Planetary energy budget during abrupt glacial climate events set by Atlantic Ocean heat valve

During Pleistocene ice ages, abrupt climate changes co-occurred with switches in Atlantic Meridional Overturning Circulation (AMOC) strength. The global impact and characteristic north-south temperature pattern of these events is typically explained via interhemispheric redistribution of heat in a conceptual framework called the ‘thermal bipolar seesaw’. Here we synthesize recent work on an emerging alternative framework centred instead on the global ocean heat content and planetary energy budget, which we illustrate using simulations of spontaneous abrupt climate change in three climate models. The strong and weak AMOC modes are associated with oceanic and planetary heat loss and heat gain, respectively, facilitated via changes to North Atlantic deep convection and radiative feedbacks that set the top-of-the-atmosphere energy budget. Antarctic and Greenland temperatures, as recorded in ice cores, reflect ocean heat content and the rate of North Atlantic heat loss, respectively. Climate instability at intermediate glacial states reflects an inability to balance global ocean heat uptake with North Atlantic heat loss for either of the two AMOC modes. Our synthesis suggests that the AMOC strength acts as a ‘heat valve’ that alters planetary temperature by changing the radiative balance. This implies amplified planetary heat uptake in response to projected future AMOC weakening.