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Abstract:
Atmospheric CO2 inversions estimate surface carbon
fluxes from an optimal fit to atmospheric CO2 measurements,
usually including prior constraints on the flux estimates.
Eleven sets of carbon flux estimates are compared,
generated by different inversions systems that vary in their
inversions methods, choice of atmospheric data, transport
model and prior information. The inversions were run for
at least 5 yr in the period between 1990 and 2010. Mean
fluxes for 2001–2004, seasonal cycles, interannual variability
and trends are compared for the tropics and northern and
southern extra-tropics, and separately for land and ocean.
Some continental/basin-scale subdivisions are also considered
where the atmospheric network is denser. Four-year
mean fluxes are reasonably consistent across inversions at
global/latitudinal scale, with a large total (land plus ocean)
carbon uptake in the north (−3.4 PgC yr−1 (±0.5 PgC yr−1
standard deviation), with slightly more uptake over land than
over ocean), a significant although more variable source over
the tropics (1.6±0.9 PgC yr−1) and a compensatory sink of
similar magnitude in the south (−1.4±0.5 PgC yr−1) corresponding
mainly to an ocean sink. Largest differences across
inversions occur in the balance between tropical land sources
and southern land sinks. Interannual variability (IAV) in carbon
fluxes is larger for land than ocean regions (standard deviation
around 1.06 versus 0.33 PgC yr−1 for the 1996–2007
period), with much higher consistency among the inversions
for the land. While the tropical land explains most
of the IAV (standard deviation 0.65 PgC yr−1), the northern
and southern land also contribute (standard deviation
0.39 PgC yr−1). Most inversions tend to indicate an increase
of the northern land carbon uptake from late 1990s to
2008 (around 0.1 Pg Cyr−1), predominantly in North Asia.
The mean seasonal cycle appears to be well constrained by
the atmospheric data over the northern land (at the continental
scale), but still highly dependent on the prior flux seasonality
over the ocean. Finally we provide recommendations to
interpret the regional fluxes, along with the uncertainty estimates.
