The oceans have taken up approximately half of the anthropogenic CO2 emissions. This uptake reduces climate change but also lowers ocean pH, with the potential to disrupt ecosystems. Climate change affects ocean biology and physics and could lead to reduced efficiency of the carbon sinks, a process that atmospheric data and ocean models indicate is already occurring in the Southern Ocean. Attempts to set a baseline stabilization target for the atmospheric CO2 concentration will ultimately depend on our understanding and prediction of oceanic CO2 sinks. While we are now close to monitoring oceanic CO2 uptake on decadal and regional scales, meaningful predictions of its future behavior are difficult. There is a critical and urgent need to better understand the ocean processes regulating CO2 uptake and to identify research and observational priorities for the future.