Deglacial temperature rise in Antarctica: myths and mechanisms

Axel Timmermann

Abstract

About 18,000 years ago temperatures in the Southern Ocean and Antarctica started to rise in concert with increasing atmospheric CO2 concentrations. Almost at the same time a major meltwater event occurred in the northern North Atlantic (Heinrich event I) that led to a disruption of North Atlantic Deep Water formation, widespread cooling in the Northern Hemisphere and a bipolar-seesaw type of response in the Southern Hemisphere. Many previous studies have attributed the deglacial temperature rise in the Southern Hemisphere to northern hemispheric causes and the atmospheric CO2 increase.

Using transient climate model simulations covering the last 21,000 years, these hypotheses are tested. The modeling results suggest that about 40-50% of the deglacial temperature rise in and around Antarctica can be attributed to increasing atmospheric CO2 concentrations. Another important factor that helped to jump-start the last glacial termination in Antarctica were orbitally-driven changes of boreal spring insolation. Sensitivity experiments conducted with a coupled climate-carbon cycle model furthermore demonstrate that a significant contribution of the initial CO2 rise ~17,000 years ago can be explained as a result of a shutdown of the Atlantic Meridional Overturning Circulation (AMOC), a subsequent shift of the Intertropical Convergence Zone and reduction of northern hemispheric terrestrial primary productivity. Furthermore, weakening of the AMOC and the associated cooling of the northern tropical oceans generate an atmospheric response that encompasses the Southern Ocean and Antarctica. A low pressure anomaly forms north of the Ross Sea and generates a temperature dipole that might explain the seemingly contradictory results from ice cores in that region.