The ocean has long been known to be both a source and sink of climate reactive trace species. Recently, satellite data and general circulation models have been used to simulate air-sea exchange inventories with high resolution and global coverage. Here, we discuss the differences between a 3-D global atmospheric CO transport model run with a homogeneous ocean to atmosphere CO flux (35 Tg CO yr-1), and a large spatially and temporally varying ocean CO flux (153 Tg CO yr-1). Both ocean sources of atmospheric CO are within previous estimates of the ocean to atmosphere flux. The model sensitivity calculations show that the ocean can account for between 5--50% of the total surface level atmospheric CO pool, over large areas of the northern and southern hemisphere oceans. Oceanic CO fluxes to the atmosphere are important in both hemispheres, with a greater relative influence occurring in the southern hemisphere. Seasonal cycle calculations indicate that area-weighted CO concentrations in the remote southern and northern hemisphere marine troposphere are most heavily influenced by the ocean source during summer months. The main source of CO in surface ocean waters is thought to be photochemical oxidation of organic matter. Any climate related changes in surface radiative fluxes, due to ozone depletion or other processes, may alter surface ocean CO concentrations and by implication atmospheric marine boundary layer CO, OH and O3 abundance. ¿ American Geophysical Union 1992