All climate models are wrong but wrong models are useful: Why, where, and how?

In the present essay, I first argue that current Earth System Models (ESMs) are not the Achilles’ heel for the design of climate change mitigation policies. In particular, the comparison of illustrative rather than predictive socio-economic scenarios does not justify the implementation of km-scale ESMs. I also claim that km-scale models is not a priority within the framework of the Coupled Model Intercomparison Project (CMIP), and that adaptation policies should be designed as risk management strategies requiring, above all, better sampling of other sources of uncertainty than the current resolution of climate models. Analyzing the impacts of climate change requires large ensembles of simulations, which also involves a trade-off with spatial resolution. Such ensembles also offer new opportunities for an improved model calibration and an easier assessment of their forced response. Conventional ESMs thus remain extremely useful as long as they are not misinterpreted as perfect replicas of the Earth system. Given the increasing emergence of climate change in observations, they will undoubtedly be far less of a hindrance to the next IPCC report than the lack of continuous and controlled instrumental data.