Summary

A host of global climate models developed for the United Nations's next major assessment of global warming, due in 2021, are now showing a puzzling but undeniable trend: They are running hotter than they have in the past. In earlier models, doubling atmospheric carbon dioxide over preindustrial levels led models to predict somewhere between 2°C and 4.5°C of warming once the planet came into balance. But in at least eight of the next-generation models, produced by leading centers in the United States, the United Kingdom, Canada, and France, that "equilibrium climate sensitivity" has come in at 5°C or warmer. Many scientists, including the model developers, are doubtful this increased warming is likely to be real. Over the next year, they will be comparing notes on what happened in their models, which in many cases simulate the Earth system better than ever before. It's also possible that climate sensitivities from models will be de-emphasized in the next U.N. climate assessment, further replaced instead by restraints from the ancient climate and modern observations.