Acceleration of Northern Ice Sheet Melt Induces AMOC Slowdown and Northern Cooling in Simulations of the Early Last Deglaciation

AbstractThe cause of a rapid change in Atlantic Ocean circulation and northern cooling atthe onset of Heinrich Stadial 1 ~18.5 ka is unclear. Previous studies have simulated the event using ice sheet and/or iceberg meltwater forcing, but these idealized freshwater fluxes have been unrealistically large. Here we use a different approach, driving a high‐resolution drainage network model with a recent time‐resolved global paleo‐ice sheet reconstruction to generate a realistic meltwater forcing. We input this flux to the Hadley Centre Coupled Model version 3 (HadCM3) climate model without adjusting the timing oramplitude and find that an acceleration in northern ice sheet melting (up to~7.5 m/kyr global mean sea level rise equivalent) triggers a 20% reduction in the Atlantic Meridional Overturning Circulation. The simulated pattern of ocean circulation and climate change matches an array of paleoclimate and ocean circulation reconstructions for the onset of Heinrich Stadial 1, in terms of both rates and magnitude of change. This is achieved with a meltwater flux that matches constraints on sea level changes and ice sheet evolution around 19–18ka. Since the rates of melting are similar to those projected for Greenland by 2200, constraining the melt rates and magnitude of climate change during Heinrich Stadial 1 would provide an important test of climate model sensitivity to future ice sheet melt.

Plain Language Summary
Atlantic Ocean circulation plays a key role in redistributing heat around Earth's surface, and thus has an important influence on our climate. Because of this, sudden shifts in Atlantic Ocean circulation can drive rapid climate changes. One such example is at the onset of “Heinrich Stadial 1”, 18.5 thousand years ago, when geological records show that Atlantic circulation weakened and the Northern Hemisphere cooled while the Southern Hemisphere warmed. At the time, huge ice sheets (several kilometers thick) covered much of North America and northern Europe. Climate model results suggest that the freshwater produced by these melting ice sheets is responsible for weakening the Atlantic Ocean circulation and triggering the abrupt climate changes captured in the geological records. This result helps to elucidate the complex interaction between ice sheets, ocean circulation, and climate, and how these interactions can lead to sudden shifts in climates of the past and, potentially, the future. Indeed, the rate of melting we adopt in the present model is comparable to the melting projected for the Greenland Ice Sheet by 2200.