This report defines the necessary components for a multi scale and multi species data assimilation ( system that targets anthropogenic CO ₂ emissions. This DA system will use multiple streams of observations, including satellite observations. Out of the many viable options to serve as the basis for such a system on the global scale, we see a hybrid 4d VAR ensemble approach, implemented in an online transport model, and operated within a Numerical Weather Prediction environment, as a fundamental building b lock. On top of this, a DA system should use multiple tracers, be adaptable to long and short windows and optimize both the atmospheric state as well as surface fluxes.