GLOBAL AND HEMISPHERIC CO2 SINKS DEDUCED FROM CHANGES IN ATMOSPHERIC O-2 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 biota(1,2 5). A similar budgeting problem has been found for the Northern Hemisp here alone(2,3), suggesting that northern land biota may be the sought -after sink, although this interpretation is not unique(2-5); to disti nguish oceanic and land carbon uptake, the budgets rely variously, and controversially, on ocean models(2,6,7), (CO2)-C-13/(CO2)-C-12 data(2 ,4,5), sparse oceanic observations of p(CO2) (ref. 3) or C-13/C-12 rat ios of dissolved inorganic carbon, (4,5,8) or single-latitude trends i n atmospheric O-2 as detected from changes in O-2/N-2 ratio.(9,10). He re we present an extensive O-2/N-2 data set which shows simultaneous t rends in O-2/N-2 in both northern and southern hemispheres and allows the O-2/N-2 gradient between the two hemispheres to be quantified. The data are consistent with a budget in which, for the 1991-94 period, t he global oceans and the northern land biota each removed the equivale nt of approximately 30% of fossil-fuel CO2 emissions, while the tropic al land biota as a whole were not a strong source or sink.