In determining global sea-to-air CO2 flux from measurements or models, the ocean margin has not been resolved from the land or the open ocean. Recent studies have indicated that shelves can be either a large sink or a source for atmospheric CO2. This CO2 sink/source term may substantially alter our current view of the global carbon budget for land and oceans. However, past fieldwork and synthesis have focused on a few shelves in the northern temperate zone while the vast majority of other shelves are ignored. By dividing the highly heterogeneous shelves into seven provinces, we suggest that the continental shelves are a sink for atmospheric CO2 at mid-high latitudes (-0.33 Pg C a-1) and a source of CO2 at low latitudes (0.11 Pg C a-1). Warm temperature and high terrestrial organic carbon input are most likely responsible for the CO2 release in low latitude shelves.