Global and hemispheric CO₂ sinks deduced from changes in atmospheric O₂ concentration

THE global budget for sources and sinks of anthropogenic CO2 has been found to be out of balance unless the oceanic sink is supplemented by an additional 'missing sink', plausibly associated with land biota1,25. A similar budgeting problem has been found for the Northern Hemisphere alone2,3, suggesting that northern land biota may be the sought-after sink, although this interpretation is not unique2–5; to distinguish oceanic and land carbon uptake, the budgets rely variously, and controversially, on ocean models2,6,7, 13CO2/12CO2 data2,4,5, sparse oceanic observations of p CO2 (ref. 3) or 13C/12C ratios of dissolved inorganic carbon,4,5,8 or single-latitude trends in atmospheric O2 as detected from changes in O2/N2 ratio.9,10 Here we present an extensive O2/N2 data set which shows simultaneous trends in O2/N2 in both northern and southern hemispheres and allows the O2/N2 gradient between the two hemispheres to be quantified. The data are consistent with a budget in which, for the 1991–94 period, the global oceans and the northern land biota each removed the equivalent of approximately 30% of fossil-fuel CO2 emissions, while the tropical land biota as a whole were not a strong source or sink