Baseline Evaluation of the Impact of Updates to the MIT Earth System Model on its Model Parameter Estimates

Abstract. For over twenty years, the Massachusetts Institute of Technology Earth System Model (MESM) has been used extensively for climate change research. The model is under continuous development with components being added or updated. To provide transparency in the model development, we perform a baseline evaluation of the newest version by comparing model behavior and properties to the previous model version. In particular, the impacts resulting from updates to the land surface model component and the input forcings used in historical simulations of climate change are investigated. We run an 1800-member ensemble of MESM historical climate simulations where the model parameters that set climate sensitivity, ocean heat uptake, and the net anthropogenic aerosol forcing are systematically varied. By comparing model output to observed patterns of surface temperature changes, the linear trend in the increase in ocean heat content, and upper-air temperature changes, we derive probability distributions for the three model parameters. Furthermore, we run a 372-member ensemble of transient climate simulations where model forcings are held fixed, absent an increase in carbon dioxide concentrations at the rate of 1 % per year. From these runs, we derive a response surface for transient climate response and thermosteric sea level rise as a function of climate sensitivity and ocean heat uptake. We compare the probability distributions and response surfaces derived using the current version of MESM to the preceding version to evaluate the impact of the updated land surface model and forcing suite. We show that the probability distributions shift towards higher climate sensitivities and weaker aerosol forcing in response to the new forcing suite. The climate response surfaces are relatively unchanged between model versions, indicating that the updated land surface model has limited impact on temperature evolution in the model.