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Abstract:
This study presents the outcome of an inverse modeling intercomparison experiment on the
use of total column CO2 retrievals from Greenhouse Gas Observing Satellite (GOSAT) for quantifying global
sources and sinks of CO2. Eight research groups submitted inverse modeling results for the first year of
GOSAT measurements. Inversions were carried out using only GOSAT data, a combination of GOSAT and
surface measurements, and using only surface measurements. As expected, the most robust flux estimates
are obtained at large scales (e.g., within 20% of the annual flux at the global scale), and they quickly
diverge toward the scale of the subcontinental TRANSCOM regions and beyond (to >100% of the annual
flux). We focus our analysis on a shift in the CO2 uptake over land from the Tropics toward the Northern
Hemisphere Extra tropics of ∼1 PgC/yr when GOSAT data are used in the inversions. This shift is largely
driven by TRANSCOM regions Europe and Northern Africa, showing, respectively, an increased uptake and
release of 0.7 and 0.9 PgC/yr. Inversions using GOSAT data show a reduced gradient between midlatitudes
of the Northern Hemisphere and the Tropics, consistent with the latitudinal shift in carbon uptake. However,
the reduced gradients degrade the agreement with background aircraft and surface measurements.
To narrow the range of inversion-derived flux, estimates will require further efforts to understand the
differences not only between the retrieval schemes but also between inverse models, as their contributions to the overall uncertainty are estimated to be of similar magnitude.
