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For the picturesque coral colonies of the Earth's tropical oceans, it's as if the very water has betrayed them.
The brilliant colors of the Great Barrier Reef and other swatches of coral depend on that water staying within a certain range of temperatures. But the past year has seen ocean temperatures rising sharply, beyond the point where the living coral effectively shuts down and turns white — a frequently-fatal process known as "bleaching."
The world is now in the middle of a historic die-off of coral, an ecological slaughter prolonged by the Pacific warming known as El Niño. And for scientists like Kim Cobb, who uses coral samples to calculate past Pacific Ocean temperatures, it's a depressing scene.
Cobb recently returned to Kiritimati, or Christmas Island, after about four months to find swaths of the surrounding reefs stripped of their vibrant hues. She said she ran into a "wall of emotion" on her second dive, realizing that 50- to 60-year-old coral was dead or dying.
"It was just shocking — so shocking it took me the better part of the first dive to take it in and comprehend what happened," she said. "It became really apparently to me that there was no portion of the reef that was spared."
"We're dealing with something off-scale, completely off-scale," said Cobb, an oceanographer at the Georgia Institute of Technology who has studied the reef for more than 18 years.
The Christmas Island reefs were among the world's healthiest, said Cobb's collaborator, biologist Julia Baum of Canada's University of Victoria. But up to 80 percent of the coral has been killed by the undersea heat wave, leaving behind dead, white skeletons that will collapse over the coming months.
"Seeing that on this trip was surreal," Baum said. "It's really hard to take in that that huge a change could have happened in just a matter of months."
The coral crash that started in 2014 is now the longest event of its kind on record, according to the US National Oceanic and Atmospheric Administration. It's the third worldwide bleaching event, a catastrophe that's becoming more frequent and more intense as the oceans get warmer, said Timothy Swain, an ecologist at Chicago's Field Museum and nearby Northwestern University.
"It's a very tangible example of what's happening due to climate change," said Swain, the lead researcher on a newly published index documenting the vulnerabilityof various coral species. "Corals are extremely sensitive to temperature changes, and they're sort of right on the cusp of what they're able to tolerate."
Coral is a symbiotic organism, making space in its own tissue for a species of algae that helps provide it with oxygen and food. That algae produces pigment that gives the coral its distinctive color. But when coral gets put under stress, it evicts the algae — leaving its white, calcium-rich skeleton exposed and weakening the living cells.
Swain said he hopes the index he and his colleagues just produced will be a tool to help people manage the risks posed to the reefs in an era of rising temperatures.
"Not only do you know that a major bleaching event is coming, you also know which corals are more likely to both bleach and succumb to the injuries that bleaching causes," he said. "We're also hoping it will be very useful to basic scientists as well, who are trying to understand the mechanisms that cause some corals to bleach and die while other corals survive."
The coral Cobb studies lives in water that's typically about 27 degrees Celsius (80 Fahrenheit). But the El Niño that emerged in 2015 drove up water temperatures by nearly 4 degrees C (7.2 F).
"That's way beyond a coral's tolerance level," she said. "It can bleach and survive over a period of two or three months, but the problem with this El Niño event is the bleaching conditions began sometime in July."
Kiritimati sits just above the equator, about 1,300 miles (2,100 km) south of Hawaii. It's been the "bulls-eye" of the El Niño event that peaked late this winter, and water temperatures in the area remain more than 1C over normal, Cobb said.
The reefs also sustain marine life that are at the root not only of natural ecosystems but national economies. Kiritimati's residents catch the brightly colored fish that live among the coral and sell them as pets for aquariums in the United States and elsewhere.
"Right now on the reefs, the fish populations still look very healthy," Baum said. But as the empty shells disintegrate over the next few months, those populations are likely to shrink — and that will cascade up the food chain to the people of the island, many of whom subsist on the sea.
"They're fishing day in, day out to feed themselves and feed their families," she said. "And if the fish populations diminish, as I expect they're going to, then there will be very serious negative consequences for the people there."
Further west, the northern end of Australia's Great Barrier Reef has seen severe bleaching, with authorities reporting "substantial" coral deaths. Heavy rain and cloudy skies over parts of the reef last month brought little relief.
"The reef continues to experience heat stress, with sea surface temperatures remaining significantly warmer than average for April," the Great Barrier Reef Maritime Park Authority announced this week.
The bleaching event also extends to the Caribbean Sea, where the reefs have been strained by a variety of natural and man-made causes for many years, Swain said. And as climate change pushes ocean temperatures upward, an event like El Niño becomes that much more likely to push the coral past their tipping points.
"Coral may be the first ecosystem that is lost to climate change," he said. "It's sort of the canary in the coal mine scenario."
Follow Matt Smith on Twitter: @mattsmithatl
Photo via XL Catlin Seaview Survey