Showing papers by "Michel Ramonet published in 1996"

Variations in modeled atmospheric transport of carbon dioxide and the consequences for CO2 inversions

Abstract: Carbon dioxide concentrations due to fossil fuel burning and CO2 exchange with the terrestrial biosphere have been modeled with 12 different three-dimensional atmospheric transport models. The models include both on-line and off-line types and use a variety of advection algorithms and subgrid scale parameterizations. A range of model resolutions is also represented. The modeled distributions show a large range of responses. For the experiment using the fossil fuel source, the annual mean meridional gradient at the surface varies by a factor of 2. This suggests a factor of 2 variation in the efficiency of surface interhemispheric exchange as much due to differences in model vertical transport as to horizontal differences. In the upper troposphere, zonal mean gradients within the northern hemisphere vary in sign. In the terrestrial 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 observed seasonal cycles in the northern extratropics suggest that the terrestrial 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 Climate Change for fossil fuel and ocean uptake and smaller than those for terrestrial fluxes. We outline approaches which may reduce this component in CO2 budget uncertainties.

Influence of two atmospheric transport models on inferring sources and sinks of atmospheric CO2

Abstract: Atmospheric transport models are a source of uncertainty in the diagnostics of the CO 2 sources and sinks. We propose here a protocol to compare two transport models: a 2-dimensional (2D) and a 3-dimensional (3D) model, based on 3 different experiments that reveal the ability of each model to account for the different components of the atmospheric carbon cycle. The 2D model we use is the one described by Tans et al. and the 3D model is the TM2 model, developed by Heimann et al. First, we conduct the same fossil fuel experiment in both models and show that the 2D model has a stronger interhemispheric mixing than the 3D model (∼ 25%), even though the 2D model presents a weaker intra-hemispheric mixing above source regions (experiment A). The influence of year-to-year variability of transport on the latitudinal profile in fossil-fuel CO 2 appears to be weak for the 1990s. We then use a set of “all but fossil fuel” fluxes, originally inferred from the 2D model, as an input to the 3D model (experiment B). Even if the main discrepancy on the resulting latitudinal CO 2 concentrations occurs between the 2D and 3D models in the tropics and at the mid-northern latitudes, the differences implied by three longitudinal distributions tested in the 3D model are important and can be explained by a few global transport mechanisms. Finally, we quantify the differences in latitudinal CO 2 concentrations observed in experiment B in terms of net carbon fluxes at the surface. To do so, an inverse calculation of the CO 2 fluxes in latitude and time is performed with the 3D model, using as an input a smoothed latitudinal profile of atmospheric measurements for the period 1990–1993 (experiment C = A + B ). We find with the 3D model that, averaged on the period 1990–1993, the equatorial release is reduced by 40 Tmol yr −1 (roughly 25% of the original source) compared with the initial 2D budget and is shifted southward by roughly 10°. The mid northern latitude sink is also reduced by 80 Tmol yr −1 (roughly 25% of the original sink). In summary, this study shows that the changes in the carbon budget required when moving from the 2D model to this 3D model are important, but they are not radical changes. DOI: 10.1034/j.1600-0889.1996.t01-2-00011.x