At the CHE-VERIFY Joint General Assembly, Pete Smith (U Aberdeen) and Philippe Peylin (CEA-LSCE) introduce the progress and plans for the VERIFY WP3. The overall goal of this WP is to build a pre-operational system and deliver the respective Algorithm Description Document ADD and Product Description Document PDD (as a component of the overall policy-relevant observation system) to deliver national and sub-national annual budgets of land based CO2 fluxes, including quantified uncertainties, and to improve our understanding of the key processes and drivers contributing to these budgets. New multi-data model fusion strategies will further reduce uncertainties of CO2 budgets and trends at regional scale, with a focus on Eastern Europe.