Abstract
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 Pg C yr−1 (±0.5 Pg C 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 Pg C yr−1) and a compensatory sink of similar magnitude in the south (−1.4 ± 0.5 Pg C 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 Pg C 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 Pg C yr−1), the northern and southern land also contribute (standard deviation ~ 0.39 Pg C yr−1). Most inversions tend to indicate an increase of the northern land carbon uptake from late 1990s to 2008 (around 0.1 Pg C yr−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.
Highlights
Atmospheric CO2 inversions offer a method by which to estimate carbon exchange between the land/ocean and atmosphere by utilizing atmospheric CO2 measurements, a key observational component of the global carbon cycle
We present a number of atmospheric CO2 inversions that employ this general approach, with differences in the detailed specification, which will be given in the inversion description (Sect. 3)
The LSCE Laboratory has been collecting carbon flux estimates from state-of-the-art inversions performed by groups around the world in an effort to construct a new atmospheric CO2 inversion intercomparison
Summary
Atmospheric CO2 inversions offer a method by which to estimate carbon exchange between the land/ocean and atmosphere by utilizing atmospheric CO2 measurements, a key observational component of the global carbon cycle (e.g. their observed temporal and spatial gradients). No exhaustive intercomparison has been performed between these recent estimates (i.e. from global to regional scales), apart from regional initiatives in Europe (Schulze et al, 2010) and North America (Hayes et al, 2012) and individual studies investigating only specific aspects of the carbon balance (e.g., Ciais et al, 2010, for the Northern Hemisphere long-term mean fluxes). In this context, the results presented here are the latest comprehensive intercomparison. The last section discusses issues involved with interpreting inversion results at regional scale
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