Abstract
Abstract. We infer CO2 surface fluxes using satellite observations of mid-tropospheric CO2 from the Tropospheric Emission Spectrometer (TES) and measurements of CO2 from surface flasks in a time-independent inversion analysis based on the GEOS-Chem model. Using TES CO2 observations over oceans, spanning 40° S–40° N, we find that the horizontal and vertical coverage of the TES and flask data are complementary. This complementarity is demonstrated by combining the datasets in a joint inversion, which provides better constraints than from either dataset alone, when a posteriori CO2 distributions are evaluated against independent ship and aircraft CO2 data. In particular, the joint inversion offers improved constraints in the tropics where surface measurements are sparse, such as the tropical forests of South America. Aggregating the annual surface-to-atmosphere fluxes from the joint inversion for the year 2006 yields −1.13±0.21 Pg C for the global ocean, −2.77±0.20 Pg C for the global land biosphere and −3.90±0.29 Pg C for the total global natural flux (defined as the sum of all biospheric, oceanic, and biomass burning contributions but excluding CO2 emissions from fossil fuel combustion). These global ocean and global land fluxes are shown to be near the median of the broad range of values from other inversion results for 2006. To achieve these results, a bias in TES CO2 in the Southern Hemisphere was assessed and corrected using aircraft flask data, and we demonstrate that our results have low sensitivity to variations in the bias correction approach. Overall, this analysis suggests that future carbon data assimilation systems can benefit by integrating in situ and satellite observations of CO2 and that the vertical information provided by satellite observations of mid-tropospheric CO2 combined with measurements of surface CO2, provides an important additional constraint for flux inversions.
Highlights
Inverse modeling has emerged as a key method for obtaining quantitative information on the global carbon cycle
Tropospheric Emission Spectrometer (TES) CO2 observations over oceans provide a weaker constraint on global CO2 surface fluxes than data from the surface flask networks, but we demonstrate that TES CO2 observations can be used together with the flask data to obtain improved estimates of CO2 surface fluxes
We showed that the spatial coverage provided by satellite observations of CO2 is an important benefit to CO2 surface flux inversions especially in regions where the surface data are sparse such as South America or Africa
Summary
Inverse modeling has emerged as a key method for obtaining quantitative information on the global carbon cycle. In this approach, CO2 measurements are combined with CO2 distributions from a 3-dimensional (3-D) transport model, weighting them according to their uncertainties in order to produce optimized estimates of surface source and sink strengths (fluxes). The terrestrial biospheric flux is the component of the global carbon cycle that currently exhibits the most interannual variability, the most geographical heterogeneity and the greatest uncertainty (Denman et al, 2007, Ch. 7, AR4). Nassar et al.: Inverse modeling of CO2 sources and sinks
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