you don't use data to show temperature from prior measuring periods ------------- you use evidence.
I'm surprised that you aren't aware of this very well known concept. ------------ Eg.
Ice cores: The ratio of oxygen isotopes in ice can provide information about past temperatures and atmospheric composition
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Glaciers: Variations in the past size of glaciers can be inferred from the location of moraines (rocks and debris deposited by glaciers) and buried soils, and in the presence of glacial features in the landscape
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Speleothems: Glacial deposits embedded within speleothems (stalactites and stalagmites) can be used to indicate periods of glacial advance
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Boreholes: It is sometimes possible to deduce past surface temperatures going back several hundred years by measuring the way temperature varies with depth in a borehole several hundred meters deep
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Climate proxies: These are evidence of past climate that can be substituted for direct measurements. Examples include tree rings, sediment cores, and chemical analyses of ice cores
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