Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> The effective radiative forcing of anthropogenic aerosols (ERF<span class="inline-formula"><sub>aer</sub></span>) is an important measure of the anthropogenic aerosol effects simulated by a global climate model. Here we analyze ERF<span class="inline-formula"><sub>aer</sub></span> simulated by the E3SM version 1 (E3SMv1) atmospheric model using both century-long free-running atmosphereâland simulations and short nudged simulations. We relate the simulated ERF<span class="inline-formula"><sub>aer</sub></span> to characteristics of the aerosol composition and optical properties, and we evaluate the relationships between key aerosol and cloud properties. In terms of historical changes from the year 1870 to 2014, our results show that the global mean anthropogenic aerosol burden and optical depth increase during the simulation period as expected, but the regional averages show large differences in the temporal evolution. The largest regional differences are found in the emission-induced evolution of the burden and optical depth of the sulfate aerosol: a strong decreasing trend is seen in the Northern Hemisphere high-latitude region after around 1970, while a continued increase is simulated in the tropics. The relationships between key aerosol and cloud properties (relative changes between pre-industrial and present-day conditions) also show evident changes after 1970, diverging from the linear relationships exhibited for the period of 1870â1969. In addition to the regional differences in the simulated relationships, a reduced sensitivity in cloud droplet number and other cloud properties to aerosol perturbations is seen when the aerosol perturbation is large. Consequently, the global annual mean ERF<span class="inline-formula"><sub>aer</sub></span> magnitude does not increase after around 1970. The ERF<span class="inline-formula"><sub>aer</sub></span> in E3SMv1 is relatively large compared to the recently published multi-model estimates; the primary reason is the large indirect aerosol effect (i.e., through aerosolâcloud interactions). Compared to other models, E3SMv1 features large relative changes in the cloud droplet effective radius in response to aerosol perturbations. Large sensitivity is also seen in the liquid cloud optical depth, which is determined by changes in both the effective radius and liquid water path. Aerosol-induced changes in liquid and ice cloud properties in E3SMv1 are found to have a strong correlation, as the evolution of anthropogenic sulfate aerosols affects both the liquid cloud formation and the homogeneous ice nucleation in cirrus clouds (that causes a large effect on longwave ERF<span class="inline-formula"><sub>aer</sub></span>). As suggested by a previous study, the large ERF<span class="inline-formula"><sub>aer</sub></span> appears to be one of the reasons why the model cannot reproduce the observed global mean temperature evolution in the second half of the 20th century. Sensitivity simulations are performed to understand which parameterization and/or parameter changes have a large impact on the simulated ERF<span class="inline-formula"><sub>aer</sub></span>. The ERF<span class="inline-formula"><sub>aer</sub></span> estimates in E3SMv1 for the shortwave and longwave components are sensitive to the parameterization changes in both liquid and ice cloud processes. When the parameterization of ice cloud processes is modified, the top-of-model forcing changes in the shortwave and longwave components largely offset each other, so the net effect is negligible. This suggests that, to reduce the magnitude of the net ERF<span class="inline-formula"><sub>aer</sub></span>, it would be more effective to reduce the anthropogenic aerosol effect through liquid or mixed-phase clouds.
Highlights
Variations in atmospheric composition play an important role in causing the observed climate changes since the preindustrial era
In terms of historical changes from the year 1870 to 2014, our results show that the global mean anthropogenic aerosol burden and optical depth increase during the simulation period as expected, but the regional averages show large differences in the temporal evolution
The largest regional differences are found in the emission-induced evolution of the burden and optical 10 depth of the sulfate aerosol: a strong decreasing trend is seen in the Northern Hemisphere high-latitude region after around 1970, while a continued increase is simulated in the tropics
Summary
Variations in atmospheric composition play an important role in causing the observed (historical) climate changes since the preindustrial era. Golaz et al (2019) reports that the present-day (average of 1995-2014) net (shortwave + longwave) effective radiative forcing of anthropogenic aerosols (ERFaer) in the atmosphere component of the Energy Exascale Earth System Model version 1 (E3SMv1) is about -1.65 W m−2. This value is outside the likely range of -0.63 to -1.37 W m−2 estimated from models participating in the Radiative Forcing Model Intercomparison Project (RFMIP, Smith et al, 2020) 45 for the World Climate Research Programme Coupled Model Intercomparison Project-Phase 6 (CMIP6, Eyring et al, 2016).
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