Northern Hemisphere ice-sheet dynamics during the last two deglaciations: responses to gradual and abrupt climate changes

The last two deglaciations mark transitions from glacial to interglacial climates, dramatically reshaping Northern Hemisphere ice sheets. Numerical modelling of these transitions provides critical insight into the processes controlling ice-sheet retreat and collapse. Comparing the last two deglaciations allows us to evaluate how different forcings and initial conditions influence ice-sheet dynamics and understand the interplay between orbital forcing, greenhouse gases, abrupt climate changes and ice sheet instabilities in driving ice sheet evolution.We use the fast yet comprehensive coupled General Circulation Atmosphere–ice-sheet model FAMOUS–BISICLES to simulate the Northern Hemisphere ice-sheet evolution during the penultimate deglaciation (140–128 thousand years ago; ka) and the last deglaciation (21-7 ka), with particular interest in the abrupt Bølling warming (14.5 ka). Our simulations follow the PMIP4 (Palaeoclimate Model Intercomparison Project 4) protocols and are forced with prescribed sea surface temperatures and sea ice from transient climate model outputs to reduce biases and force millennial abrupt climate changes.First, we compare the penultimate and last deglaciations to assess how orbital forcing, greenhouse gas concentrations, and uncertain model parameters and SST inputs shape both the pace and spatial patterns of ice retreat. Results indicate a faster ice retreat during the penultimate deglaciation. Sensitivity experiments show that the rate of deglaciation is particularly sensitive to processes that impact the surface mass balance, but ice dynamics also play an important role. Sub-shelf melt rate is less significant; however, it can be important where confined ice shelves are able to form. Although insolation drives the deglaciations, rising greenhouse gases and warming SSTs significantly amplify the ice-sheet response to orbital forcing.Second, we focus on the abrupt Bølling warming (~14.5 ka). Our simulations show accelerated deglaciation during this event, though the magnitude of response depends on the ice-sheet topography during the warming and on the pattern of abrupt SST increase prescribed. Marine-based sections, particularly the Barents–Kara ice sheet, exhibit the greatest sensitivity to prescribed ocean changes.