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

Abstract

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^1,25. A similar budgeting problem has been found for the Northern Hemisphere alone^2,3, suggesting that northern land biota may be the sought-after sink, although this interpretation is not unique^2–5; to distinguish oceanic and land carbon uptake, the budgets rely variously, and controversially, on ocean models^2,6,7, ¹³CO₂/¹²CO₂ data^2,4,5, sparse oceanic observations of $p_{\text{C}{\text{O}}_{\text{2}}}$ (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.