Effective radiative forcing of anthropogenic aerosols in E3SM version 1: historical changes, causality, decomposition, and parameterization sensitivities
Authors: Zhang Kai , Zhang Wentao , Wan Hui , Rasch Philip J. , Ghan Steven J. , Easter Richard C. , Shi Xiangjun , Wang Yong , Wang Hailong , Ma Po-Lun , Zhang Shixuan , Sun Jian , Burrows Susannah M. , Shrivastava Manish , Singh Balwinder , Qian Yun , Liu Xiaohong , Golaz Jean-Christophe , Tang Qi , Zheng Xue , Xie Shaocheng , Lin Wuyin , Feng Yan , Wang Minghuai , Yoon Jin-Ho , Leung L. Ruby 
Published: July 2022

Abstract. The effective radiative forcing of anthropogenic aerosols (ERFaer) is an important
measure of the anthropogenic aerosol effects simulated by a global climate model.
Here we analyze ERFaer simulated by the E3SM version 1 (E3SMv1) atmospheric model using
both century-long free-running atmosphere–land simulations and short nudged simulations.
We relate the simulated ERFaer 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 1870–1969. 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 ERFaer magnitude does not increase
after around 1970. The ERFaer 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 aerosol–cloud 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 ERFaer). As suggested by a previous study, the large ERFaer 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 ERFaer.
The ERFaer 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 ERFaer, it would be
more effective to reduce the anthropogenic aerosol effect through
liquid or mixed-phase clouds.