Global Ocean Cooling of 2.3°C During the Last Glacial Maximum.

Quantitative constraints on past mean ocean temperature (MOT) critically inform our historical understanding of Earth's energy balance. A recently developed MOT proxy based on paleoatmospheric Xe, Kr, and N2 ratios in ice core air bubbles is a promising tool rooted in the temperature dependences of gas solubilities. However, these inert gases are systematically undersaturated in the modern ocean interior, and it remains unclear how air‐sea disequilibrium may have changed in the past. Here, we carry out 30 tracer‐enabled model simulations under varying circulation, sea ice cover, and wind stress regimes to evaluate air‐sea disequilibrium in the Last Glacial Maximum (LGM) ocean. We find that undersaturation of all three gases was likely reduced, primarily due to strengthened high‐latitude winds, biasing reconstructed MOT by −0.38 ± 0.37°C (1σ). Accounting for air‐sea disequilibrium, paleoatmospheric inert gases indicate that LGM MOT was 2.27 ± 0.46°C (1σ) colder than the pre‐industrial era. Plain Language Summary: The ocean plays a central role in Earth's climate system as a major reservoir of heat. Understanding how ocean heat content (OHC) changed in the past is therefore key to unraveling the history of global climate. Xenon, krypton, and nitrogen trapped in ice core air bubbles offer a means of reconstructing past OHC, because changes in global ocean temperature affect the solubilities of these gases in seawater, leading to corresponding changes in their atmospheric abundances. For example, a colder ocean can hold more xenon, meaning less xenon resides in the atmosphere. However, these gases in the ocean today are slightly out of equilibrium with the atmosphere (i.e., they are undersaturated), and it remains unclear to what extent this disequilibrium could have changed in the past. We carried out global atmosphere‐ocean model simulations, finding that undersaturation was likely reduced in the Last Glacial Maximum (LGM), a colder era of global climate ∼20,000 years ago. Our analysis suggests that a small component of the additional xenon in the colder LGM ocean arose from this change in air‐sea disequilibrium. After accounting for this effect, ice core noble gas measurements suggest a slightly warmer LGM ocean than previously thought. Key Points: Global ocean air‐sea gas exchange simulations suggest reduced undersaturation of inert gases in the Last Glacial Maximum (LGM) oceanReduced LGM undersaturation indicates a small cold bias in mean ocean temperature (MOT) reconstruction from ice core noble gasesA revised estimate of MOT, accounting for air‐sea disequilibrium, is 2.27 ± 0.46°C colder than the pre‐industrial era [ABSTRACT FROM AUTHOR]