Decomposing the global and regional aerosol effective radiative forcing associated with strong versus weak air quality policies by Mid-21st century

Allen, Robert J and Wilcox, Laura J and Samset, Bjørn H and Ahmadi, Sharar and Ekman, Annica M L and Elling, Maxwell T and Fraser-Leach, Luke and Griffiths, Paul and Keeble, James and Koshiro, Tsuyoshi and Kushner, Paul and Lewinschal, Anna and MacRae, Molly and Makkonen, Risto and Merikanto, Joonas and Nabat, Pierre and Nazarenko, Larissa and O’Donnell, Declan and Oshima, Naga and Paynter, David and Persad, Geeta and Rumbold, Steven T and Swart, Neil and Takemura, Toshihiko and Tsigaridis, Kostas and von Salzen, Knut and Westervelt, Daniel M (2026) Decomposing the global and regional aerosol effective radiative forcing associated with strong versus weak air quality policies by Mid-21st century. Environmental Research: Climate, 5 (2): 025014. ISSN 2752-5295

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Abstract

The Regional Aerosol Model Intercomparison Project (RAMIP) is designed to quantify the forcing and climate impacts of mid-21st century anthropogenic aerosol and precursor gas (AA) emissions reductions (both industrial and biomass burning), by comparing a weak (SSP3-7.0) versus strong (SSP1-2.6) level of air quality control aerosol emissions pathway. AA emissions reductions experiments include global (GLO), East Asia (EAS), South Asia, Africa and the Middle East (AFR), and North America and Europe (NAE). Here, we use RAMIP time-slice simulations with fixed sea surface temperatures and sea-ice distributions from nine models to quantify the aerosol effective radiative forcing (ERF), including aerosol radiation (ERFari) and aerosol cloud interactions (ERFaci). The multi-model global mean net ERFari+aci is 0.77±0.25 W m−2 for GLO, and three of the four regional perturbations yield a significant positive net ERFari+aci (up to 0.15±0.07 W m−2 for EAS). In all cases, net ERFari+aci is dominated by aerosol-cloud interactions, which are largely due to reduced cloud scattering. Of the four regions, NAE yields the largest forcing efficiency whereas AFR yields the weakest. Although the areas outside our four target regions contribute 25% to the GLO aerosol optical depth reduction, they disproportionately contribute 44% to the GLO net ERFari+aci. The multimodel regional mean net ERFari+aci for three regional perturbations is much larger (up to 1.64±1.36 W m−2 for EAS) than the corresponding global mean value. However, these regional values are even larger (up to 2.69±1.72 W m−2 for EAS) under global aerosol reductions, implying remote emission reductions represent a sizable contribution (up to 1.05±0.56 W m−2 for EAS). These large regional ERFs will in turn drive relatively large regional climate impacts, which continue to be underappreciated in most policy discussions.