Glacial cooling and climate sensitivity revisited

The Last Glacial Maximum (LGM), one of the best-studied paleoclimatic intervals, offers a prime opportunity to investigate how the climate system responds to changes in greenhouse gases (GHGs) and the cryosphere. Previous work has sought to constrain the magnitude and pattern of glacial cooling from paleothermometers, but the uneven distribution of the proxies, as well as their uncertainties, has challenged the construction of a full-field view of the LGM climate state. Here, we combine a large collection of geochemical proxies for sea-surface temperature with an isotope-enabled climate model ensemble to produce a field reconstruction of LGM temperatures using data assimilation. The reconstruction is validated with withheld proxies as well as independent ice core and speleothem d18O measurements. Our assimilated product provides a precise constraint on global mean LGM cooling of -5.9˚C (-6.3 – -5.6˚C, 95% CI). Given assumptions concerning the radiative forcing of GHGs, ice sheets, and aerosols, this cooling translates to an equilibrium climate sensitivity (ECS) of 3.2˚C (2.2 – 4.3˚C, 95% CI), a value that is higher than previous estimates and but consistent with the traditional consensus range of 2 – 4.5˚C.