Evaluation of CNRM Earth System Model, CNRM‐ESM2‐1: Role of Earth System Processes in Present‐Day and Future Climate

• Main Authors: Roland Séférian, Pierre Nabat, Martine Michou, David Saint‐Martin, Aurore Voldoire, Jeanne Colin, Bertrand Decharme, Christine Delire, Sarah Berthet, Matthieu Chevallier, Stephane Sénési, Laurent Franchisteguy, Jessica Vial, Marc Mallet, Emilie Joetzjer, Olivier Geoffroy, Jean‐François Guérémy, Marie‐Pierre Moine, Rym Msadek, Aurélien Ribes, Matthias Rocher, Romain Roehrig, David Salas‐y‐Mélia, Emilia Sanchez, Laurent Terray, Sophie Valcke, Robin Waldman, Olivier Aumont, Laurent Bopp, Julie Deshayes, Christian Éthé, Gurvan Madec
• Format: Article
• Language: English
• Published: American Geophysical Union (AGU) 2019-12-01
• Series: Journal of Advances in Modeling Earth Systems
• Subjects: climate modeling; Earth system modeling; biogeochemical cycles; aerosols; CMIP6; future projections
• Online Access: https://doi.org/10.1029/2019MS001791

Abstract This study introduces CNRM‐ESM2‐1, the Earth system (ES) model of second generation developed by CNRM‐CERFACS for the sixth phase of the Coupled Model Intercomparison Project (CMIP6). CNRM‐ESM2‐1 offers a higher model complexity than the Atmosphere‐Ocean General Circulation Model CNRM‐CM6‐1 by adding interactive ES components such as carbon cycle, aerosols, and atmospheric chemistry. As both models share the same code, physical parameterizations, and grid resolution, they offer a fully traceable framework to investigate how far the represented ES processes impact the model performance over present‐day, response to external forcing and future climate projections. Using a large variety of CMIP6 experiments, we show that represented ES processes impact more prominently the model response to external forcing than the model performance over present‐day. Both models display comparable performance at replicating modern observations although the mean climate of CNRM‐ESM2‐1 is slightly warmer than that of CNRM‐CM6‐1. This difference arises from land cover‐aerosol interactions where the use of different soil vegetation distributions between both models impacts the rate of dust emissions. This interaction results in a smaller aerosol burden in CNRM‐ESM2‐1 than in CNRM‐CM6‐1, leading to a different surface radiative budget and climate. Greater differences are found when comparing the model response to external forcing and future climate projections. Represented ES processes damp future warming by up to 10% in CNRM‐ESM2‐1 with respect to CNRM‐CM6‐1. The representation of land vegetation and the CO2‐water‐stomatal feedback between both models explain about 60% of this difference. The remainder is driven by other ES feedbacks such as the natural aerosol feedback.