The situation at Kiritimati came to light during an expedition last month headed by researchers Julia Baum from the University of Victoria and Kim Cobb of Georgia Tech. They expected bad news before they even arrived — after all, the island is in the part of the world most strongly affected by the past year’s unusually severe El Niño event. Abnormally warm water temperatures have plagued the region for months, and as recently as November, research expeditions had observed widespread coral bleaching, disease and even some coral death as a result.

But Baum, who has studied coral in the area for years, was not wholly prepared for the devastation she met with upon arrival. She and her team estimate that about 80 percent of all the coral around the island are now dead. Another 15 percent appear to be bleached, but still alive — and Baum estimates that only around 5 percent are actually healthy. Much of the dead coral has been covered over with a fuzzy, red algae, giving the reef a haunting appearance.

“It was a horror show,” Baum said. “Rationally I know what’s happened, but emotionally it’s very hard to accept it. It seems like it can’t possibly be real that this vibrant, healthy reef that I’ve been working on so long and studying so intensely — specifically because it was one of the healthiest reefs in the world — that it could just be dramatically transformed in a matter of months into this graveyard.”

Scientists believe that the mass die-off around Kiritimati — also known as Christmas Island — is one of the worst casualties in a larger coral bleaching event that’s taking place all over the world. And now, some experts are saying the events could signal an even more disturbing revelation — the idea that we’re reaching a point where many coral reef ecosystems may not be able to adapt to the relentless progression of climate change.

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“Dangerous” climate change

A primary concern of world leaders is keeping global warming from climbing above a threshold that would constitute “dangerous” climate change — the point at which damaging and likely irreversible events would become widespread on Earth. That goal has been a global focus since the 1992 United Nations Framework Convention on Climate Change (UNFCCC) was first established, with language calling for “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.”

But that’s not all the convention says. In the next sentence, it also says such stabilization should happen in a time frame “sufficient to allow ecosystems to adapt naturally to climate change.”

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The recent deterioration of the world’s coral reefs suggest that this objective may have already been breached, notes Thomas Lovejoy, a conservationist and a professor at George Mason University. “Abrupt changes in ecosystems like tropical coral reefs show we are already beyond ecosystems adapting naturally,” he said in an email to The Washington Post.

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Lovejoy isn’t the only scientist willing to make this point.

“From a global perspective, the level of warming is about to enter the danger zone for coral reefs and it is certainly arguable that it is already too late for some of these systems to ‘adapt naturally,'” said Michael Oppenheimer, a professor of geosciences and international affairs at Princeton University. Oppenheimer is one of many scientists who have in fact long pointed toward coral reefs as one of the first places that truly irreversible climate changes might manifest themselves.

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“We and others thought coral reefs would be the first global indicator of emergence of dangerous warming and events have borne out that expectation,” Oppenheimer said.

Bleaching is a common reaction to environmental stress. While corals are living animals themselves, they survive by maintaining a symbiotic relationship with certain types of algae, which actually live inside the corals and are responsible for their brilliant colors. If a coral becomes stressed, though — for instance, if the surrounding water becomes too warm — it will expel its algae, bleaching a ghostly white in the process.

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Bleaching doesn’t kill the corals right away — in fact, if the environment stabilizes in enough time, the corals can regrow their algae and return to business as usual. But bleaching does leave corals weakened and more susceptible to disease, and if the bleaching condition lasts long enough — or if too many bleaching events occur in a row — then the corals may die.

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This type of “single-species interaction,” according to Lovejoy — in this case, the reliance of coral on algae — is part of what makes ecosystems so vulnerable. “These responses are fairly hard to predict I would think, but suggest that ecosystems are far more sensitive to climate change than climate models can actually predict,” he noted. “In other words we are in for a lot of unpleasant surprises.”

The current bleaching event has affected reefs throughout the tropics — including much of the Pacific and parts of the Indian Ocean, the Atlantic and the Caribbean basin — and is largely thanks to the onset of a particularly severe El Niño event in 2015, which has resulted in unusually warm water temperatures in many regions.

