More than a third of industrial-age fossil fuel CO2 emissions have been absorbed by the ocean. This uptake has had implications for ocean chemistry and marine life by driving ocean acidification, but has also mitigated atmospheric CO2 concentrations and their impact on climate. However, despite its critical importance for climate and understanding climate–carbon feedbacks, the dynamics of the marine carbon sink are not well constrained.
Galen McKinley from Columbia University and Lamont Doherty Earth Observatory and colleagues investigate the mechanisms behind the slow-down of marine carbon uptake in the 1990s through models and observations. They find that uptake slowed due to a lower growth rate of atmospheric CO2, which influenced the air–sea flux. Changes in sea surface temperature caused by the volcanic eruption of Mount Pinatubo modified the timing of the sink within the decade. These results highlight the role of factors external to the ocean in driving variability in the ocean carbon sink and indicate that this sink will decrease as soon as the growth rate of atmospheric CO2 is reduced by reductions in anthropogenic emissions.