Eco-Guilt

I suspect the main climate change paleothermometry is done via ¹⁸O:¹⁶O ratio studies.

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In paleosciences, ¹⁸O:¹⁶O data from foraminifera and ice core are used as a proxy for temperature.

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and

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Foraminifera shells of calcium carbonate (CaCO₃), having oxygen in them, and being found in many common geological features, are most commonly tested. The ratio of ¹⁸O:¹⁶O to ¹⁶O is used to tell the temperature of the surrounding water of the time solidified, indirectly. The ratio varies slightly depending on the temperature of the surrounding water, as well as other factors such as the water's salinity, and the volume of water locked up in ice sheets.

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δ¹⁸O:¹⁶O also reflects local evaporation and freshwater input, as rainwater is ¹⁶O enriched - a result of ¹⁶O's preferential evaporation from seawater. Consequently, the surface ocean contains greater amounts of ¹⁸O:¹⁶O around the subtropics and tropics where there is more evaporation, and lesser amounts of ¹⁸O:¹⁶O in the mid-latitudes where it rains more.

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Similarly, when water vapor condenses, heavier water molecules holding ¹⁸O:¹⁶O atoms tend to condense and precipitate first. The water vapor gradient heading from the tropics to the poles gradually becomes more and more depleted of ¹⁸O:¹⁶O. Snow falling in Canada has much less H2¹⁸O:¹⁶O than rain in Florida; similarly, snow falling in the center of ice sheets has a lighter δ¹⁸O:¹⁶O signature than that at its margins, since heavier ¹⁸O:¹⁶O precipitates first.

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Changes in climate alter global patterns of evaporation and precipitation therefore change the background δ¹⁸O:¹⁶O ratio.

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A little more info available at the cite, including some time/temperature graphs.It sounds like quite a few complications have to be considered to get proper conclusions.Boy, doing all those superscripts is a pain. I hope I didn't screw any up.Added by edit: Thought I better explain that the ¹⁸O and ¹⁶O's refer to different isotopes of Oxygen.MooseEdited by Minnemooseus, : Screwed 1 up.Edited by Minnemooseus, : See ABE above.