It is well known that Antarctica's temperature and atmospheric CO2 levels were much lower during the last ice age than today. As the world transitioned from the last ice age into the current warm period, Antarctica warmed and CO2 rose. The Southern Ocean has been identified as the main ´control valve´, ventilating CO2 from the ocean to the atmosphere.

The new study published in Science Advances provides evidence that Antarctic warming and the outgassing of CO2 from the Southern Ocean to the atmosphere were tightly coupled with a rapid reduction of the Antarctic sea ice cover, although the summer sea ice had already started to decline from its glacial maximum extent at least ~2,000 years earlier. “The unprecedented reconstruction of past spring/summer sea ice conditions off East Antarctica documents that sea ice decline and associated feedback mechanisms played a leading role for Southern Ocean circulation changes and deglacial climate changes”, states the study´s main author, Henrik Sadatzki, who did research in the Marine Geology section at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) while working on the study and has now moved to the MARUM – Center for Marine Environmental Sciences, University of Bremen.

Molecular evidence from sea floor sediments

Sadatzki and his colleagues investigated a sediment core from offshore East Antarctica, revealing that the upper three meters of sediments were deposited over the last 40 thousand years, thus including the time when Earth transitioned from the last ice age into the modern warm period. The researchers analyzed biomarkers, specific organic molecules in the sediments, to reconstruct sea ice conditions during the last 40 thousand years.

The molecular biomarker evidence was combined with other sediment core data and compared with model simulation results. Thereby, Sadatzki and his colleagues provided an unprecedentedly detailed and comprehensive investigation of the past Antarctic sea ice dynamics, their drivers, and linkages with Southern Ocean circulation, climate and atmospheric CO2 changes.

Past sea ice and climate changes

The study provides robust evidence of near-perennial sea ice conditions with extensive summer sea ice at the core location off East Antarctica during the last ice age, and reduced seasonal sea ice conditions during the deglaciation and current warm period.

The results indicate that Antarctic summer sea ice decline initiated early, at least 2,000 years before the onset of the deglaciation, which was probably induced by an increasing local summer insolation. This illustrates the leading role of processes in the high-latitude Southern Hemisphere for driving deglacial climate changes.

“Our records indicate that sea ice decline culminated rapidly and contemporaneously with enhanced deep mixing and outgassing of CO2 in the Southern Ocean at the onset of the last deglaciation, associated with a reduction of the Atlantic Meridional Overturning Circulation”, Sadatzki says. 

The proxy data and model simulation presented in the new study document that Antarctic sea ice reduction, Southern Ocean circulation changes, atmospheric CO2 rise and high-latitude Southern Hemisphere climate warming were tightly coupled and reinforced each other.

The results of this study support hypotheses that a reduction of the sea ice lid around Antarctica contributed to the opening of the Southern Ocean ´control valve´ and amplified atmospheric CO2 rise and climate warming during the deglaciation. The findings also highlight the sensitivity of Antarctic sea ice to atmospheric CO2 rise and weakening of the Atlantic Meridional Overturning Circulation in the past, which could point to a potential for Antarctic sea ice decline under future climate boundary conditions.