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.