NOVA Online | Tracking El Niño

Coral Reefs

Yes. Like trees, coral reefs grow at different rates during different times of the year, and deposit calcium carbonate in layers of different density that appear as annual rings in a core sample. How much a reef grows depends on many factors, including the water temperature and whether it is sunny or cloudy overhead. By looking at the ratio of oxygen isotopes in the coral, scientists can determine the water temperature when the coral was growing. In ideal temperatures, the coral will grow a lot. But if water temperatures are too warm (which is what happens in the eastern Pacific when an El Niño event occurs), or too cold, the coral will grow less and exhibit this with a thinner ring, as seen in the X-ray. (The thin band in this coral resulted from the water being too cold.) This coral sample is only a small part of a much larger sample taken in 1995.

Coral bleaching hotspots as of February 10, 1998. For the most current listing and more information on the methodology, see the full-size Coral Bleaching Hotspots chart.

During the 1982-83 El Niño, the water temperatures around Costa Rica, Panama, Colombia, and the Galapagos Islands became too warm, causing much coral to expel the algae that live in their insides and help to keep them alive. This algae, called zooxanthellae, is what gives the coral its color, so without it the coral turned white, a process known as bleaching. Some 70 to 95 percent of corals in those areas died because of this phenomena.

More about coral reefs:

How are coral reefs made?
Why are reefs only found in shallow waters?
What else can coral tell you?