Abstract
Abstract. We present one of the first estimates of the global distribution of CO2 surface fluxes using total column CO2 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 CO2 measurements to use as baselines for comparing GOSAT data-derived fluxes. Assimilating only GOSAT data, we can reproduce the observed CO2 time series at surface and TCCON sites in the tropics and the northern extra-tropics. In contrast, in the southern extra-tropics GOSAT XCO2 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 XCO2 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 bias-corrected GOSAT data on top of surface CO2 data (a) reduces the estimated global land sink of CO2, and (b) shifts the terrestrial net uptake of carbon from the tropics to the extra-tropics. It is concluded that while GOSAT total column CO2 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.
Highlights
The traditionalMtopo-ddoewlnDapepvroeaclhopfomr qeuannttifying sourcesM produce the observed CO2 time series at surface and TC- and sinks of CO2 is an atmospheric inversion of CO2 con-CON sites in the tropics and the northern extra-tropics
In centrations measured at the earth’s surface
The posterior atmospheric CO2 fields from both inversions are consistent with Total Carbon Column Observing Network (TCCON) XCO2 measurements over North America, because Chevallier et al (2011) assimilated TCCON XCO2 while we assimilated GOSAT XCO2 which were validated against TCCON XCO2. This suggests that at present, different XCO2 measurements consistent with the same set of TCCON XCO2 can yield dramatically different posterior flux distributions, even over regions such as North America, which has several TCCON stations. In this manuscript we have presented optimized global source–sink estimates of CO2 using the RemoTeC retrieval of GOSAT XCO2 over eighteen months from 1 June 2009 to 1 December 2010
Summary
M produce the observed CO2 time series at surface and TC- and sinks of CO2 is an atmospheric inversion of CO2 con-. Several observation system simulation experiments have explored the added benefit of assimilating satellite measurements in atmospheric inversions of CO2 (Rayner and O’Brien, 2001; Park and Prather, 2001; Houweling et al, 2004; Chevallier et al, 2007; Miller et al, 2007; Hungershoefer et al, 2010) These studies have shown that space-based measurements of CO2 are not as accurate as surface layer measurements, the increased spatial coverage can provide information not available from the sparse surface network (Buchwitz et al, 2007). They showed that assimilating TES CO2 in addition to surface CO2 measurements improves constraints on surface fluxes, especially over regions absent from the surface network, such as the tropical forests of South America (Nassar et al, 2011) It follows that a satellite instrument sensitive to the lower troposphere would be even more useful for CO2 source–sink inversions.
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