Allen, Robert J., Laura J Wilcox, Bjørn H Samset, Sharar Ahmadi, Annica M L Ekman, Maxwell T Elling, Luke Fraser-Leach, Paul T Griffiths, James Keeble, Tsuyoshi Koshiro, Paul J Kushner, Anna Lewinschal, Molly MacRae, Risto Makkonen, Joonas Merikanto, Pierre Nabat, Larissa Nazarenko, Declan O'Donnell, Naga Oshima, David J Paynter, Geeta Persad, Steven T Rumbold, Neil C Swart, Toshihiko Takemura, Kostas Tsigaridis, Knut von Salzen, and Daniel M Westervelt, 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), DOI:10.1088/2752-5295/ae5418. 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 + or - 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 + or - 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 + or - 1.36 W m−2 for EAS) than the corresponding global mean value. However, these regional values are even larger (up to 2.69 + or - 1.72 W m−2 for EAS) under global aerosol reductions, implying remote emission reductions represent a sizable contribution (up to 1.05 + or - 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.
Despite distinct geographic distributions of top-of-the-atmosphere radiative forcing, anthropogenic greenhouse gases and aerosols have been found to produce similar patterns of climate response in atmosphere-and-ocean coupled climate model simulations. Understanding surface energy flux changes, a crucial pathway by which atmospheric forcing is communicated to the ocean, is a vital bridge to explaining the similar full atmosphere-and-ocean responses to these disparate forcings. Here we analyze the fast, atmosphere-driven change in surface energy flux caused by present-day greenhouse gases vs aerosols to elucidate its role in shaping the subsequent slow, coupled response. We find that the surface energy flux response patterns achieve roughly two-thirds of the anti-correlation seen in the fully coupled response, driven by Rossby waves excited by symmetric changes to the land–sea contrast. Our results suggest that atmosphere and land surface processes are capable of achieving substantial within-hemisphere homogenization in the climate response to disparate forcers on fast, societally-relevant timescales.
East Asia has some of the largest concentrations of absorbing aerosols globally, and these, along with the region’s scattering aerosols, have both reduced the amount of solar radiation reaching the Earth’s surface regionally (“solar dimming”) and increased shortwave absorption within the atmosphere, particularly during the peak months of the East Asian Summer Monsoon (EASM). This study analyzes how atmospheric absorption and surface solar dimming compete in driving the response of regional summertime climate to anthropogenic aerosols, which dominates, and why—issues of particular importance for predicting how East Asian climate will respond to projected changes in absorbing and scattering aerosol emissions in the future. These questions are probed in a state-of-the-art general circulation model using a combination of realistic and novel idealized aerosol perturbations that allow analysis of the relative influence of absorbing aerosols’ atmospheric and surface-driven impacts on regional circulation and climate. Results show that even purely absorption-driven dimming decreases EASM precipitation by cooling the land surface, counteracting climatological land-sea contrast and reducing ascending atmospheric motion and on-shore winds, despite the associated positive top-of-atmosphere regional radiative forcing. Absorption-driven atmospheric heating does partially offset the precipitation and surface evaporation reduction from surface dimming, but the overall response to aerosol absorption more closely resembles the response to its surface dimming than to its atmospheric heating. These findings provide a novel decomposition of absorbing aerosol’s impacts on regional climate and demonstrate that the response cannot be expected to follow the sign of absorption’s top-of-atmosphere or even atmospheric radiative perturbation.
Persad, Geeta, Yi Ming, and V Ramaswamy, September 2014: The Role of Aerosol Absorption in Driving Clear-Sky Solar Dimming over East Asia. Journal of Geophysical Research: Atmospheres, 119(17), DOI:10.1002/2014JD021577. Abstract
Surface-based observations indicate a significant decreasing trend in clear-sky downward surface solar radiation (SSR) over East Asia since the 1960s. This “dimming" is thought to be driven by the region's long-term increase in aerosol emissions, butlittle work has been done to quantify the underlying physical mechanisms or the contribution from aerosol absorption within the atmospheric column. Given the distinct climate impacts that absorption-driven dimming may produce, this constitutes an important, but thus far rather neglected, line of inquiry.
We examine experiments conducted in the Geophysical Fluid Dynamics Laboratory's Atmospheric General Circulation Models, AM2.1 and AM3, in order to analyze the model-simulated East Asian clear-sky SSR trends. We also use the models’ standalone radiation module to examine the contribution from various aerosol characteristics in the two models (such as burden, mixing state, hygroscopicity, and seasonal distribution) to the trends. Both models produce trends in clear-sky SSR that are comparable tothat observed, but via disparate mechanisms. Despite their different aerosol characteristics, the models produce nearly identical increases in aerosol absorption since the 1960s, constituting as much as half of the modeled clear-sky dimming. This is due to a compensation between the differences in aerosol column burden and mixing state assumed in the two models, i.e. plausible clear-sky SSR simulations can be achieved via drastically different aerosol parameterizations. Our novel results indicatethat trends in aerosol absorption drive a large portion of East Asian clear-sky solar dimming in the models presented here and for the time periods analyzed, and that mechanistic analysis of the factors involved in aerosol absorption is an important diagnostic in evaluating modeled clear-sky solar dimming trends.
Absorbing aerosols affect the Earth's climate through direct radiative heating of the troposphere. We analyze the tropical tropospheric-only response to a globally uniform increase in black carbon, simulated with an atmospheric general circulation model, in order to gain insight into the interactions that determine the radiative flux perturbation. Over the convective regions, heating in the free troposphere hinders the vertical development of deep cumulus clouds, resulting in the detrainment of more cloudy air into the large-scale environment and stronger cloud reflection. A different mechanism operates over the subsidence regions, where heating near the boundary layer top causes a substantial reduction in low cloud amount thermodynamically by decreasing relative humidity and dynamically by lowering cloud top. These findings, which align well with previous general circulation model and large eddy simulation calculations for black carbon, provide physically based explanations for the main characteristics of the tropical tropospheric adjustment. The implications for quantifying the climate perturbation posed by absorbing aerosols are discussed.
Ming, Yi, V Ramaswamy, and Geeta Persad, July 2010: Two opposing effects of absorbing aerosols on global-mean precipitation. Geophysical Research Letters, 37, L13701, DOI:10.1029/2010GL042895. Abstract
Absorbing aerosols affect global-mean precipitation primarily in two ways. They give rise to stronger shortwave atmospheric heating, which acts to suppress precipitation. Depending on the top-of-the-atmosphere radiative flux change, they can also warm up the surface with a tendency to increase precipitation. Here, we present a theoretical framework that takes into account both effects, and apply it to analyze the hydrological responses to increased black carbon burden simulated with a general circulation model. It is found that the damping effect of atmospheric heating can outweigh the enhancing effect of surface warming, resulting in a net decrease in precipitation. The implications for moist convection and general circulation are discussed.
