Using earth system modelling to understand abrupt climate changes in the last glacial period: transient events and ocean oscillations

The Last Glacial Period and ensuing deglaciation were marked by rapid transitions between cold and warm climates. Despite decades of research, uncertainty remains about the mechanisms driving these events, though it is widely agreed that reorganisations of the Atlantic Meridional Overturning Circulation (AMOC) play a crucial role. However, the precise climatic controls on AMOC remain unclear.

To explore AMOC evolution and its role in abrupt climate change, I employ earth system modelling. First, I compare transient simulations of the last deglaciation under various meltwater scenarios to assess the influence of ice sheet meltwater on the occurrence of abrupt events like Heinrich Stadial 1. Finding that most models fail to produce an abrupt interstadial-stadial switch when forced by current reconstructions of ice sheet meltwater, I then investigate AMOC oscillations in glacial climate simulations with a constant meltwater flux. By varying background conditions, I determine that CO₂ and orbital changes strongly influence the periodicity, magnitude, and occurrence of AMOC oscillations. Lastly, using a coupled climate-ice sheet model, I examine how the Bølling Warming impacts Northern Hemisphere ice sheets, revealing that surface ablation increases due to the abrupt warming, but the scale of this melt is modulated by ocean forcing and initial ice sheet topography.

The results of this thesis demonstrate the sensitivity of the AMOC to subtle shifts in background conditions. These findings underscore the need for further high-resolution climate and ice sheet simulations to unravel the chain of events governing past abrupt climate transitions in the Last Glacial Period.