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 fuels1,2. 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 years6–8 the availability of Δ14C data with an accuracy of better than 1.5‰ was thought to be essential to test these9,10. 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.