How Do We Know Temperatures from Thousands of Years Ago?

Researchers estimate ancient temperatures using data from climate proxy records, i.e., indirect methods to measure temperature through natural archives, such as coral skeletons, tree rings, glacial ice cores, and so on. For example, the ice sheets of polar regions (Greenland, Antarctic, and the North American regions) can help in evaluating historical temperatures. With every year’s snowfall, a new layer of ice builds up on top of the previous one. This layer, and the ice sheets of every subsequent year, are preserved in visible layers. Since layers of snow formed at different temperatures have distinct chemical properties, all of these layers can give us some insight into the existing temperature conditions of that time.