Author: Nicholas Bakalar
Source: New York Times 
July 5, 2012
"Increasing sea temperatures caused by global warming are harming coral reefs, but a new study suggests this is not the first time it has happened.
Researchers took core samples from reefs off the coast of Panama, estimating their age at different levels using radiocarbon dating and other methods. They found that the reefs stopped growing about 4,000 years ago and did not recover for 2,500 years.
According to the researchers, whose work was published online on Thursday in the journal Science, the beginning of that 2,500-year period coincided with the start of a series of extremely strong El Niño effects, elevations of water temperature every three to seven years that cause long-term changes in weather. Then, about 3,800 to 3,200 years ago, there was a cycle of La Niña events — periods of unusually cold ocean temperatures — which could also have affected coral reef health."
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