What is a climate proxy?

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Photograph of a coral reef. Corals are some of the most vulnerable organisms affected by ocean acidification, as more acidic ocean waters make it harder for corals to grow their skeletons. Photograph by Ilsa Kuffner, U.S. Geological Survey

The Earth's climate has changed greatly through geologic time, and even in recent centuries. The study of past climates is called paleoclimatology ("paleo-" means "early" or "past").

Something that represents something else indirectly is called a proxy. In some elections, a voter can choose another person to cast the vote. That vote is called a proxy. There are many proxies for past climate. They provide a lot of information, although none is perfect. Some, like kinds of past plants and animals, are easy to understand. Some important proxies, involving the chemical element oxygen, are more difficult to understand.