Developments in the MPI‐M Earth System Model version 1.2 (MPI‐ESM1.2) and Its Response to Increasing CO2

• Main Authors: Thorsten Mauritsen, Jürgen Bader, Tobias Becker, Jörg Behrens, Matthias Bittner, Renate Brokopf, Victor Brovkin, Martin Claussen, Traute Crueger, Monika Esch, Irina Fast, Stephanie Fiedler, Dagmar Fläschner, Veronika Gayler, Marco Giorgetta, Daniel S. Goll, Helmuth Haak, Stefan Hagemann, Christopher Hedemann, Cathy Hohenegger, Tatiana Ilyina, Thomas Jahns, Diego Jimenéz‐de‐la‐Cuesta, Johann Jungclaus, Thomas Kleinen, Silvia Kloster, Daniela Kracher, Stefan Kinne, Deike Kleberg, Gitta Lasslop, Luis Kornblueh, Jochem Marotzke, Daniela Matei, Katharina Meraner, Uwe Mikolajewicz, Kameswarrao Modali, Benjamin Möbis, Wolfgang A. Müller, Julia E. M. S. Nabel, Christine C. W. Nam, Dirk Notz, Sarah‐Sylvia Nyawira, Hanna Paulsen, Karsten Peters, Robert Pincus, Holger Pohlmann, Julia Pongratz, Max Popp, Thomas Jürgen Raddatz, Sebastian Rast, Rene Redler, Christian H. Reick, Tim Rohrschneider, Vera Schemann, Hauke Schmidt, Reiner Schnur, Uwe Schulzweida, Katharina D. Six, Lukas Stein, Irene Stemmler, Bjorn Stevens, Jin‐Song vonStorch, Fangxing Tian, Aiko Voigt, Philipp Vrese, Karl‐Hermann Wieners, Stiig Wilkenskjeld, Alexander Winkler, Erich Roeckner
• Format: Article
• Language: English
• Published: American Geophysical Union (AGU) 2019-04-01
• Series: Journal of Advances in Modeling Earth Systems
• Subjects: coupled climate model; model development; climate sensitivity
• Online Access: 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.