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.