The progress made in these first phases of the TransCom project can now be directly applied to resolving some of the discrepancies in estimates of the global carbon budget. To that end, we are currently running a CO2 inversion calculation intercomparison. Participating transport models will be used to simulate the atmospheric response to an agreed-upon set of surface emission "basis functions" representing regional emissions and uptake of CO2 due to various processes (industrial emissions, ecosystem metabolism, air-sea gas exchange, biomass burning, etc). The focus of this inversion intercomparison activity will be to produce a formal estimate of the degree of uncertainty in such an inversion calculation that arises directly from the uncertainty in the model transport, the inversion methodology, and the observational dataset used. This activity has produced preliminary results and will be completed in late-2001. This final phase of TransCom will use a set of sensitivity experiments to isolate the components of the models and methods that are most responsible for the different behavior they exhibit, using the results to recommend priorities for future model development and observational network improvements to reduce uncertainty.
The proposed experimental design reflects the tension between greater participation and greater diagnostic detail. We have chosen to specify a minimum experimental protocol that is straightforward to implement (to maximize participation by keeping the entry barrier low), and allow for more detailed experiments by those groups with sufficient resources and interest. This strategy is best pursued by centralizing many of the tasks to be performed, so that participation in the experiment requires a minimum of effort by a CTM group.