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Estimates of the natural CO2 flux over Europe inferred from in situmeasurements of atmospheric CO2 mole fraction have been used previouslyto check top-down flux estimates inferred from space-borne dry-air CO2column (XCO2) retrievals. Several recent studies have shown thatCO2 fluxes inferred from XCO2 data from the JapaneseGreenhouse gases Observing SATellite (GOSAT) and the Scanning ImagingAbsorption Spectrometer for Atmospheric CHartographY (SCIAMACHY) have largerseasonal amplitudes and a more negative annual net CO2 balance thanthose inferred from the in situ data. The cause of this elevated Europeanuptake of CO2 is still unclear, but some recent studies have suggestedthat this is a genuine scientific phenomenon. Here, we put forward analternative hypothesis and show that realistic levels of bias in GOSAT datacan result in an erroneous estimate of elevated uptake over Europe. We use aglobal flux inversion system to examine the relationship between measurementbiases and estimates of CO2 uptake from Europe. We establish a referencein situ inversion that uses an Ensemble Kalman Filter (EnKF) to assimilateconventional surface mole fraction observations and XCO2retrievals from the surface-based Total Carbon Column Observing Network(TCCON). We use the same EnKF system to assimilate two independent versionsof GOSAT XCO2 data. We find that the GOSAT-inferred Europeanterrestrial biosphere uptake peaks during the summer, similar to thereference inversion, but the net annual flux is1.40 ± 0.19 GtC a−1 compared to a value of0.58 ± 0.14 GtC a−1 for our control inversion that uses only insitu data. To reconcile these two estimates, we perform a series of numericalexperiments that assimilate observations with added biases or assimilatesynthetic observations for which part or all of the GOSAT XCO2data are replaced with model data. We find that for our global fluxinversions, a large portion (60–90 %) of the elevated European uptakeinferred from GOSAT data in 2010 is due to retrievals outside the immediateEuropean region, while the remainder can largely be explained by a sub-ppmretrieval bias over Europe. We use a data assimilation approach to estimatemonthly GOSAT XCO2 biases from the joint assimilation of in situobservations and GOSAT XCO2 retrievals. The inferred biasesrepresent an estimate of systematic differences between GOSATXCO2 retrievals and the inversion system at regional orsub-regional scales. We find that a monthly varying bias of up to 0.5 ppmcan 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 regionalobservation biases, which have not been fully characterized by the currentobservation network. Without further dedicated measurements we cannot proveor disprove that European ecosystems are taking up a larger-than-expectedamount of CO2. More robust inversion systems are also needed to inferconsistent fluxes from multiple observation types

atmospheric chemistry and physics

Estimates of European uptake of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; inferred from GOSAT X&amp;lt;sub&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;&amp;lt;/sub&amp;gt; retrievals: sensitivity to measurement bias inside and outside Europe