Simulations of the flux of atmospheric carbon dioxide into the ocean show that changes in flux associated with human activities are currently masked by natural climate variations, but will be evident in the near future. See Letter p.469 The world's oceans have taken up vast amounts of amount of carbon produced by fossil fuel burning during the industrial era. These authors use a large ensemble of a single Earth system climate model, the Community Earth System Model–Large Ensemble (CESM–LE), to assess variability and change in the ocean carbon cycle in recent decades and through to 2100. This approach allows for a separation between trends in the air–sea carbon flux due to anthropogenic climate change and those due to internal climate variability. The study reveals how the ocean carbon sink may be expected to change throughout this century in different oceanic regions. The findings suggest that a large internal climate variability makes it unlikely that changes in the rate of anthropogenic carbon uptake can be directly observed in most oceanic regions at present, but that this may become possible between 2020 and 2050 in some regions.