[AstralTraveller (David)]

Originally Posted by renormalised

If you look at the CO2 levels in pre-ice age ice cores from Antarctica and the isotope ratios present in ancient lake sediment cores, the amount of CO2 was high beforehand and the C isotope was heavy C13, but when the planet began to cool, the isotope C12 became more prevalent in the cores cf. C13 and atmospheric CO2 dropped dramatically.

What has this got to do with whether the planet will cool? The 12C/13C ratio can certainly be used as an indicator of past environments but it is not a causal agent. The ratio of the two stable isotopes is pretty constant at about 1.11% 13C, give or take a few parts per thousand. That's why we need such fancy instruments to measure the differences.

BTW these days we can watch the 13C/12C ratio drop as the CO2 concentration increases. This is because we are burning fossilized plant remains and photosynthesis discriminates against 13C, meaning that fossil fuels are isotopically 'light'. The difference between the 13C concentration of the present atmosphere and plant remains is about 20 parts per thousand. So we are adding CO2 which has relatively little 13C to the atmosphere and so diluting the 13C.