VARIATIONS IN MODELED ATMOSPHERIC TRANSPORT OF CARBON-DIOXIDE AND THECONSEQUENCES FOR CO2 INVERSIONS

Carbon dioxide concentrations due to fossil fuel burning and CO2 excha nge with the terrestrial biosphere have been modeled with 12 different three-dimensional atmospheric transport models, The models include bo th on-line and off-line types and use a variety of advection algorithm s and subgrid scale parameterizations, A range of model resolutions is also represented, The modeled distributions show a large range of res ponses. For the experiment using the fossil fuel source, the annual me an meridional gradient at the surface varies by a factor of 2, This su ggests a factor of 2 variation in the efficiency of surface interhemis pheric exchange as much due to differences in model vertical transport as to horizontal differences. In the upper troposphere, zonal mean gr adients within the northern hemisphere vary in sign, In the terrestria l biotic source experiment, the spatial distribution of the amplitude and the phase of the seasonal cycle of surface CO2 concentration vary little between models, However, the magnitude of the amplitudes varies similarly to the fossil case, Differences between modeled and observe d seasonal cycles in the northern extratropics suggest that the terres trial biotic source is overestimated in late spring and underestimated in winter. The annual mean response to the seasonal source also shows large differences in magnitude. The uncertainty in hemispheric carbon budgets implied by the differences in interhemispheric exchange times is comparable to those quoted by the Intergovernmental Panel on Clima te Change for fossil fuel and ocean uptake and smaller than those for terrestrial fluxes. We outline approaches which may reduce this compon ent in CO2 budget uncertainties.