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 range^2,3. 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.