Elevated uptake of CO2 over Europe inferred from GOSAT XCO2 retrievals: a real phenomenon or an artefact of the analysis?

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 (XCO2

) retrievals. Recent work

has shown that CO2 fluxes inferred from XCO2

5 data from the Japanese Greenhouse

gases Observing SATellite (GOSAT) have a larger seasonal amplitude and a more

negative annual net CO2 balance than those inferred from the in situ data. The causes

of this enhanced European CO2 uptake have since become the focus of recent studies.

We show this elevated uptake over Europe could largely be explained by mis-fitting

10 data due to regional biases. We establish a reference in situ inversion that uses an

Ensemble Kalman Filter (EnKF) to assimilate surface flask data and the XCO2

data

from the surface-based Total Carbon Column Observing Network (TCCON). The same

EnKF system is also used to assimilate two, independent versions of GOSAT XCO2

data. We find that the GOSAT-inferred European terrestrial biosphere uptake peaks

15 during the summer, similar to the reference inversion, but the net annual flux is 1.18-

0.1GtCa-1 compared to a value of 0.56-0.1GtCa-1 for our control inversion that

uses only in situ data. To reconcile these two estimates, we have performed a series of

numerical experiments that assimilate observations with biases or assimilate synthetic

observations for which part or all of the GOSAT XCO2 data are replaced with model

20 data. We find that 50–80% of the elevated European uptake in 2010 inferred from

GOSAT data is due to retrievals outside the immediate European region, while most

of the remainder can be explained by a sub-ppm retrieval bias over Europe. We have

used data assimilation techniques to estimate monthly GOSAT XCO2

biases from the

joint assimilation of in situ observations and GOSAT XCO2

retrievals. We find a monthly

25 varying bias of up to 0.5 ppm can explain an overestimate of the annual sink of up to

0.18 GtCa-1.