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Global assessment of aerosol radiative effects: New insights from observations, reanalysis, and climate models.

K. Ansari,S. Ramachandran,R. Cherian

Published 2025 in Science of the Total Environment

ABSTRACT

Aerosols continue to be the most uncertain forcing factor in quantifying the present-day radiative forcing. An integrated analysis of crucial aerosol optical and radiative properties across the globe using multi-source (ground-based, satellite, reanalysis, and climate model) data and quantifying the biases on regional and temporal scales can substantially reduce this uncertainty. In a first-of-its-kind study, a comprehensive investigation using AErosol RObotic NETwork (AERONET), MODerate resolution Imaging Spectroradiometer (MODIS), Ozone Monitoring Instrument (OMI), Clouds and the Earth's Radiant Energy System (CERES), Modern-Era Retrospective Analysis for Research and Applications-2 (MERRA-2), and Coupled Model Intercomparison Project Phase 6 (CMIP6) datasets, revealed that aerosol direct radiative effect (DRE) is highest over South Asia and lowest over Australia, followed by North America and Europe. AERONET retrieved aerosol DRE at surface (DRESFC: ∼ - 70 Wm-2), top-of-atmosphere cooling (DRETOA: ∼ - 30 Wm-2), and atmospheric heating (DREATM: ∼40 Wm-2; HR: ∼0.80 Kday-1) are strongest over South Asia - I with significant spatiotemporal variations. These values of DREs arise due to high aerosol optical depth (AOD: ∼0.57) and low single scattering albedo (SSA: ∼0.91). In contrast, AOD and DRE (absolute) in North America, Europe, and Australia are ∼2-4 times lower than in Asia, with less spatiotemporal variability. Notably, low SSA enhances both atmospheric heating and surface cooling efficiencies over South Asia - I and biomass-burning regions. High underestimation of MERRA-2 AOD in high AOD conditions leads to high underestimations in MERRA-2 DREs (in absolute terms). Underestimations in MERRA-2 DREs are pronounced over Asia (∼25 %), with large biases over South Asia (∼33 %). Taylor diagram analysis and collocated validation reveal that MERRA-2 outperforms CERES in reproducing AERONET DRE globally; however, both datasets exhibit substantial biases over Asia. On an annual scale, CMIP6 multi-model mean underestimates AOD in South and Southeast Asia (factor of ∼1.4), and the differences between CMIP6 aerosol direct radiative forcing (DRF) and AERONET DRE at each level (top-of-atmosphere, atmosphere: >10 Wm-2; surface: >20Wm-2) are higher over most regions in Asia. This study provides crucial global insights into aerosol direct radiative effects by utilizing multi-platform datasets, quantifying the regional and seasonal biases, which are essential to fine-tune and improve the aerosol properties and processes in regional and global climate models for accurately assessing the aerosol-climate interactions.

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