Towards robust regional estimates of CO₂ sources and sinks using atmospheric transport models

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.