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How do corals and cave stalagmites help paleoclimatologists study past climates?

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World map with continents shaded in colors representing climate zones, from warm reds and yellows near the equator to cool blues and whites at the poles. — Climate Even in our warming world, there will be some chilly evenings.

Paleoclimatologists use corals and cave stalagmites as natural archives, or "proxy" evidence, to reconstruct past climates because these features record environmental conditions as they grow over time.12

Corals

Corals build skeletons of calcium carbonate that incorporate trace elements and isotopes from the surrounding seawater. By analyzing the oxygen isotopic composition within these skeletons, researchers can infer changes in water temperature that occurred during the coral's growth period. Because some corals provide annually dated records, they allow scientists to reconstruct climate variables on a yearly or even seasonal basis. Additionally, certain fossil corals have been used to study the Earth's historical rotation rate by counting daily growth ridges.324

Stalagmites

Stalagmites are speleothems that form in caves, which provide a stable environment where temperature and humidity can remain constant for millennia. As water drips into a cave, it deposits calcium carbonate, creating layers that accumulate over time. These deposits serve as climate proxies because their growth and chemical composition are influenced by the water infiltrating from the surface, which in turn reflects past precipitation and temperature patterns.561