THE concentration of carbon dioxide in the atmosphere is increasing, largely because of fossil-fuel combustion, but the rate of increase is only about half of the total emission rate(1). The balance of the carbon must be taken up in the oceans and the terrestrial biosphere, but the relative importance of each of these sinks-as well as their geographical distribution and the uptake mechanisms involved-are still a matter of debate(1-4). Measurements of CO2 concentrations at remote marine sites(5-9) have been used with numerical models of atmospheric transport to deduce the location, nature and magnitude of these carbon sinks(2,10-19). One of the most important constraints on such estimates is the observed interhemispheric gradient in atmospheric CO2 concentration. Published models that simulate the transport of trace gases suggest that the gradient is primarily due to interhemispheric differences in fossil-fuel emissions, with small contributions arising from natural exchange of CO2 with the various carbon reservoirs. Here we use a full atmospheric general circulation model with a more realistic representation of turbulent mixing near the ground to investigate CO2 transport. We find that the latitudinal (meridional) gradient imposed by the seasonal terrestrial biota is nearly half as strong as that imposed by fossil-fuel emissions. Such a contribution implies that the sinks of atmospheric CO2 in the Northern Hemisphere must be stronger than previously suggested.