This document reports on progress to date within WP3 ‘Coordinating efforts on uncertainty trade-off for fossil fuel emissions’ of the CO2 Human Emissions (CHE) project. The aim of WP3 is ultimately to evaluate and coordinate possible improvements in the quantification of fossil fuel CO2 emission uncertainty estimates from enhanced space-borne and in-situ observations. To this end, this work package involves both bottom-up inventory compilers and top-down observation analysts and atmospheric modellers to assess uncertainties in anthropogenic emissions and biogenic CO2 fluxes, estimate the flux error covariances, and establish an optimal inversion strategy based on observing system simulation experiments (OSSEs) and quantitative network design (QND). All tasks within this WP have considerably progressed according to their work description in the Grant Agreement and no deviations from this work description have been identified. In Task 3.1, a dataset of global biogenic fluxes (gross primary production and net ecosystem exchange) have been derived for the period 2008-2017 and delivered to ECMWF. Task 3.2 has summarized the IPCC guidelines for CO2 emission uncertainties and has compiled a table of the uncertainties on a per sector and per country basis. Task 3.3 has created a test dataset for tropospheric NO2 column data from TROPOMI and is developing an inverse algorithm to derive TROPOMI-based NO2 emission estimates. Task 3.4 has prepared an inversion system for OSSE studies, which currently assimilates synthetic in-situ and space-borne CO2 observations. Finally, Task 3.5 has started configuring towards a coupled carbon cycle and fossil fuel data assimilation system for QND experiments to explore the benefit of different observations for monitoring and constraining fossil fuel emissions.