(CNN) For the third time since records have been kept, the world is experiencing a global coral bleaching event.

It's a major environmental incident, predicted by the U.S. National Oceanic and Atmospheric Administration and now confirmed as underway by teams of scientists and reef mappers across the globe.

Coral bleaching is triggered by stresses on coral reefs. During bleaching, the coral expel the algae that live within them, exposing the coral's white skeleton. The symbiotic algae not only provide coral with its color, they also provide crucial nutrients. Without them, the coral eventually will starve.

When coral bleaching spanning 100 kilometers (63 miles) or more is found in the Pacific, Atlantic and Indian ocean basins, it's designated a global coral bleaching event. That's what NOAA, the XL Catlin Seaview Survey, Reef Check and the University of Queensland say is happening now.

NOAA says that in all three global events seen so far, the coral stress has been due mainly to rising ocean temperatures. All three events have occurred in El Niño years.

This bleaching event could kill over 12,000 square kilometers (about 4,600 square miles) of reefs by the end of the year, and NOAA warns that with forecasts of El Niño remaining strong until early 2016, the worst may be yet to come.

"Coral reefs are the litmus test of our oceans, a visual representation of the health of our seas," said CNN anchor and meteorologist Derek Van Dam. "When coral becomes bleached or white in color, this sensitive ecosystem is negatively impacted, which creates a profound ripple effect on the world's food chain."

The changing face of the world's coral reefs has been out of sight for most people, but now it's being systematically mapped by the XL Catlin Seaview survey.

Richard Vevers, the survey's executive director, said the project is trying to create a global baseline of coral reefs than can be used to monitor change over time.

It uses a custom-made camera that maps the ocean in 360 degrees, while also acting an underwater scooter for its operator.

The team has visited reefs around 26 different countries so far, in some cases mapping coral before and after a bleaching event has taken place.

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Vevers said that in some reefs, bleached coral bounces back as water temperatures cool. Other reefs aren't so lucky. When the crew revisited one bleached reef off American Samoa, it found that about 95% of the coral had died.

The survey wants to raise awareness of what's happening under the oceans. It's uploading some of its data to Google Street View, letting anyone with Internet access take a virtual dive.

Vevers said coral bleaching hasn't yet grabbed the world's attention.

"This is the equivalent of rainforest turning white and no one noticing."

He said the survey is collecting the type of images that make people sit up and take notice, and he's hoping that will help make positive change in December at the U.N. Climate Change Conference (COP21) in Paris.

"The XL Catlin Seaview Survey team will be at COP21 with this imagery," Vevers said. "We're also using virtual reality technology with Google Cardboard so we can take the policy makers and the media to go see the coral bleaching firsthand."