Scientists can estimate the anthropogenic contribution to global temperature change by comparing recent temperatures to historical values. A physical test for this comparison is done using:

Changes in isotopic oxygen found in ice cores.

Explanation:

It says here that in polar ice cores, the measurement is relatively simple which brings less oxygen in the frozen water meaning the temperature were cooler.

Oxygen isotopes in ice cores extracted from polar regions exhibit a decreasing trend in the ratio of the heavy to light isotopes from the beginning of the Ice Age to its end, at which point the trend reverses sharply and then remains fairly constant for several thousand years.

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