Southern Ocean upwelling, Earth’s obliquity, and glacial-interglacial atmospheric CO2 change

By Xuyuan E. Ai. et al.

Controlling atmospheric carbon dioxide

The atmospheric concentration of carbon dioxide (CO2) has varied substantially over the past million years in tandem with the glacial cycle. Although it is widely agreed that upwelling of Southern Ocean water is a key factor, the finer details about what caused these CO2 variations are of great importance for understanding climate. Ai et al. identified three modes of change in Southern Ocean upwelling, adding a third to two previously recognized ones. This new mode can help explain better the relative timing of the glacial and CO2 cycles.


Previous studies have suggested that during the late Pleistocene ice ages, surface-deep exchange was somehow weakened in the Southern Ocean’s Antarctic Zone, which reduced the leakage of deeply sequestered carbon dioxide and thus contributed to the lower atmospheric carbon dioxide levels of the ice ages. Here, high-resolution diatom-bound nitrogen isotope measurements from the Indian sector of the Antarctic Zone reveal three modes of change in Southern Westerly Wind–driven upwelling, each affecting atmospheric carbon dioxide. Two modes, related to global climate and the bipolar seesaw, have been proposed previously. The third mode—which arises from the meridional temperature gradient as affected by Earth’s obliquity (axial tilt)—can explain the lag of atmospheric carbon dioxide behind climate during glacial inception and deglaciation. This obliquity-induced lag, in turn, makes carbon dioxide a delayed climate amplifier in the late Pleistocene glacial cycles.

The full article appeared on the Science website at https://science.sciencemag.org/content/370/6522/1348


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