Impacts Of Reductions In Non-methane Short-lived Climate Forcers On Future Climate Extremes And The Resulting Population Exposure Risks In Asia

Non-methane short-lived climate forcers（SLCFs）,including aerosols,ozone,and their precursors,are important climate forcings and primary air pollutants.Future stringent SLCF emissions controls to mitigate air pollution will substantially impact regional climate change.With high population density and air pollutant emissions,Asia is also one of the most vulnerable regions in the world to the risks of climate change.However,few studies have considered the impact of future combined emission reductions in non-methane SLCFs on climate risks.This study investigates the changes in future（2031～2050 versus 1995～2014）air quality and climate extremes due to non-methane SLCF emissions reductions aiming to improve air quality using multi-model ensemble（MME）simulations under two future scenarios with the same greenhouse gases（GHGs）emissions but with“weak”（SSP3-7.0）versus“strong”（SSP3-7.0-low NTCF）levels of air quality control measures from the Aerosol and Chemistry Model Intercomparison Project（Aer Chem MIP）,which is endorsed by the Coupled Model Intercomparison Project phase 6（CMIP6）.This study also assesses the resulting population exposure risks in Asia due to non-methane SLCF emissions reductions combined with future population distribution data under shared socioeconomic pathway（SSP3）.The main findings are summarized as follows:（1）Under the SSP3-7.0-low NTCF scenario,reductions of non-methane SLCFs will generally improve air quality across Asia,with average PM_2.5（particles with aerodynamic equivalent diameter≤2.5μm）and ozone concentrations decreasing by2.7±0.4μg/m³ and 4.0±1.7 ppb,respectively.PM_2.5 decreased by 8.8±2.8μg/m³,10.6±1.9μg/m³,and 2.5±0.4μg/m³ in southern Asia,eastern China（EC）,and southeastern Asia,respectively.Global warming can offset part of decrease in PM_2.5concentrations caused by emission reductions.The improvement of ozone pollution at regional and seasonal scales due to emission reductions is limited,especially in EC,where the winter ozone concentrations increase by 4.0±4.5 ppb despite of emission reductions.（2）Future reductions in non-methane SLCF emissions lead to an increase of 0.23±0.16 W/m² in global annual mean effective radiative forcing at the top of the atmosphere and an increase in global average surface air temperature of 0.19±0.1 K during 2031～2050,thereby magnifying GHG-induced global surface warming.The additional warming caused by the non-methane SLCF reductions increases the hottest days（TXx）by 0.3±0.1 K,the percentage of warm days（TX90p）by 4.8±2.2%,the number of tropical nights（TR）by 1.7±0.8 days,the warm spell duration（WSDI）by1.0±0.4 days,the number of heavy precipitation days（R10）by 1.0±0.5 days,the maximum consecutive 5-day precipitation（RX5day）by 1.0±0.3 mm,and the total wet-day precipitation（R95p）by 16.4±7.3 mm during 2031～2050.The largest regional increases of TXx,TX90p,and WSDI occur in northern India（NIN）and northern China（NC）.Relatively large increases in TR are projected in NC and the Sichuan Basin（SCB）,reaching 5.1±2.5 days and 4.9±3.3 days,respectively.For precipitation extremes,the regional changes are greatest in southern China（SC）,particularly southwestern China（SWC）,where reductions of non-methane SLCF emissions increase R10 by 2.5±1.9days,RX5day by 2.5±1.5 mm,and R95p by 37.5±22.6 mm.Moreover,future emission reductions of non-methane SLCFs also increase the occurrence of compound flood-heat wave extreme in Asia,with stronger impacts on NC and northeast China.（3）Future reductions in non-methane SLCFs increase the population exposure to climate extremes to varying degrees.The NIN is the most affected region,where the population exposure to temperature,precipitation,and compound extremes increase（32.2±11.4）×10⁷,（4.6±6.1）×10⁶,and（2.5±2.2）×10⁶ person·days,respectively.Increased risk of population exposure to climate extremes caused by the non-methane SLCFs in some areas is comparable to or even stronger than that caused by the GHG forcing under higher GHG emission scenarios（SSP3-7.0）.