Knowledge of recent global changes in the rare isotopic spicies 13CO2 in atmospheric carbon dioxide does not now distinguish whether the land biosphere is a source or sink for fossil fuel carbon dioxide. The question is critically dependent on isotopic fractionation between the atmosphre and ocean water: if the isotopic fractionation factor CO2 uptake at the air-ocean boundary, &agr;am, is approximately 0.986, as is the case for strongly alkaline solutions, the 13CO2 content of atmospheric CO2 is insensitive changes in the biosphere; if &agr;am is approximately unity, as now seems likely for ocean water, very accurate measurements, over the next decade or longer, will permit an estimate to be made of the net global loss or gain of biospheric carbon.