A two-dimensional global atmospheric diffusion simulation model is used to establish zonal and monthly average net surface fluxes of carbon dioxide (CO2) which are consistent with the variations in CO2 concentration observed at six monitoring stations distributed globally. These fluxes represent the zonally averaged net ecosystem production of primarily terrestial biosphere. Annually, the global net ecosystem production during the photosynthetically more active season removed 6.8¿1012 kg of carbon from the atmosphere, returning it during the less active winter months. This turnover represents about 14% of the annual continental net primary production of carbon and 0.8% of the total terrestrial biomass. During the growing season, net ecosystem production appears to be relatively independent of latitude in the northern hemisphere (~6¿10-9 kg carbon m-2 s-1). The model indicates that time correlations between the CO2 concentration at different altitudes of the northern hemisphere with the interhemispheric advection of air results in a net interhemisphereic CO2 flux which becomes zero when the South Pole CO2 concentration is on average 0.92 ppmv above that at Mauna Loa, Hawaii. The influence of both the net and the gross atmospheric biospheric exchange of carbon on the atmospheric stable carbon isotope ratio is modeled. On time scales of a year the gross turnover of carbon has an insignificant effect, and all isotopic effects can be related to the net exchanges. It is argued that as with concentration, long-term trends in isotopic composition will be best observed in the southern hemisphere.