A global synthesis inversion analysis of recent variability in CO2 fluxes using GOSAT and in situ observations
Authors: Wang James S. , Kawa S. Randolph , Collatz G. James , Sasakawa Motoki , Gatti Luciana V. , Machida Toshinobu , Liu Yuping , Manyin Michael E. 
Published: August 2018

Abstract. The precise contribution of the two major sinks for anthropogenic CO2
emissions, terrestrial vegetation and the ocean, and their location and
year-to-year variability are not well understood. Top-down estimates of the
spatiotemporal variations in emissions and uptake of CO2 are expected
to benefit from the increasing measurement density brought by recent in situ
and remote CO2 observations. We uniquely apply a batch Bayesian
synthesis inversion at relatively high resolution to in situ surface
observations and bias-corrected GOSAT satellite column CO2 retrievals
to deduce the global distributions of natural CO2 fluxes during
2009–2010. The GOSAT inversion is generally better constrained than the in
situ inversion, with smaller posterior regional flux uncertainties and
correlations, because of greater spatial coverage, except over North America
and northern and southern high-latitude oceans. Complementarity of the
in situ and GOSAT data enhances uncertainty reductions in a joint inversion;
however, remaining coverage gaps, including those associated with spatial and
temporal sampling biases in the passive satellite measurements, still limit
the ability to accurately resolve fluxes down to the sub-continental
or sub-ocean basin scale. The GOSAT inversion produces a shift
in the global CO2 sink from the tropics to the north and south
relative to the prior, and an increased source in the tropics of
∼ 2 Pg C yr−1 relative to the in situ inversion, similar to
what is seen in studies using other inversion approaches. This result may be
driven by sampling and residual retrieval biases in the GOSAT data, as
suggested by significant discrepancies between posterior CO2
distributions and surface in situ and HIPPO mission aircraft data. While the
shift in the global sink appears to be a robust feature of the inversions,
the partitioning of the sink between land and ocean in the inversions using
either in situ or GOSAT data is found to be sensitive to prior uncertainties
because of negative correlations in the flux errors. The GOSAT inversion
indicates significantly less CO2 uptake in the summer of 2010 than in
2009 across northern regions, consistent with the impact of observed severe
heat waves and drought. However, observations from an in situ network in
Siberia imply that the GOSAT inversion exaggerates the 2010–2009 difference
in uptake in that region, while the prior CASA-GFED model of net ecosystem
production and fire emissions reasonably estimates that quantity. The prior,
in situ posterior, and GOSAT posterior all indicate greater uptake over North America in spring to
early summer of 2010 than in 2009, consistent with wetter conditions. The
GOSAT inversion does not show the expected impact on fluxes of a 2010 drought
in the Amazon; evaluation of posterior mole fractions against local aircraft
profiles suggests that time-varying GOSAT coverage can bias the estimation of
interannual flux variability in this region.