[inigyou]

I'm not an ice core scientist. I suggest you ask them for detailed specifications. But the way you have posed the question strongly suggests that you will not be satisfied with any possible answer. Anything short of your eyes laying upon a thermometer will always be "not reliable enough", and thermometers only record the current temperature.

[qsera]

It is not really surprising that skeptics exist when the answers they get are of these sort...