Assimilation of MERIS FAPAR into a Terrestrial Vegeitation Model and Mission Design

The current and future strength of the terrestrial carbon sink has a crucial influence on the expected degree of climate warming humanity is going to face. Usually, Earth Observation (EO) by its very nature focuses on diagnosing the current state of the planet. However, it is possible to use EO products in data assimilation systems to improve not only the diagnostics of the current state, but also the accuracy of future predictions. This contribution reports from an on-going ESA funded study (see http://rs.ccdas.org) in which the MERIS FAPAR product is assimilated into a terrestrial biosphere model within the global Carbon Cycle Data Assimilation System (see http://CCDAS.org). Using methods of variational data assimilation, CCDAS relies on first and second derivatives of the underlying model for estimating process parameters with uncertainty ranges. In a subsequent step these parameter uncertainties are mapped forward onto uncertainty ranges for predicted carbon fluxes. In this contribution, we quantify how MERIS data improve the accuracy of the current and future (net and gross) carbon flux estimates for a range of sites spanning the major biomes of the globe. We further present first assimilation experiments of MERIS FAPAR at the global scale together with in situ observations of atmospheric CO2 in a coarse-resolution setup of CCDAS and address the systematic application of CCDAS for the design of future space missions. As an example application we demonstrate that even with considerably higher accuracy MERIS-like products can only provide a weak constraint on long-term carbon fluxes.