"temperature is inferred from ratios of isotopes."
I don't know what that's about.
Here's some articles on the subject that can help in understanding what they do:
Since molecules made up of differing isotopes of the same elements have different masses, natural processes tend to favor some molecules over others in depositing them in snow, rain, incorporating them into plant tissues etc.
The ratios of these Oxygen isotopes can be used as an indirect measure of temperature and humidity/precipitation for example, as the rate such is deposited is dependent on these factors.
Other isotopes such as 10Be & 14C are created in nuclear interactions between cosmic rays and atmospheric nitrogen, since the geomagnetic field shielding the earth from cosmic rays is modulated by solar magnetic fields such isotope concentrations vary with the solar activity cycle and thus are indicators of solar factors on the climate such as solar brightness & other impacts on the atmosphere through ionization like cloud formation.
Using mass spectrometers, vary small concentrations can be measured and used to create proxies for the variables involved with creation and depositing of these isotopes in the geological record.
Thanks. I was more the details and particulars I was referring to. I'd have to read more about the ice core work to know and understand it.