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So far, the corals at Christmas Island appear to have suffered the most worldwide, but severe bleaching has also been observed in Florida, Hawaii, American Samoa and Australia, among other places. Just last month, in fact, a new survey concluded that Australia’s Great Barrier Reef was experiencing its worst bleaching event in history.

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There are likely several reasons this year’s bleaching is so severe. First, the recent El Niño has been unusually strong. But there’s also the fact that this is the third major bleaching event in the past couple of decades, with each one leaving the affected corals a little weaker.

The first of the last three events took place in 1998, corresponding with another unusually strong El Niño. The next event hit in 2010 in conjunction with another El Niño event, albeit a much more moderate one. And finally, last year, the third event struck.

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“These bleaching events are coming more quickly, they are more severe and there are a number of coral reefs around the world that just are not being given enough time to truly recover between events,” said Mark Eakin, a coral reef specialist with the National Oceanic and Atmospheric Administration. “You’re looking at ecosystems that are being changed substantially.”

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This is where climate change comes into play. It may appear that the last few El Niño events are to blame for all the bleaching events, but experts are increasingly certain that climate change is operating in combination with the effects of El Niño. And even after this year’s event abates, the oceans will continue warming, meaning it may not take such a strong El Niño event in the future to trigger these kinds of widespread bleachings.

Thus the major problem now is that these environmental changes are occurring too quickly for may reefs to catch up. Even when reefs recover — which can take years or even decades — their overall community structure is often different than it was before a bleaching event. Reefs that were once dominated by large, slow-growing species, for instance, may be repopulated after a bleaching event with different, faster-growing species.

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“The ecosystems are being changed — they are not adapting in the sense that they’re not able to respond to these high temperature conditions and still be the same robust ecosystems they were,” Eakin said. “What’s happening is the ecosystems are being broken down in many ways and you’re being left with a coral reef that is not as diverse, that is not able to function as well, that basically is not as healthy as it was previously.”

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When it comes to stabilizing emissions fast enough to “allow ecosystems to adapt naturally,” “generally I would say that we are at a point where we may already have breached that goal,” Eakin said.

A look ahead

It may look like a grim prognosis for many of the world’s corals, but experts aren’t predicting a death sentence for all the planet’s reefs. Depending on an ecosystem’s composition and the other environmental factors affecting it — fishing pressure and pollution, for instance — some will likely fare better than others. Furthermore, scientists believe that even some of the worst-affected places may actually hold clues that could help conservationists better protect reefs in the future.

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At Christmas Island, Baum noticed that while a majority of the corals had died off, a few species — which she dubs “miracle corals” — seemed to fare better than others. A coral known as Porites, for instance, seemed to be among the most resilient.

“How is it that those 5 percent of the corals in the exact same conditions as all the other corals around them that have died — how is it that these ones have managed to survive?” Baum said. “That’s the million-dollar question.”

Baum and her team have taken hundreds of samples from the corals around Christmas Island and brought them to the Hawaii Institute of Marine Biology for analysis. They hope that the samples may hold some clues to what makes some species so resilient. It’s possible that certain types of symbiotic algae are more heat-resistant than others, or that some of the hardier species of corals had more energy reserves built up before they bleached.

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How the Christmas Island reefs will fare in the future remains uncertain. The area may repopulate with coral over a period of years or even decades, and Baum’s “miracle corals” will likely play a major role in this recovery if it’s successful.

“The big question is how long is that going to take and will this reef look fundamentally different after this event than it did before,” said Kim Cobb, a professor at Georgia Tech who specializes in using coral samples to reconstruct climate patterns and who has also conducted work at Christmas Island for years. “And that’s a very important question that speaks to the longer-term evolution of coral reefs under continued temperature extremes.”

Meanwhile, because Christmas Island has suffered some of the worst impacts of this year’s El Niño event, it’s possible that its fate could also serve as a warning for what may await other reefs in less-affected parts of the world as the climate continues to change.

“This happens to be an example that, as a record-breaking El Niño on top of continued ocean warming related to climate change, I do think is an appropriate case study and a very important case study for understanding the long-term evolution of reefs under continued climate change,” Cobb said.

Chris Mooney contributed to this report.

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