Abstract
Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO2 concentrations via inverse modelling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models. Here we report estimates of surface-atmosphere CO2 fluxes from an intercomparison of atmospheric CO2 inversion models (the TransCom 3 project), which includes 16 transport models and model variants. We find an uptake of CO2 in the southern extratropical ocean less than that estimated from ocean measurements, a result that is not sensitive to transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO2. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO2 observation network within the tropics.
Highlights
Powered by the California Digital Library University of California letters to nature supported metal clusters from extended metal surfaces. These effects are most pronounced for the smallest clusters, and may be negligible for large supported metal particles for which only a small fraction of the metal atoms are bonded to the support
Geometric effects can distinguish the catalytic activity seen with small metal clusters from that seen with bulk metal or larger supported particles, by limiting the structures that can bond to a very small cluster and subsequently react on it
We estimateuxes for 11 land and 11 ocean regions as differences frombackground' ̄uxes that are run separately through each transport model and represent fossil-fuel emissions[17,18], seasonally varying air±sea gas exchange[10] and an annually balanced, seasonally varyingux due to terrestrial photosynthesis and respiration[19]
Summary
The authors declare that they have no competing ®nancial interests. Kevin Robert Gurney*, Rachel M. Comparing the uncertainties between regions indicates where the inversion would bene®t most from new observations, and where model improvements are most needed In this particular inversion, new measurements would be most useful over tropical continents and in the South America and South Atlantic regions, while the focus for resolving transport differences would be the northern and tropical land regions. Our sensitivity tests ®nd that the near-uniformity of observed concentration in the Southern Hemisphere and the small uncertainty associated with those measurements make this result robust to the choice of observing network, priorux estimates, global ocean constraint, and transport (see Fig. 2 in Supplementary Information). The discrepancy cannot be explained by a systematic bias in transport models, as the north±south transport has been investigated in a recent intercomparison[12] where successful simulations of the observed meridional gradient in SF6 suggested reasonable veracity in gross interhemispheric transport
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