This project is producing a strategy for a global observing system for verifying national claims of greenhouse gas emissions. The strategy will define the requirements of a system that would yield top-down emission estimates simultaneously consistent with all reliable observations, relevant theoretical models of natural oceanic, atmospheric and terrestrial processes, and data on economies and trade flows involved in these emissions. The system will use coupled models of the oceanic, terrestrial and atmospheric carbon and water cycles, and circulation and other relevant physical properties for the ocean and atmosphere derived previously from assimilation of observations into general circulation models. It will use state estimation principles, control theory algorithms and data assimilation approaches that couple models and observations, in order to "optimally" estimate greenhouse gas fluxes (plus uncertain parameters in GHG models) as functions of position (or process) and time.