Coral reefs record small variations in the climate that can tell scientists about growing conditions in the oceans. Corals that grow near the ocean surface provide annual records of tropical climates that extend back over the past several centuries. They therefore serve as proxies of the upper ocean environment for sea-surface temperature and salinity.

When scientists want to inspect a core of coral, they must carefully extract a sample using a hydraulic drill connected to a compressor on a ship. Their goal is to drive a perpendicular core path into the coral so that they obtain a sample of the coral's maximum growth record. (NOAA Paleoclimatology Paleo Slide Set; Maris Kazmers, SharkSong Photography, Okemos, Michigan, photographer)

This is a positive X-radiograph collage of a Galápagos Pavona clavus coral. It is a sample of a coral skeleton composed of calcium carbonate (CaCO3). A coral skeleton formed in the winter has a different density than one formed in the summer because of variations in growth rates due to temperature and cloud cover conditions, creating growth bands. The bands are made more visible in an X-ray. (NOAA Paleoclimatology Paleo Slide Set; Jerry Wellington, department of biology, University of Houston, photographer)

Corals build their hard skeletons from calcium carbonate they extract from seawater. The calcium carbonate contains isotopes of oxygen and trace metals scientists can use to determine the temperature of the water in which the corals grew. Similar to the way annual rings on a tree form, the living tissue of corals is generated on the outermost layer, which is where the annual growth band occurs. As with dendrochronology, it is the thickness and thinness of the rings that climatologists use to infer past climate conditions. The coral responds to variations in temperature and cloud cover, and the thickness of the bands is determined by the ocean's temperature and salinity.

When the water is warm, coral growth accelerates, and the growth layer is wide and porous. Conversely, when the water is cooler, the layers are denser. In coral banding, the lighter bands are those laid down in summer during intervals of fast growth, and the darker layers are those formed during winter, when growth is slow. There is a temperature threshold, however, and when the water temperature gets too warm, it can actually harm the coral by drastically slowing its growth or killing it in a process called coral bleaching. This is a problem now affecting the world's natural reef systems, and many scientists believe it is caused by global warming. Scientists also use chemical ratios within coral to determine past climate because ocean temperature has a direct correlation to coral's constituents.

According to scientists at NOAA, coral reefs today are in jeopardy globally, especially those located on shallow shelves near heavily populated areas. Not only are warmer temperatures damaging coral habitats, but so are human activities such as pollution, scuba diving, and boating.

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