Max Planck Institut's MPI-M Earth System Model v1.2: To prevent a sensitivity of 7C in average temperature instead of the right value (2.77C), an atmospheric parameter had to be tuned by a factor of 10 (!)

Mauritsen, T., Bader, J., Becker, T., Behrens, J., Bittner, M., Brokopf, R., et al. (2019). Developments in the MPI-M Earth System Model version 1.2 (MPI-ESM1.2) and its response to increasing CO2. Journal of Advances in Modeling Earth Systems, 11, 998–1038. https://doi.org/10.1029/2018MS001400

Abstract: A new release of the Max Planck Institute for Meteorology Earth System Model version 1.2 (MPI ESM1.2) is presented. The development focused on correcting errors in and improving the physical processes representation, as well as improving the computational performance, versatility, and overall user friendliness. In addition to new radiation and aerosol parameterizations of the atmosphere, several relatively large, but partly compensating, coding errors in the model's cloud, convection, and turbulence parameterizations were corrected. The representation of land processes was refined by introducing a multilayer soil hydrology scheme, extending the land biogeochemistry to include the nitrogen cycle, replacing the soil and litter decomposition model and improving the representation of wildfires. The ocean biogeochemistry now represents cyanobacteria prognostically in order to capture the response of nitrogen fixation to changing climate conditions and further includes improved detritus settling and numerous other refinements. As something new, in addition to limiting drift and minimizing certain biases, the instrumental record warming was explicitly taken into account during the tuning process. To this end, a very high climate sensitivity of around 7 K caused by low-level clouds in the tropics as found in an intermediate model version was addressed, as it was not deemed possible to match observed warming otherwise. As a result, the model has a climate sensitivity to a doubling of CO2 over preindustrial conditions of 2.77 K, maintaining the previously identified highly nonlinear global mean response to increasing CO2 forcing, which nonetheless can be represented by a simple two-layer model.

3.6 Atmospheric Model Tuning

A major retuning of the model was required because all modifications taken together caused a decrease of the global top-of-atmosphere radiation budget by about 10 W/m2 due mainly to the corrected cloud fraction scheme (section 3.1) and also because the model climate sensitivity had roughly doubled to around 7 K, which would have prevented a reasonable match to the instrumental record warming. If the latter had not been addressed, the model's historical warming would have roughly exceeded that observed by a factor of 2. When reducing the historical warming in a model there are essentially three options: reduce forcing, increase deep ocean heat uptake efficiency, or reduce the climate sensitivity. The forcing can be reduced by increasing aerosol cooling by enhancing the indirect effect, but at the time (2014–2015) we did not have such a parameterization in the model, which was developed after that (section 3.3). Further, ocean heat uptake already exceed that observed (Giorgetta et al., 2013), and so we were left with reducing the climate sensitivity. Since the predecessor MPI-ESM model warmed slightly more than observed, and it had a sensitivity of 3.5 K, we decided to aim at an equilibrium climate sensitivity of around 3 K. The reduction of the model's sensitivity was primarily achieved by increasing the entrainment rate for shallow convection by a factor of 10, from 3 × 10−4 m−1 in ECHAM6.1 to 3 × 10−3 m−1 in ECHAM6.3, with the purpose to reduce tropical low-level cloud feedback. But also other convective cloud parameters, mixed-phase cloud processes, and the representation of stratocumulus were found to be important.