Although poorly understood, the north–south distribution of the natural component of atmospheric CO offers 2 information essential to improving our understanding of the exchange of CO between the atmosphere, oceans, and 2 biosphere. The natural or unperturbed component is equivalent to that part of the atmospheric CO distribution which is 2 controlled by non-anthropogenic CO fluxes from the ocean and terrestrial biosphere. Models should be able to reproduce 2 the true north–south gradient in CO due to the natural component before they can reliably estimate present-day CO 2 2 sources and sinks and predict future atmospheric CO . We have estimated the natural latitudinal distribution of atmospheric 2 CO , relative to the South Pole, using measurements of atmospheric CO during 1959–1991 and corresponding estimates of 2 2 Ž . anthropogenic CO emissions to the atmosphere. Key features of the natural latitudinal distribution include: 1 CO 2 2 Ž . concentrations in the northern hemisphere that are lower than those in the southern hemisphere; 2 CO concentration 2 Ž . Ž . differences that are higher in the tropics associated with outgassing of the oceans than those currently measured; and 3 CO concentrations over the southern ocean that are relatively uniform. This natural latitudinal distribution and its sensitivity 2 to increasing fossil fuel emissions both indicate that near-surface concentrations of atmospheric CO in the northern 2 hemisphere are naturally lower than those in the southern hemisphere. Models that find the contrary will also mismatch present-day CO in the northern hemisphere and incorrectly ascribe that region as a large sink of anthropogenic CO . 2 2 q 2000 Elsevier Science B.V. All rights reserved.