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Abstract:
A new release of the Max Planck Institute for Meteorology Earth System Model (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 such that the atmospheric physics now conserve energy. Likewise, a coding error in the parameterization of fractional cloud cover, causing earlier versions of the model to have primarily zero or unity cloud fractions, was corrected. The representation of land processes was refined by introducing a multi-layer 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 oceanbiogeochemistry 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. Updated boundary conditions were implemented in order to participate in the sixth phase of the coupled model intercomparison project (CMIP6), and the model has been tuned and spun up to yield a stable pre-industrial climate. We inspect a first set of model simulations, verifying a close match to the observed mean state and instrumental record warming. We find that the model has a climate sensitivity to a doubling of CO2 over pre-industrial conditions of 2.77 K with a highly non-linear global mean response to increasing CO2 forcing, which nonetheless can be represented by a simple two-layer model.
