Southern Ocean drives multidecadal atmospheric CO2 rise during Heinrich Stadials.

Wendt, Kathleen A; Nehrbass-Ahles, Christoph; Niezgoda, Kyle; Noone, David; Kalk, Michael; Menviel, Laurie; Gottschalk, Julia; Rae, James W B; Schmitt, Jochen; Fischer, Hubertus; Stocker, Thomas F; Muglia, Juan; Ferreira, David; Marcott, Shaun A; Brook, Edward; Buizert, Christo (2024). Southern Ocean drives multidecadal atmospheric CO2 rise during Heinrich Stadials. Proceedings of the National Academy of Sciences of the United States of America - PNAS, 121(21) National Academy of Sciences 10.1073/pnas.2319652121

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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 CO2 measurements 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 CO2 concentrations 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 CO2 are concurrent with jumps in atmospheric CH4 and 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 CO2 uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future.

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