Global CO2 fluxes estimated from GOSAT retrievals of total column CO2

We present one of the first estimates of the global distribution of CO2 surface fluxes using total column CO 2 measurements retrieved by the SRON-KIT RemoTeC algorithm from the Greenhouse gases Observing SATellite (GOSAT). We derive optimized fluxes from June 2009 to December 2010. We estimate fluxes from surface CO 2 measurements to use as baselines for comparing GOSAT dataderived fluxes. Assimilating only GOSAT data, we can reproduce the observed CO 2 time series at surface and TCCON sites in the tropics and the northern extra-tropics. In contrast, in the southern extra-tropics GOSAT X CO2 leads to enhanced seasonal cycle amplitudes compared to independent measurements, and we identify it as the result of a land– sea bias in our GOSAT X CO2 retrievals. A bias correction in the form of a global offset between GOSAT land and sea pixels in a joint inversion of satellite and surface measurements of CO2 yields plausible global flux estimates which are more tightly constrained than in an inversion using surface CO2 data alone. We show that assimilating the biascorrected GOSAT data on top of surface CO 2 data (a) reduces the estimated global land sink of CO 2, and (b) shifts the terrestrial net uptake of carbon from the tropics to the extratropics. It is concluded that while GOSAT total column CO 2 provide useful constraints for source–sink inversions, small spatiotemporal biases – beyond what can be detected using current validation techniques – have serious consequences for optimized fluxes, even aggregated over continental scales.