Early Last Interglacial ocean warming drove substantial ice mass loss from Antarctica

The future response of Antarctic ice sheets to rising temperatures remains highly uncertain, with their projected contribution to twenty-first century global mean sea level (GMSL) ranging from negligible to several metres. The need to reduce this uncertainty is pressing. A valuable analogue for assessing the sensitivity of Antarctica to warming is the Last Interglacial (LIG; 129-116 kyr), which experienced warmer polar temperatures and higher GMSL (+6 to 9 m) relative to present day. LIG sea level cannot be fully explained by Greenland Ice Sheet melt (<2 m) and ocean thermal expansion (<0.4±0.3 m), suggesting substantial Antarctic mass loss which may have been initiated by enhanced upwelling of the southern limb of the Atlantic Meridional Overturning Circulation in response to surface freshening in the North Atlantic. Here we report a new highly-resolved 134 kyr blue-ice record of regional environmental change and ice dynamics from a site close to the grounding line of the marine-based West Antarctic Ice Sheet which is underlain by major methane hydrate reserves. Constrained by a widespread volcanic horizon immediately prior to the onset of the LIG, ancient microbial DNA analyses provides evidence for elevated marine methane concentrations during sustained warming and preceding a significant ice flow reconfiguration. Our results imply elevated offshore temperatures led to ice-sheet retreat and destabilisation of hydrate reserves. To derive a temperature stability threshold, we use a global network of LIG marine records to constrain an ice-sheet model. We find that a millennium of polar ocean temperatures only 2˚C warmer than 1981-2010 are sufficient for Antarctica to increase GMSL by 3.2 metres. We suggest that relatively limited subsurface warming can substantially enhance ice-sheet mass loss, making Antarctica highly vulnerable to projected increases in Southern Ocean temperatures, and initiating ice-climate feedbacks that may further amplify warming.