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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.

Spring and autumn multiannual measurements of the partial pressure of CO2 in the surface ocean and atmosphere in the equatorial Pacific region give a valuable insight into the impact of El Nino events on the size of the equatorial Pacific source of CO2. Spring flux data indicate significant interannual effects of El Nino on CO2 exchange in the equatorial Pacific. The total fall in CO2 outgassing from the equatorial oceans during the prolonged El Nino period of 1991 to 1994 was greater than during the severe El Nino of 1982 to 1983.

The amount of anthropogenic CO(sub2) sequestered by the ocean is usually estimated based on numerical ocean carbon-cycle models. The models often have marked differences, and the CO(sub2) flux must be verified by field measurements. Recent improvements in dissolved inorganic carbon (DIC) measurement methods have been used to reveal changes in DIC concentrations in the Indian Ocean between 1978 and 1995.

The interannual variability in fossil-fuel CO2 uptake from the atmosphere by the terrestrial biosphere and ocean is a poorly quantified process. Current estimates are based on atmospheric measurements. The interannual variability in global net air-sea CO2 flux has been estimated using alterations in wind speeds and partial pressure of CO2 (pco2) in the overlying air of surface sea water. Results indicated that the main cause of variations in the rate of atmospheric CO2 accumulation is variable sequestration of carbon by the terrestrial biosphere.

The Redfield ratio puts the molar C:N ratio in the organic material exported to deeper waters from the euphotic zones in the ocean at 6.6. However, a study conducted on the C:N consumption ratio in coastal waters yielded time-series data showing that net organic carbon production in these areas are way above levels predicted based on the Redfield ratio and nitrate consumption. Similar deviations were found in certain North Atlantic regions during eutrophic periods. These indicate that extrapolations using the Redfield ratio can result in substantial underestimations of organic carbon export.