Glacial cooling and climate sensitivity revisited

The Last Glacial Maximum (LGM) has long been a key paleoclimate target for PMIP modeling efforts, as a data-rich climate state substantially different from today. Over the years there have been several estimates of LGM tropical cooling and global cooling, many of which fall on the low side of expectations based on the classical IPCC range of climate sensitivity (1.5-4.5 K per CO₂ doubling). Here, we reanalyze LGM climatology using an updated network of nearly 1000 sea-surface temperature (SST) proxies, Bayesian forward models, and data assimilation with isotope-enabled timeslice simulations conducted with iCESM1.2. We find that the tropics cooled by ca. 2.5C, as opposed to the CLIMAP and MARGO estimate of 1.7C. Global SSTs cooled by ca. 3C, corresponding to a global mean air temperature decrease of 5-7C. These updated estimates imply a LGM climate sensitivity of ca. 3.5 K per CO₂ doubling, squarely in the IPCC range. Our LGM reanalysis provides a new validation target for CMIP6 models.