Global glacier-based climate reconstruction of the Last Glacial Termination

Abstract

Glaciers provide a physically interpretable record of past climate, yet their behaviour through Termination 1 appears paradoxical: alpine glaciers in both hemispheres have been reported to retreat most rapidly during Greenland stadials, when Northern Hemisphere mean-annual temperature reconstructions indicate cooling. This apparent mismatch has fuelled debate over conventional interpretations of deglaciation as paced primarily by Northern Hemisphere forcing (e.g., orbitally driven summer insolation and North Atlantic-driven bipolar-seesaw dynamics). In this thesis, I develop a globally standardised reconstruction of moraine-based equilibrium line altitudes (ELAs) from the Last Glacial Maximum through Termination 1 to resolve consensus regional and global glacier behaviour. Using a transient climate model simulation of the deglaciation, I then test whether North Atlantic-driven bipolar-seesaw dynamics can reproduce the timing and structure of observed glacier change. The moraine-based ELA reconstruction resolves synchronous midlatitude deglaciation onset at 17.9 ± 0.5 ka in the Southern Hemisphere and 17.7 ± 0.5 ka in Northern Hemisphere. In both hemispheres, ELA rise accelerated during Heinrich Stadial 1 and again during the Younger Dryas stadial, with reduced rates of change during the Bølling–Allerød interstadial. Transient ELA modelling then demonstrates how this coherent stadial retreat is physically plausible. Northern Hemisphere midlatitude glaciers are shown to be primarily sensitive to summer temperatures, and Northern Hemisphere summer temperatures are shown to rise during stadials in response to increasing atmospheric CO₂ despite mean-annual cooling. In this framework, CO₂ increases provide a unifying forcing that can align glacier retreat between hemispheres through the termination. Together, these results reconcile coherent interhemispheric glacier retreat with opposing orbital forcing and bipolar-seesaw climate variability. They show that synchronous stadial retreat can be understood within a Northern Hemisphere–paced framework when glacier sensitivities and the seasonal expression of stadial climate anomalies are treated explicitly. More broadly, this work highlights the importance of diagnosing seasonality in paleoclimate interpretations and in assessing future climate responses to North Atlantic-driven climate variability.