Scientists have been recording climate data for around 150 years. Therefore, when they need to estimate climate conditions from long before that in the geologic past, they study climate data that has been preserved in the environment. These environmental records are called climate proxies and can indicate to scientists the climatic conditions that occurred many millions of years ago.

Some climate proxies reveal general climatic patterns that occurred over the entire Earth such as the chemistry of fossil marine organisms or layers of sediment in the ocean that become sedimentary rock.

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Some climate proxies are more helpful in understanding seasonal changes in specific regions such as tree rings and coraline red algae.