Not just a seesaw: Global warming caused by the net radiative effect of Atlantic Meridional Overturning Circulation disruption

Abstract:

Each of the dramatic global warmings that ended the Pleistocene ice ages began with an interruption of the Atlantic Meridional Overturning Circulation (AMOC). These AMOC interruptions have been invoked to explain the observed deglacial ‘bipolar seesaw’, whereby the northern hemisphere was relatively cool at times when the southern hemisphere was relatively warm, and it is also suspected that the AMOC interruptions contributed to the outgassing of CO₂ from the ocean. Here, we use an ensemble of climate model simulations to show that, in addition to the previously-established bipolar seesaw and CO₂ effects, an interruption of the AMOC causes the global climate to warm, through a direct physical mechanism. Reduced overturning causes both the absorbed shortwave and outgoing longwave radiation to decrease, but with a larger effect for the latter. The result is a positive radiative imbalance of 0.3 - 1 W m^-2 that is accommodated by heat accumulation in the subsurface ocean. A gradual retreat of Southern Ocean sea ice increases shortwave absorption, counteracting the increased outgoing longwave radiation that arises from southern hemisphere warming, and maintaining a significant positive radiative imbalance on a millennial timescale. These results suggest that the direct radiative impact of AMOC disruptions was a previously unrecognized, but critical ingredient in tipping the planet out of a stable glacial state.