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Determining the feedbacks that modulate Southern Ocean carbon dynamics is key to understanding past and future climate. The global pause in rising atmospheric CO2 during the period of mid- to high-latitude southern surface cooling known as the Antarctic Cold Reversal (ACR, 14,700-12,700 years ago) provides an opportunity to disentangle competing influences. We present highly-resolved and precisely-aligned ice and marine reconstructions that capture a previously unrecognized increase in microbial diversity and ocean primary productivity during the ACR. Transient climate modeling across the last glacial suggests this period corresponds to a maximum seasonal difference in sea-ice extent. Our results indicate that this increased seasonal sea-ice variability drove changes in high-latitude light, temperature and nutrient availability, turning the southern seasonal sea-ice zone into a globally significant carbon sink.

Sea levels have been rising since the early 20th century due to the increase in global temperatures predominantly caused by anthropogenic forcing. Although sea level rise is one of the major challenges we face today, the magnitude of future sea level rise remains uncertain due to a lack of understanding of dynamic feedbacks and tipping points in both the climate system and cryosphere. As proxy records and current observations are fragmentary and have limited spatial and temporal coverage, numerical modelling can help to explore these crucial areas. It is thought that projected global mean surface temperatures will be ∼1-4◦C above pre- industrial values by the end of this century, and whilst no past period truly reflects the potential future under anthropogenic climate change, past warm periods can be useful process analogues for future change. The Last Interglacial (LIG) was the last warm period before the present day, occurring 129-116k years ago. It is especially useful as a process analogue due to the relative abundance of proxy data from this time. Importantly, with amplified temperatures at high latitudes (polar amplification) global average temperatures during this period were up to 3◦C warmer than pre-industrial times. It is thought that global mean sea level (GMSL) during the LIG was 5 to 10 m higher than present. The Greenland Ice Sheet is believed to have contributed 0.6 to 4.3 m to LIG GMSL, and -0.2 to 0.4 m may be attributed to thermal expansion. With 60 m sea level equivalent (SLE) ice volume, the Antarctic Ice Sheets are the largest potential contributor to sea level rise, but they are also associated with the largest unknowns. Climate models consistently underestimate the level of warming during the LIG, and ice sheet modelling studies have been unable to reconstruct the apparent LIG sea level shown in the palaeo-records without significant changes to the model physics. LIG CO2 levels from ice core records show a variation of up to σ4, where σ represents standard deviation. It can take many years for bubbles of trace gases to become enclosed in the ice and so these uncertainties may be even greater if peak CO2 is not captured due to low precipitation and long closure periods. This study uses both climate and ice sheet modelling to assess the impact of elevated greenhouse gas (GHG) concentrations on Antarctic climate and ice sheet dynamics under LIG orbital forcing. The climate modelling aspect of this study uses CSIRO Mk3L to investigate the interaction between LIG orbital forcing, elevated greenhouse gas levels, sea ice cover, and the dynamics of the Southern Ocean. These climate model outputs are used to drive the PISM hybrid ice sheet model to discover the relationship between increased greenhouse gas levels under LIG orbital forcing and Antarctic ice sheet dynamics and sea level contribution, with a focus on tipping points and the impact of increased GHG concentrations. Ice losses range from 16 to 648 cm SLE (sea level equivalent) over 20 k years of constant climate forcing. This study shows that a 5.3% increase in peak CO2 under LIG conditions is sufficient to generate a 4.5 to 6.5 m contribution to sea level on a millennial scale, with the majority of this sea level rise stemming from the collapse of the West Antarctic Ice Sheet (WAIS). This mass loss was found to be driven by ocean-warming and is constrained by topography, with marine ice-sheet instability having significant impacts across key sectors of Antarctica. A reduction of sea ice on the east coast and increased air temperatures lead to an increase in precipitation; some sectors, such as the Dry Valleys, see small mass gains of 1 to 5 cm SLE.

The future response of the Antarctic ice sheets to rising temperatures remains highly uncertain. A valuable analogue for assessing the sensitivity of Antarctica to warming is the Last Interglacial (129-116 kyr), when global sea level peaked 6 to 9 meters above present. Here we report a blue-ice record of ice-sheet and environmental change from the periphery of the marine-based West Antarctic Ice Sheet (WAIS). Constrained by a widespread volcanic horizon and supported by ancient microbial DNA analyses, we provide the first direct evidence for Last Interglacial WAIS collapse, driven by ocean warming and associated with destabilization of sub-glacial hydrates. Ice-sheet modelling supports this interpretation and suggests a 2˚C warming of the Southern Ocean over a millennia could trigger a ~3.2 meter rise in global sea levels. Our data indicate Antarctica is highly vulnerable to projected increases in ocean temperatures and may drive ice-climate feedbacks that further amplify warming.