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Abstract The fate of the West Antarctic Ice Sheet (WAIS)¹is the largest cause of uncertainty in long-term sea-level projections. In the last interglacial (LIG) around 125,000 years ago, data suggest that sea level was several metres higher than today^2–4, and required a significant contribution from Antarctic ice loss, with WAIS usually implicated. Antarctica and the Southern Ocean were warmer than today^5–8, by amounts comparable to those expected by 2100 under moderate to high future warming scenarios. However, direct evidence about the size of WAIS in the LIG is sparse. Here we use sea salt data from an ice core from Skytrain Ice Rise, adjacent to WAIS, to show that, during most of the LIG, the Ronne Ice Shelf was still in place, and close to its current extent. Water isotope data are consistent with a retreat of WAIS⁹, but seem inconsistent with more dramatic model realizations¹⁰in which both WAIS and the large Antarctic ice shelves were lost. This new constraint calls for a reappraisal of other elements of the LIG sea-level budget. It also weakens the observational basis that motivated model simulations projecting the highest end of projections for future rates of sea-level rise to 2300 and beyond. 

The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here, we present CO₂measurements from the West Antarctic Ice Sheet Divide ice core across Heinrich Stadials 2 to 5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO₂concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 y) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO₂are concurrent with jumps in atmospheric CH₄and abrupt changes in the water isotopologs in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologs can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal- to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO₂uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future. 

Abstract. A new ice core was drilled in West Antarctica on Skytrain Ice Rise in field season 2018/2019. This 651 m ice core is one of the main targets of the WACSWAIN (WArm Climate Stability of the West Antarctic ice sheet in the last INterglacial) project. A present-day accumulation rate of 13.5 cm w.e. yr−1 was derived. Although the project mainly aims to investigate the last interglacial (115–130 ka), a robust chronology period covering the recent past is needed to constrain the age models for the deepest ice. Additionally, this time period is important for understanding current climatic changes in the West Antarctic region. Here, we present a stratigraphic chronology for the top 184.14 m of the Skytrain ice core based on absolute age tie points interpolated using annual layer counting encompassing the last 2000 years of climate history. Together with a model-based depth–age relationship of the deeper part of the ice core, this will form the ST22 chronology. The chemical composition, dust content, liquid conductivity, water isotope concentration and methane content of the whole core was analysed via continuous flow analysis (CFA) at the British Antarctic Survey. Annual layer counting was performed by manual counting of seasonal variations in mainly the sodium and calcium records. This counted chronology was informed and anchored by absolute age tie points, namely, the tritium peak (1965 CE) and six volcanic eruptions. Methane concentration variations were used to further constrain the counting error. A minimal error of ±1 year at the tie points was derived, accumulating to ± 5 %–10 % of the age in the unconstrained sections between tie points. This level of accuracy enables data interpretation on at least decadal timescales and provides a solid base for the dating of deeper ice, which is the second part of the chronology.