Coral reefs across the Pacific Ocean are being wiped out by this year’s massive El Nino -- a global phenomenon that may be intensified by the effects of climate change.

Scientists just back from tropical areas of the Pacific say they are seeing up to half the reefs in some locations killed by bleaching. This happens when the water gets too hot and the coral expels the algae with which it lives.

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“We weren’t expecting to see the levels of the destruction that we saw on the reef,” said Kim Cobb, professor of Earth and atmospheric sciences at Georgia Tech, who just returned from a two-week trip to Christmas Island, a large coral atoll about halfway between Hawaii and Tahiti.

“It was shocking,” Cobb said. “To dive on a reef where we had installed monitoring equipment six to eight months ago that was in perfect condition, and to see it with 50 percent death was astounding.”

Cobb has been visiting the same research site for the past 18 years, and says this is the worst she has even seen it. This year’s El Nino, an event that occurs every 7 to 10 years, is one of the biggest on record, according to government scientists. The last big one was during 1997-98.

During El Nino years, equatorial winds that usually blow westward across the Pacific inexplicably die, and as a result, a big slosh of warm water moves eastward toward the coastline of North and South America.

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El Nino has caused torrential rains and mudslides in California, for example, where officials in San Diego are asking to declare the region a federal disaster area.

Along the way, the warm water is also changing ecosystems, pushing tropical fish and other marine life northward, while forcing other animals to flee or die. Cobb has published several research papers in recent years linking the intensity El Nino events to climate change.

“The atmosphere is more sensitive to El Nino anomalies under climate change,” Cobb said. “This may already be afoot in the El Ninos of the recent past. It remains a challenging problem because each El Nino is so variable. The records we have are so short that (it's like) chasing a small signal in a very noisy system.”

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Hawaii’s corals, too, have been hurt by El Nino’s spa-like waters, according to Andrea Grottoli, head of the division of water, climate and environment at Ohio State University.

“This El Nino is causing bleaching throughout the Hawaiian Islands,” Grottoli said in an interview from her base on Oahu.

The 1997-98 El Nino killed 16 percent of corals worldwide, she said. “It’s too early to know which corals will recover this time.”

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Grottoli and her team are trying to understand why some species, and some individuals, are more resilient to the effects of bleaching from warm temperatures as compared to others. This will help in understanding the long-term effects of bleaching.

In addition to hot water -- temperatures around Christmas Island, for example, jumped from 81 degrees to 88 degrees -– corals also face threats from increasing acidification of tropical waters, pollution and overfishing.

Some researchers are trying to understand the future of the world’s coral reefs, massive structures that are home to a rich ecosystem of fish, plants and other marine life.

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“Some species will dwindle and others will survive,” said Mark Warner, professor of marine ecology at the University of Delaware, and an expert in coral physiology. The big solid corals that comprise Australia’s Great Barrier Reef or the Florida Keys, for example, may be replaced by sponges, algae and other heat-resistant plants.

“The reefs we see today, 100 to 150 years from now, will be very different,” Warner said. “It’s matter of time if we will have enough warming to push corals over the edge.”