New paper led by Jiang Zhu, w/ @mountain_gaucho and @cjpoulsen demonstrates that ECS in the CESM2 climate model is just too damn high , leading to a very chilly simulation of the last Ice Age https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL091220

This figure from the paper says it all. Global temperatures drop to 11˚C cooler than preindustrial way below the proxy estimate of 6˚C for the Last Glacial Maximum. This is all attributable to the SW cloud feedback in CAM6 (CESM2 w/ CAM5 is fine)

This is a simple example of how past climates can help constrain climate sensitivity. Now if only we could do paleoclimate experiments *while* we develop the next-gen climate models, we could improve future projections of climate change.