryland lake warming.(2)Lake warming may lead to stronger lake heatwaves,and during lake heatwaves air temperature transfers more heat to the LSWT leading to increased warming efficiency,both of which form a positive feedback mechanism.With the rapid warming of LSWT in China,lake heatwaves became stronger and lasted longer during 1950-2020,especially in drylands.Lake heatwaves in dryland were shorter on average but 31% more intense than those in humid regions.During lake heatwave periods,warming efficiencies were higher in drylands than in humid regions,while during non-heatwave periods the four climate zones performed similarly.Moreover,the warming efficiency in drylands was higher during lake heatwave periods than during non-heatwave periods.The higher warming efficiency induced stronger lake heatwaves and LSWT warming.Further combining the effects of air temperature on LSWT and lake heat waves,the study found that LSWT warming and lake heatwaves can form a positive feedback relationship and reinforce each other.(3)The effect of lake warming on lake drought trends.The lakes in China show a continuous aridification,where the trend of lake drought is stronger in drylands than in humid regions.The changing patterns of lake drought events at different time scales show consistency,with the frequency of occurrence increasing and the duration tending to increase by 1-2 months.Lake drought peaks in the recent two decades,which is primarily driven by the increase in lake potential evaporation(PET)contributing 41% and 80% in the first and second 10 years,respectively.In 2000-2009 and 2010-2020,the contribution of lake PET to lake drought in China was 48%and 40%,respectively,and the contribution to dryland lakes was 62% and 48%.Using multiple linear regression to analyze of the factors influencing the increase in PET of lakes in China and the four climatic zones,it is found that increasing air temperatures and incident shortwave radiation dominated the effect on PET of lakes,contributing more than 30%,and air emissivity and relative humidity also contributes positively.Further analyzed from a regional perspective,relative humidity and incident shortwave radiation have a non-negligible influence on the increase of PET in dryland lakes compared with the humid regions.The intensification of drought trend of dryland lakes in China is related to increased air temperature,increased incoming shortwave flux and reduced relative humidity.