Plausible Last Interglacial Antarctic Ice Sheet Changes Do Not Fully Explain Antarctic Ice Core Water Isotope Records

Antarctic ice cores can help determine ice mass loss from the Antarctic Ice Sheet (AIS) during past warm periods. We compile Last Interglacial (LIG) δ18O ${\delta }^{18}O$ measurements from eight Antarctic cores and compare these to new isotope‐enabled LIG simulations, which explore three plausible LIG AIS elevation and extent scenarios. We find that these simulations capture less than 10% of East Antarctic core‐mean δ18O ${\delta }^{18}O$ changes. Although our simulations do not fully explain the changes, they capture some inter‐core geographical δ18O ${\delta }^{18}O$ variations. Some LIG AIS configurations show higher skill than PI AIS configurations in simulating the inter‐core differences. The remaining discrepancies between the simulated and observed core‐mean water isotope changes suggest that LIG simulations also need to include the influences of reduced Antarctic sea ice, a warmer Southern Ocean, and resultant shifts in vapor source regions to produce a more satisfactory match to δ18O ${\delta }^{18}O$ observed at ice core sites. Plain Language Summary Reconstructions of Antarctic Ice Sheet (AIS) retreat during past warm periods provide important constraints for projections of future ice mass loss and sea level rise. The Last Interglacial (LIG) is a particularly useful example, when global temperatures were 1±1oC $\pm {1}^{o}C$ warmer than preindustrial (PI) conditions, to explore future sea level rise projections associated with the successful implementation of the Paris Agreement. Here, we use data from six East Antarctic ice cores and two new, unpublished records from West Antarctica that capture LIG conditions. Water isotopes in ice cores are sensitive to changes in temperature, AIS elevation, atmospheric circulation pattern, sea ice area, and ocean conditions. We compare model simulations of the LIG with ice core data and find that elevation changes, an important indicator of ice mass loss, explain only around 9% of the isotope change captured in East Antarctic ice cores. Due to the limitation of our simulations in the coastal regions, our West Antarctic coastal sites are more challenging. In summary, we find that while the range of our simulations does not fully explain average East Antarctic PI‐to‐LIG isotope changes, they do capture some of the geographical variations in isotope change patterns. Key Points A compilation of Last Interglacial δ18O ${\delta }^{18}O$ ice core records shows anomalies of + ${+}$1.5‰ and + ${+}$3.3‰ for 127 kyr B