Two Modes of Change in Southern Ocean Productivity Over the Past Million Years

The variability of atmospheric carbon dioxide concentrations over glacial cycles, which are central aspects of the climate cycle, was documented decades ago. However, it has been difficult to identify which mechanisms have driven CO2 variability. Attention has focused on the Southern Ocean, because of its unique combination of hydrology and biology, although it has not been clear how the different behaviors of its Antarctic and Subantarctic zones might be reconciled with the observations of atmospheric CO2 change. Jaccard et al. (p. 1419) present a record of productivity from the Atlantic Antarctic Zone that extends back in time far enough to cover the last 10 glacial cycles. The findings show how the combination of effects in the Antarctic and Subantarctic zones can explain most of the atmospheric CO2 record over the past million years.