evidence: The atmospheric CO2 increase over the past few decades is less than the input from human activities because a fraction of the added CO2 is removed by oceanic and terrestrial processes. Until recently, the partitioning of the carbon sink between the land and sea has been highly uncertain, but recent high-precision measurements of the atmospheric oxygen:nitrogen (O2:N2) ratio have provided a crucial constraint: fossil fuel burning and terrestrial uptake processes have different O2:CO2 ratios, whereas the ocean CO2 sink has no significant impact on atmospheric O2. The atmospheric CO2 increase for the 1990s was about half the CO2 emission from fossil fuel combustion, with the oceans and land both serving as important repositories of the excess carbon, i.e., as carbon sinks.