The oceans are a major sink for atmospheric carbon dioxide (CO 2 Historically, observations have been too sparse to allow accurate tracking of changes in rates of CO 2 uptake over ocean basins, so little is known about how these vary. Here, we show observations indicating substantial variability in the CO 2 uptake by the North Atlantic on time scales of a few years. Further, we use measurements from a coordinated network of instrumented commercial ships to define the annual flux into the North Atlantic, for the year 2005, to a precision of about 10%. This approach offers the prospect of accurately monitoring the changing ocean CO 2 sink for those ocean basins that are well covered by shipping routes.