Predicted shift in the ¹³<i>C</i>/¹²<i>C</i> ratio of atmospheric carbon dioxide

Abstract

Knowledge of recent global changes in the rare isotopic species 13 CO 2 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 atmosphere and ocean water: if the isotopic fractionation factor for CO 2 uptake at the air‐ocean boundary, α am , is approximately 0.986, as is the case for strongly alkaline solutions, the 13 CO 2 content of atmospheric CO 2 is insensitive to changes in the biosphere; if α 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.