Insights into recent aerosol trends over Asia from observations and CMIP6 simulations

Aerosols continue to contribute the largest uncertainty in climate change. Over Asia, a global aerosol hotspot, spatial patterns of aerosol emissions are changing mainly because of changes in anthropogenic emissions, producing a dipole in atmospheric aerosol loading between East (decrease in emissions) and South Asia (increase in emissions). The resultant aerosol radiative effects are expected to be different as compared to the last decades of the 20^th century because of this emerging Asian aerosol dipole. The projection and assessments of radiative and climate impacts of aerosols rely on simulating accurately the aerosol properties, thus, making it imperative that current climate models involved in climate assessments including the Intergovernmental Panel on Climate Change Assessment Reports, simulate well the magnitude and trends in changing aerosol properties. For the first time, in this study we analyze trends in aerosol properties over Asia from satellite and ground-based observations, and simulations from climate models in Coupled Model Intercomparison Project Phase 6 (CMIP6) experiment with state-of-the-art treatment of aerosol chemistry, physics and meteorology. The results reveal large inter-model differences in model estimates, and discrepancies between model simulations and observations as most models are not able to capture the recent observed magnitudes and trends in aerosol optical depth (AOD) and single scattering albedo (SSA) over Asia. The absolute and the relative (percent) trends (positive and/or negative) in AOD are significantly higher than the trends in SSA. The aerosol-induced effective radiative forcing within the atmosphere simulated with three CMIP6 models show a positive (increasing) trend over Asia. A positive trend in atmospheric heating due to aerosols in model simulations is consistent with model simulated trends in AOD (positive) and SSA (negative). These results on model-observations comparison need to be taken into account while examining the projected/expected future climate impacts due to aerosols, and potential value of various mitigation measures, in particular on regional and decadal climate change in Asia which is largely uncertain. MAIN FINDING: Our analysis of satellite and ground-based observations, and simulations from climate models in CMIP6 experiment with state-of-the-art treatment of aerosol chemistry, physics and meteorology reveal large difference in model calculations, and most models are not able to capture the recent observed trends in aerosol optical depth and single scattering albedo over Asia during 2000-2018.