Tightening the IPCC’s Climate Projections: Jerry Meehl’s Multi-Decadal Contribution

In 1990 the IPCC published their First Assessment Report that provided quantitative climate change projections as simulated by the first generation of coupled atmosphere-ocean climate models. In the years before, Jerry Meehl was already among a small team of scientists that carried out climate model simulations and so became a contributing author of two key chapters of that report. What has started as an effort of a handful of scientists from few institutions, guided by the Steering Group on Global Climate Modelling, of which Meehl was a member, has grown into a firmly established, worldwide operation, the Coupled Model Intercomparison Project (CMIP). CMIP is a comprehensive and inclusive, community-driven operation under the auspices of the World Climate Research Programme of WMO, without which the IPCC could not assess on an comprehensive, objective, open and transparent basis the scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change, its potential impacts and options for adaptation and mitigation (quote from the Principles Governing IPCC Work).

Jerry Meehl’s scientific oeuvre combines model development and simulations, climate process analysis, and assessment work in the IPCC over five complete IPCC cycles from 1990 to 2013. This is an invaluable contribution to understanding the consequences of anthropogenic climate change. Jerry was a staunch defender of multi-model means in IPCC reports, affirming "model democracy" and enabling straightforward filtering of errors and anomalies of individual climate models. This approach enabled a broad consensus among the policymakers and laid the basis for science-informed political agreements. However, recent climate events, such as the so-called “global warming pause” or “hiatus” of 1998 to 2012, or the “global ocean heat anomaly” of 2023/2024 have launched a debate on whether such out-of-experience climate variability and phases are simulated by the current generation of coupled climate models. The analysis of the CMIP database and other ensemble simulations demonstrates that such episodes are simulated by individual models and hence are not surprises. This finding, corroborating the power and realism of current climate models, should be communicated more effectively and proactively to the public and the policymakers.