By measuring the ratios of different water isotopes in polar ice cores, we can determine how temperature in Antarctica and Greenland has changed in the past.
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Ice core measurements allow us to extend this way back into the past.
In an Antarctic core (Law Dome) with a very high snowfall rate, it has been possible to measure concentrations in air from as recently as the 1980s that is already enclosed in bubbles within the ice.
It extends back 800,000 years and shows a succession of long, cold ‘glacial’ periods, interspersed roughly every 100,000 years by warm ‘interglacial’ periods (of which the last 11,000 years is the most recent).
This succession of events is well-known from other records, and the coldest periods in Antarctica are the times when we had ice ages.
Ice sheets extended over North America as far south as Wisconsin, and over Britain to south of The Wash.