Trends and variations in stable carbon isotopes of atmospheric carbon dioxide

The paper reviews knowledge about atmospheric variations in 13C/12C in atmospheric CO2. Regionally and on a short time-scale the 13C/12C ratio of atmospheric CO2 is closely linked to concentration through a simple equation. For a source 13δ value of -26 per mille the addition of 1 ppm of CO2 to a present day atmosphere with C0 = 350 ppm has an effect on 13δ of -0.052 per mille/ppm. This effect is observed during day-night variations of CO2 concentration in forests. The same effect is observed during seasonal variations in CO2, for instance over Alaska: -0.8 per mille in 13δ synchronous with a peak to peak CO2 concentration amplitude of 15 ppm. The long term trend in 13δ, however, is quite differently related to the slowly increasing CO2 concentration; for Mauna Loa the changes over the period 1980-1990 are -0.2 per mille and about +14 ppm respectively, equivalent to -0.015 per mille/ppm. This is due to the buffering effect of the oceans. Furthermore, distinct differences in 13δ and C are observed between air over northern and southern high latitudes, again consistent with a slow mixing process between air over the Northern Hemisphere, where 95% of the emission of fossil fuel CO2 occurs, and air of the Southern Hemisphere. 12 refs, 6 figs, 2 tabs