It may also be possible that the rise in temperature has triggered the increase in CO2 rather than the other way round.
Or, they may both be driven by an entirely different phenomenon; what hard evidence exists, and I mean that in the strictest sense, to indicate any causal relationship whatsoever between the two? In order to establish such for present purposes requires very precise data, on the order of:
1. 0.5° C
2. x-ppm CO2
3. +/- 50 years
#3 would not necessarily be the case, were we not being asked to believe that the current state of affairs is entirely due to human factors arising from the advent of the industrial revolution. Meaning, were we simply trying to establish a rough correlation between temp & CO2 on a millenium scale, we would not require such fine resolution in the time component; but that is not the case here, so we do.
In the case of #1, it is clear, if you research this much at all, that esablishing an authoritative global temperature is not an easy thing to do, even in the here and now. To do so going back into history becomes a stretch, to put it lightly. Again, this difficulty ONLY arises from the need to be so precise, but that is what we need if we are to determine which, if either, is the leading trend.
In the case of #2, I think that the time-scale accuracy is probably the only real sticking point. I should think it quite simple to evaporate frozen ice and get a very good reading on the amount of CO2 present in the atmosphere at the time of freezing. As I say though, timing is the main issue here, and your mum, as you say, could probably shed some light on this; what are the standard margins of error for dating various classes of artifact? Ironically, I might think that using Carbon-14 here would tend to put is into a bit of a chicken vs. egg scenario. She being a geologist, I would be curious to hear her thoughts on this.
In the end, I think that wisdom should lead us to err on the side of caution and to therefore remember that we haven't always got these types of things right; it was three-hundred odd years ago that Redi first questioned the idea of spontaneous generation, and Pasteur was still working on proving him right two-hundred years later. Most of what we regard as common knowledge now, we should remember, was anything but, in the not-so-distant past.