An anomalous Suess effect above Europe

Abstract

The major impact that man is likely to have on climate comes from the artificial increase of the atmospheric CO₂ level, caused by the combustion of fossil fuels^1,2. To predict its magnitude, the future rise of the global atmospheric CO₂ level must be calculated using carbon reservoir models. Because fossil fuel CO₂ lacks ¹⁴C and has a ¹³C/¹²C ratio which is 18‰ lower than the present average atmospheric ¹³C/¹²C ratio, the validity of such geochemical models can be tested using past temporal variations in the atmospheric concentration of these isotopes. However, the past variation of the ¹³C/¹²C ratio in global reservoirs is very difficult to infer³. Using Δ¹⁴C data of tree rings Suess measured a depletion in the atmospheric ¹⁴C concentration over the past century of <10‰ (ref. 4). Some years later more accurate data gave a value for this ‘Suess effect’ of 15–25‰ (ref. 5). As the models have been improved considerably over the past few years^6–8 the availability of Δ¹⁴C data with an accuracy of better than 1.5‰ was thought to be essential to test these^9,10. The present results indicate a good correlation with model predictions taking into account natural ¹⁴C variations. However, deviations between American and European tree ring data after AD 1920 suggest a European continental CO₂ effect.