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.