Role of climate model dynamics in estimated climate responses to anthropogenic aerosols

Significant discrepancies remain in estimates of
climate impacts of anthropogenic aerosols between different
general circulation models (GCMs). Here, we demonstrate
that eliminating differences in model aerosol or radiative
forcing fields results in close agreement in simulated
globally averaged temperature and precipitation responses
in the studied GCMs. However, it does not erase
the differences in regional responses. We carry out experiments
of equilibrium climate response to modern-day anthropogenic
aerosols using an identical representation of anthropogenic
aerosol optical properties and the first indirect effect
of aerosols, MACv2-SP (a simple plume implementation of
the second version of the Max Planck Institute Aerosol CLimatology),
in two independent climate models (NorESM,
Norwegian Earth System Model, and ECHAM6). We find
consistent global average temperature responses of 􀀀0:48
(0:02) and 􀀀0:50 (0:03)K and precipitation responses of
􀀀1:69 (0:04)% and 􀀀1:79 (0:05)% in NorESM1 and
ECHAM6, respectively, compared to modern-day equilibrium
climate without anthropogenic aerosols. However, significant
differences remain between the two GCMs’ regional
temperature responses around the Arctic circle and the Equator
and precipitation responses in the tropics. The scatter in
the simulated globally averaged responses is small in magnitude
when compared against literature data from modern
GCMs using model intrinsic aerosols but same aerosol emissions
􀀀(0.5–1.1)K and 􀀀(1.5–3.1)% for temperature and
precipitation, respectively). The Pearson correlation of regional
temperature (precipitation) response in these literature
model experiments with intrinsic aerosols is 0.79 (0.34).
The corresponding correlation coefficient for NorESM1 and
ECHAM6 runs with identical aerosols is 0.78 (0.41). The
lack of improvement in correlation coefficients between
models with identical aerosols and models with intrinsic
aerosols implies that the spatial distribution of regional climate
responses is not improved via homogenizing the aerosol
descriptions in the models. Rather, differences in the atmospheric
dynamic and snow/sea ice cover responses dominate
the differences in regional climate responses. Hence,
even if we would have perfect aerosol descriptions inside
the global climate models, uncertainty arising from the differences
in circulation responses between the models would
likely still result in a significant uncertainty in regional climate
responses.