5 Climate response
5.1 Climate sensitivity
MPI-ESM has been explicitly tuned to have an equilibrium climate
sensitivity (ECS) of 3.0°C (Müller et al., 2018). The ECS is inferred
from linear regression of the Top-of-Atmosphere (TOA) imbalance against
the temperature response in the 4xCO2 simulation. AWI-CM
uses the same atmospheric component without any extra tuning so that
differences both in the ECS and in the transient climate response (TCR),
computed as average response over the 30 years around year 70 from the
1pctCO2 simulation, are only due to the different ocean component.
For AWI-CM, the ECS amounts to 3.2°C (Fig. 12, half of the
4xCO2 equilibrium temperature difference). This is
similar to the average over the CMIP5 models (IPCC, 2014) and slightly
larger than for the CMIP6 version of MPI-ESM (3.0°C, Müller et al.,
2018, Mauritsen et al., 2019, Tokarska et al., 2020). The TCR amounts to
2.1°C, which is slightly stronger than the average over the CMIP5 models
(1.8°C, IPCC, 2014) and the CMIP6 version of MPI-ESM (1.7°C, Tokarska et
al., 2020). Note that by considering changes in the TOA flux and
the global-mean near-surface temperature (delta approach) our estimates
for the ECS and the TCR are not affected by the imbalances reported in
section 3 (apart from possible non-linear effects).
It seems that AWI-CM absorbs energy in the deep ocean more slowly
compared to MPI-ESM. However, this hypothesis needs to be confirmed
through a thorough analysis in a joint effort with the Max Planck
Institute for Meteorology. Ideally, the ECS should not be affected.
However, since the Gregory method to compute ECS is only an
approximation, small differences can still occur.
Changes in the energy budget and the role of shortwave feedback in the
historical and scenario simulations are detailed in Sect. 5.5.