[PAOCQ] Peter Huybers (Harvard)

Accurately determining the global mean surface temperature (GMST) during the Little Ice Age (LIA) is important for characterizing the degree of disequilibrium from which modern warming began. While the IPCC AR6 estimates approximately 0.5°C of cooling relative to the 1950s, other credible sources suggest up to 1°C of cooling. Uncertainties arise due to challenges in calibrating paleoclimate time series into temperature units and globally mapping these calibrated proxies. Building on previous observations that the fraction of Earth’s surface experiencing anomalous warmth in a given year closely tracks GMST anomalies (Fried and Huybers, 2024), we demonstrate that the proportion of paleoclimate records indicating above-average temperatures can be inverted to estimate GMST despite their noise and sparsity. This approach allows us to bypass the need for calibrating or mapping of proxies. Independently applying our reconstruction technique to tree rings, ice cores, lake sediments, and marine sediment temperature proxies yields consistent indications of LIA GMST anomalies near 1°C of cooling. We conclude with some speculation about the origins of LIA cooling.