Estimates of European uptake of CO₂ inferred from GOSAT X_CO2 retrievals: sensitivity to measurement bias inside and outside Europe

Abstract. Estimates of the natural CO₂ flux over Europe inferred from in situ measurements of atmospheric CO₂ mole fraction have been used previously to check top-down flux estimates inferred from space-borne dry-air CO₂ column (X_CO2) retrievals. Several recent studies have shown that CO₂ fluxes inferred from X_CO2 data from the Japanese Greenhouse gases Observing SATellite (GOSAT) and the Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY (SCIAMACHY) have larger seasonal amplitudes and a more negative annual net CO₂ balance than those inferred from the in situ data. The cause of this elevated European uptake of CO₂ is still unclear, but some recent studies have suggested that this is a genuine scientific phenomenon. Here, we put forward an alternative hypothesis and show that realistic levels of bias in GOSAT data can result in an erroneous estimate of elevated uptake over Europe. We use a global flux inversion system to examine the relationship between measurement biases and estimates of CO₂ uptake from Europe. We establish a reference in situ inversion that uses an Ensemble Kalman Filter (EnKF) to assimilate conventional surface mole fraction observations and X_CO2 retrievals from the surface-based Total Carbon Column Observing Network (TCCON). We use the same EnKF system to assimilate two independent versions of GOSAT X_CO2 data. We find that the GOSAT-inferred European terrestrial biosphere uptake peaks during the summer, similar to the reference inversion, but the net annual flux is 1.40 ± 0.19 GtC a⁻¹ compared to a value of 0.58 ± 0.14 GtC a⁻¹ for our control inversion that uses only in situ data. To reconcile these two estimates, we perform a series of numerical experiments that assimilate observations with added biases or assimilate synthetic observations for which part or all of the GOSAT X_CO2 data are replaced with model data. We find that for our global flux inversions, a large portion (60–90 %) of the elevated European uptake inferred from GOSAT data in 2010 is due to retrievals outside the immediate European region, while the remainder can largely be explained by a sub-ppm retrieval bias over Europe. We use a data assimilation approach to estimate monthly GOSAT X_CO2 biases from the joint assimilation of in situ observations and GOSAT X_CO2 retrievals. The inferred biases represent an estimate of systematic differences between GOSAT X_CO2 retrievals and the inversion system at regional or sub-regional scales. We find that a monthly varying bias of up to 0.5 ppm can explain an overestimate of the annual sink of up to 0.20 GtC a⁻¹. Our results highlight the sensitivity of CO₂ flux estimates to regional observation biases, which have not been fully characterized by the current observation network. Without further dedicated measurements we cannot prove or disprove that European ecosystems are taking up a larger-than-expected amount of CO₂. More robust inversion systems are also needed to infer consistent fluxes from multiple observation types.