Impact of climate change and variability on the global oceanic sink of CO₂

About one quarter of the CO 2 emitted to the atmosphere by human activities is absorbed annually by the ocean. All the processes that influence the oceanic uptake of CO 2 are controlled by climate. Hence changes in climate (both natural and human‐induced) are expected to alter the uptake of CO 2 by the ocean. However, available information that constrains the direction, magnitude, or rapidity of the response of ocean CO 2 to changes in climate is limited. We present an analysis of oceanic CO 2 trends for 1981 to 2007 from data and a model. Our analysis suggests that the global ocean responded to recent changes in climate by outgassing some preindustrial carbon, in part compensating the oceanic uptake of anthropogenic CO 2 . Using a model, we estimate that climate change and variability reduced the CO 2 uptake by 12% compared to a simulation where constant climate is imposed, and offset 63% of the trend in response to increasing atmospheric CO 2 alone. The response is caused by changes in wind patterns and ocean warming, with important nonlinear effects that amplify the response of oceanic CO 2 to changes in climate by > 30%.