Letter | Published:

Natural atmospheric 14C variation and the Suess effect

Abstract

THE dilution of the atmospheric 14CO2 concentration by large amounts of fossil-fuel derived CO2 which do not contain any 14C is commonly called the Suess effect. Its magnitude can be calculated with the same geochemical models as the global carbon cycle that also predict the future rise of atmospheric CO2 to be caused by the combustion of fossil fuels. Validation of a CO2 predictive model with the Suess effect 14C data is important because these two phenomena have a common cause, and therefore register model responses at roughly the same frequencies. Measurements of the Suess effect yield values between −15‰, and −25‰ in Δ 14C (in 1950)1, while different model predictions also cover about this range2,3. The requirement that a model correctly reproduces the Suess effect becomes a strong constraint when the accuracy of the measurement is improved to better than 2‰. 14C measurements in tree rings to an accuracy of 1.2‰ are reported here. The results indicate that the natural fluctuations of atmospheric 14C as yet preclude determination of the Suess effect to the accuracy required by the models.

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