This talk by Marko Scholze (Lund University) at the 1st CHE General Assembly presented an overview of the activities in Work Package 3. WP3 (Coordinating Efforts on Uncertainty trade-off for fossil fuel emissions) will evaluate the current status and possible improvements from enhanced space-borne and in-situ observation scenarios for quantifying fossil fuel CO2 emissions. This will be based on Observation System Simulation Experiments (OSSEs) and Quantitative Network Design (QND) studies using different approaches (high-resolution inverse transport modelling of CO2 and co-emitted species, advanced carbon cycle-fossil fuel data assimilation systems integrating CO2, radiocarbon, terrestrial and socioeconomic datasets). For the inverse modelling and data assimilation systems, prior anthropogenic emissions and their uncertainties are needed. In addition, the transport model inversions also require high-resolution prior biogenic fluxes with quantified uncertainties. Based on these experiments the WP will report on a set of integrated monitoring strategies that blend bottom-up and top-down approaches for estimating fossil CO2 emissions.