| Summary: | In contrast to the open ocean, the sources and sinks for atmospheric carbon dioxide (CO 2 ) in the coastal seas are poorly constrained and understood. Here we address this knowledge gap by analyzing the spatial and temporal variability of the coastal air‐sea flux of CO 2 ( F CO 2 ) using a recent high‐resolution (0.25°) monthly climatology for coastal sea surface partial pressure in CO 2 ( p CO 2 ). Coastal regions are characterized by CO 2 sinks at temperate and high latitudes and by CO 2 sources at low latitude and in the tropics, with annual mean CO 2 flux densities comparable in magnitude and pattern to those of the adjacent open ocean with the exception of river‐dominated systems. The seasonal variations in F CO 2 are large, often exceeding 2 mol C m −2 year −1 , a magnitude similar to the variations exhibited across latitudes. The majority of these seasonal variations stems from the air‐sea p CO 2 difference, although changes in wind speed and sea ice cover can also be significant regionally. Globally integrated, the coastal seas act currently as a CO 2 sink of −0.20 ± 0.02 Pg C year −1 , with a more intense uptake occurring in summer because of the disproportionate influence of high‐latitude shelves in the Northern Hemisphere. Combined with estimates of the carbon sinks in the open ocean and the Arctic, this gives for the global ocean, averaged over the 1998 to 2015 period an annual net CO 2 uptake of −1.7 ± 0.3 Pg C year −1 .
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