How is causality between past CO$_2$ levels and fossilized plant pores proxies established?

Question

According to Mann and Kump (2015),

Geologic proxies are characteristics of sedimentary rocks that we can use to infer ancient environmental conditions. Climate scientists study proxies because there are no thermometer records or samples of ancient air from the geologic past. Proxies may even include the pores on fossil leaves: when atmospheric CO₂ levels are low, plants need more pores to bring in more CO₂. Under the microscope, well-preserved fossils reveal the number of pores; comparing this pore density to that of living plants allows scientists to establish CO₂ levels in the geologic past.

However, in order to use this comparison to make inferences about CO₂ levels, it seems to me that this would require establishing that everything else is held equal. If this is not established, it seems to me that this would not allow one to use plant pores as proxies for past CO₂ levels and thus climate change. Therefore, my question is, how do researchers attempt to hold all else equal when comparing the pore density of fossilized plants to living plants to establish a causal relation between past CO₂ levels and plant pores density?

Reference:

Mann, M. E. and L. R. Kump (2015). Dire Predictions: Understanding Climate Change 2nd Edition. DK Publishing.

Answer 1

ONE WAY to judge atmospheric CO2 of primitive prehistoric environments is to gauge geologic fossil plants by taxonomic type and geographic source (Taking into account continental drift) Finding tropical variant plant types in areas with now temperate/cold climate.