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Coral reefs around the world are at risk as the ocean's temperature continues to rise. Those trends could kill not only coral, but the fish and other species that depend on the reefs. Those reefs are important for people, as well. But a study in Science magazine shows that under the right circumstances coral reefs can also bounce back from disaster.

NPR's Richard Harris reports.

RICHARD HARRIS, BYLINE: The beautiful coral reefs off the Pacific coast of Panama are seemingly a permanent fixture of that coastline. But it turns out they've had a troubled history. Richard Aronson at the Florida Institute of Technology discovered that by taking a core sample of the reef, just as a tree expert takes a sample of tree rings.

RICHARD ARONSON: By cored what I mean is that we jammed 17-foot-long irrigation pipes down into the reef and pulled out a history, a section of the reef, that told us what the ups and downs of the reef had been.

HARRIS: It turns out that this 6,000-year-old reef hadn't simply been growing steadily throughout its history. In fact, the reef had actually died off for quite some time.

ARONSON: These reefs were shut down for 2,500 years and the reefs have only been living for 6,000 years, so that represents about 40 percent of their entire history, so that's really shocking.

HARRIS: Aronson and his colleagues suspect that a natural climate shift was responsible for this terrible episode. During the 2,500 years in question, other evidence shows the ocean water cycled frequently between way-too-hot El Nino conditions to the other extreme, La Nina. And the reef didn't grow well in either extreme.

ARONSON: So it wasn't just hot temperatures from El Nino. It was also more and more extreme La Nina events, which bring cold water.

HARRIS: But then the climate shifted again to be much more like it is today. El Ninos and La Ninas still appeared but not with such devastating extremes. And lo and behold, when the climate changed, the reefs sprang back to life.

ARONSON: It seemed to fairly instantaneous. About 2,000 years ago or so, some corals that are not the main reef-building corals started up, and then maybe 500 years later, around 1,500 years ago, the main coral started growing again very rapidly.

HARRIS: All the fish and other reef creatures, which had apparently sought refuge elsewhere in the Pacific, returned. And the reef became much more like the lively place it is today.

Of course, Aronson is interested in the history of these reefs because he fears for the future. Climate change is likely to bring more devastating heat to these reefs, and threaten them once again. On the other hand, his study finds that these reefs are remarkably resilient.

ARONSON: What that tells me is that these reefs do have hope. And if we are able to get a handle on climate change, then we might be able to save coral reefs.

HARRIS: Of course, that hope depends upon being able to arrest climate change, which is a tough task. John Ogden, a leading voice for reef conservation at the University of South Florida, says he doesn't take too much comfort from this new study.

JOHN OGDEN: What we're facing in terms of climate change is unprecedented around the globe, and so we really don't know whether coral reefs are going to be able to recover from future disturbances of the like that they have discovered in the geologic past.

HARRIS: And even if reefs can recover over a period of a few hundred of years, that's not great news for the next few generations. They may well witness sickly, bleached-white corals off the shores of Panama and elsewhere, instead of the ecosystems filled with colorful fish that we see today.

Richard Harris, NPR News.

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