EC-Earth3-AerChem: a global climate model with interactive aerosols and atmospheric chemistry participating in CMIP6
Authors: van Noije Twan , Bergman Tommi , Le Sager Philippe , O'Donnell Declan , Makkonen Risto , Gonçalves-Ageitos María , Döscher Ralf , Fladrich Uwe , von Hardenberg Jost , Keskinen Jukka-Pekka , Korhonen Hannele , Laakso Anton , Myriokefalitakis Stelios , Ollinaho Pirkka , Pérez García-Pando Carlos , Reerink Thomas , Schrödner Roland , Wyser Klaus , Yang Shuting 
Published: September 2021

Abstract. This paper documents the global climate model EC-Earth3-AerChem,
one of the members of the EC-Earth3 family of models participating in the
Coupled Model Intercomparison Project Phase 6 (CMIP6). EC-Earth3-AerChem has
interactive aerosols and atmospheric chemistry and contributes to the
Aerosols and Chemistry Model Intercomparison Project (AerChemMIP). In this
paper, we give an overview of the model, describe in detail how it
differs from the other EC-Earth3 configurations, and outline the new features compared with the previously documented version of the model (EC-Earth
2.4). We explain how the model was tuned and spun up under preindustrial
conditions and characterize the model's general performance on the basis of
a selection of coupled simulations conducted for CMIP6. The net energy
imbalance at the top of the atmosphere in the preindustrial control
simulation is on average −0.09 W m−2 with a standard deviation due to
interannual variability of 0.25 W m−2, showing no significant drift.
The global surface air temperature in the simulation is on average 14.08 ∘C with an interannual standard deviation of 0.17 ∘C, exhibiting a small drift of 0.015 ± 0.005 ∘C per century.
The model's effective equilibrium climate sensitivity is estimated at 3.9 ∘C, and its transient climate response is estimated at 2.1 ∘C. The
CMIP6 historical simulation displays spurious interdecadal variability in
Northern Hemisphere temperatures, resulting in a large spread across
ensemble members and a tendency to underestimate observed annual surface
temperature anomalies from the early 20th century onwards. The observed
warming of the Southern Hemisphere is well reproduced by the model. Compared
with the ECMWF (European Centre for Medium-Range Weather
Forecasts) Reanalysis version 5 (ERA5), the surface air temperature climatology for 1995–2014 has an
average bias of −0.86 ± 0.05 ∘C with a standard deviation
across ensemble members of 0.35 ∘C in the Northern Hemisphere and
1.29 ± 0.02 ∘C with a corresponding standard deviation of
0.05 ∘C in the Southern Hemisphere. The Southern Hemisphere warm
bias is largely caused by errors in shortwave cloud radiative effects over
the Southern Ocean, a deficiency of many climate models. Changes in the
emissions of near-term climate forcers (NTCFs) have significant effects on
the global climate from the second half of the 20th century onwards. For the
SSP3-7.0 Shared Socioeconomic Pathway, the model gives a global warming at
the end of the 21st century (2091–2100) of 4.9 ∘C above the
preindustrial mean. A 0.5 ∘C stronger warming is obtained for
the AerChemMIP scenario with reduced emissions of NTCFs. With concurrent
reductions of future methane concentrations, the warming is projected to be
reduced by 0.5 ∘C.