Improving Aerosol Radiative Forcing and Climate in DOE’s E3SM
Anthropogenic aerosols cool the Earth by reflecting solar radiation back into space and increasing cloud reflectivity. Aerosol forcing is one of the largest uncertainties in climate projection. Shan et al. (2024) reduce the aerosol forcing uncertainty.
European Space Agency
Anthropogenic aerosols cool the Earth by reflecting solar radiation back into space and increasing cloud reflectivity. This is known as direct and indirect aerosol radiative forcings. They can effectively offset the warming effects of greenhouse gases. It plays a crucial role in simulating climate. Many Earth system models, including the U.S. Department of Energy's Energy Exascale Earth System Model (E3SM), exhibit excessive aerosol effective forcing. This is primarily due to the challenges in accurately simulating aerosols, clouds, and their complex interactions. Researchers have improved aerosol and cloud treatments in developing E3SM version 3 (v3). It mitigates the aerosol and cloud biases and achieves more accurate aerosol radiative forcings.
The overly strong aerosol forcing is a primary reason for E3SM (including both version 1 and 2) failed to simulate surface temperature trends over the industrial period. This casts doubt on its reliability for projecting future climate change. The researchers’ developments significantly mitigate the aerosol and cloud biases. This leads to aerosol direct and indirect forcings that agree well with the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR6). These developments play a major role in the reproduction of surface temperature trends over the industrial period by E3SM v3, an unattainable task in the previous E3SM versions. This is a game changer for increasing the fidelity of E3SM in climate projection.
Numerous Earth system models exhibit excessive aerosol effective forcing, including the U.S. Department of Energy's Energy Exascale Earth System Model (E3SM). Here, in the context of the E3SM version 3 effort, the predicted particle property (P3) stratiform cloud microphysics scheme and an enhanced deep convection parameterization suite (ZM_plus) are implemented into E3SM. The ZM_plus includes a convective cloud microphysics scheme, a multi-scale coherent structure parameterization for mesoscale convective systems, and a revised cloud base mass flux formulation considering the impacts of the large-scale environment. The P3 scheme improved cloud and radiation, particularly over the Northern Hemisphere and the frequency of heavy precipitation over the tropics, and the ZM_plus improved clouds in the tropics. P3 decreases aerosol effective forcing by 0.15 W m−2, while the ZM_plus increases it by 0.27 W m−2, resulting from excessive direct (0.31 W m−2) and indirect forcing (−1.79 W m−2). The excessive aerosol forcings are due to aerosol overestimation associated with insufficient aerosol wet removal. By improving the physical treatments in the aerosol wet removal, researchers effectively mitigate anthropogenic aerosol overestimation and thus attenuate direct (0.09 W m−2) and indirect aerosol forcing (−1.52 W m−2). Adjustment to primary organic matter hygroscopicity reduces direct and indirect forcing to more reasonable values: −0.13 W m−2 and −1.31 W m−2, respectively.