The number of stomata on leafs (breathing pores) of certain plant species appears to be sensitive to the CO2 level in the atmosphere. If it is higher, it appears that these plants does not bother to make many stomata, whereas CO2 is scarce, it simply makes more stomata to extract the CO2 from the atmosphere. So if we have fossil leafs of these species, with some accurate dating, it might be possible to reconstruct CO2 levels from the past and perhaps much better than ice cores, since there are several problems associated with the accuracy of ice cores. A first thread about that concept is here. The University of Utrecht in the Netherlands appears to be the mark leader in this specialty and this is their newest product: The study refers to earlier work in Washington on Tsuga needles, which is reported in this PhD thesis. Note that the graphs of fig 1c and fig 5.4 on page 57 (Pdf:61) of the thesis appear to correlate reasonably both with maximums around 1000AD and 1350AD and a minimum around 1200AD giving it a certain robustness. Furthermore, they make a temperature reconstruction based on the alleged dependence of temperature on CO2 (Fig 2) with a 0,1-0,2C order of magnitude temperature variation. See a concluding remark: I find it curious however that they did not test their work to other temperature reconstructions in that period, If we do that, for instance with the new & improved hockey stick discussed here, with an minimum at around 1350AD, instead of a maximum, one would tend to think that it would support the CO2 cooling hypothesis rather than the CO2 warming hypothesis.