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.
