Sinking in Slowly As the Arctic warms and its sea ice continues to melt, more of the ocean surface will be exposed, creating the potential for greater uptake of carbon dioxide from the atmosphere. Cai et al. (p. 556 , published online 22 July) present results from a series of Arctic Ocean transects that show that the amount of CO 2 in the surface waters has increased greatly recently. This will act as a barrier to future CO 2 uptake and suggests that the Arctic Ocean will not become the large CO 2 sink that some have predicted.