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A massive and unprecedented coral bleaching event may finally be coming to an end

The process of coral bleaching is triggered by warm ocean water. Here, a turtle swims over bleached coral at Heron Island on the Great Barrier Reef. (XL Catlin Seaview Survey via Agence France-Presse)
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An ongoing global coral bleaching event, one that’s affected more than 70 percent of tropical reefs worldwide, may finally be coming to a close. A new forecast from the National Oceanic and Atmospheric Administration suggests that the high ocean temperatures that lead to bleaching are no longer widespread in the Indian Ocean, potentially signaling the end of what’s been a worldwide event for the past three years.

Still, “that’s not to say that all coral reefs are in the clear and none of them are experiencing bleaching anymore,” said Jennifer Koss, director of NOAA’s coral reef conservation program. There’s still ongoing heat stress in the Atlantic and Pacific oceans, meaning corals in these parts of the world — including the long-suffering Great Barrier Reef, as well as reefs around the United States and the Caribbean — may not be out of the woods yet.

NOAA declared the global bleaching event — the third one on record — in 2015, when an unusually strong El Niño phase, coupled with the ongoing progression of climate change, caused elevated ocean temperatures throughout much of the world. Even then, bleaching already had been occurring in certain parts of the world for at least a year prior, according to Mark Eakin, coordinator of NOAA’s Coral Reef Watch program.

‘An enormous loss’: 900 miles of the Great Barrier Reef have bleached severely since 2016

The process of coral bleaching is triggered by warm ocean water. Corals have a symbiotic relationship with algae, called zooxanthellae, that engage in photosynthesis and provide the corals with nutrients. But the warm water causes the algae to be ejected from the corals’ bodies, and the corals turn white. If the stress continues, the corals can die.

Before 2015, the last global bleaching event occurred in 2010, and the first in 1998, coinciding with another particularly severe El Niño. And experts say the current event may be the worst yet. According to Koss, “it lasted far longer than the two previous ones, and the level of damage is probably a lot greater.” 

Scientists are still collecting data on the total amount of damage the global bleaching event has caused, Eakin said. The 1998 bleaching event, previously considered the worst on record, killed about 16 percent of corals on reef worldwide — and while experts can’t say for sure yet, Eakin believes the latest event may rival it.

“I will say that it is among the most damaging global bleaching events that we’ve had,” he said. “If you were to ask me to bet on it, I would bet it is the most damaging.”

What scientists can say for sure is that just about all tropical reefs throughout the world have experienced higher-than-usual temperatures in the past several years, and 70 percent of them have seen conditions that lead to bleaching. They also know that this event has lasted far longer than previous global bleaching events, which have only lasted about a year each, according to Eakin.

The most recent event’s long duration has posed a particular problem because it’s caused some reefs to bleach multiple times in rapid succession over the past few years. This is “especially bad,” he said, because corals are more sensitive to stress when they’re still recovering from a previous bleaching event.

U.S. reefs have endured some of the most prolonged stress, with some experiencing bleaching conditions since before the global event was even declared in 2015. Florida reefs have seen two years of severe bleaching, according to the agency, while the Commonwealth of the Mariana Islands in the Pacific Ocean has seen three and Guam is undergoing its fourth.

Meanwhile, Australia’s Great Barrier Reef has experienced particularly devastating losses over the past two years as well. In 2016, the northern sector of the reef saw about two-thirds of corals die off, and scientists say similar losses may occur in the middle section of the reef this summer. Altogether, about 900 miles of the 1,400-mile reef have experienced bleaching at some point in the past two years.

Similar devastation has been occurring elsewhere around the world. Reefs throughout the Caribbean, including those surrounding Cuba, the Bahamas, Haiti and the Dominican Republic, began to bleach in 2015, and bleaching-level heat stress has continued to recur in certain parts of the region since. Japan’s largest coral reef has suffered severe casualties, with 70 percent of its corals reported dead in January and 90 percent having bleached. Other reefs throughout the Pacific and Indian oceans have also experienced significant levels of both bleaching and death.

Scientists discover another coral reef that has been devastated by global warming

So it’s welcome news that, at least for some parts of the world, the heat stress is beginning to level off. However, scientists caution that there’s still cause for concern.

For one thing, the end of a “global” bleaching event doesn’t mean that bleaching conditions disappear in every part of the world. The NOAA forecast still predicts high temperatures throughout parts of the Atlantic and Pacific basins for the next few months.

“We’re concerned for certain areas — places like Micronesia and Guam and the Mariana Islands,” Eakin said. But he noted that the situation is “not nearly as bad as it was the last two years.”

However, scientists also say that the continuing progression of climate change will likely become even more of a threat as time goes on, potentially causing global bleaching events to become even more common. That could be bad news for reefs around the world, scientists have pointed out, because coral requires long periods of time in between bleaching events to fully recover.

That said, some scientists have argued that there’s no need to panic yet. A paper published earlier this month by more than a dozen experts from around the world argued that coral reefs may change in dramatic ways in the future — they may become dominated by different species than we see today, or they may migrate to more hospitable parts of the world — but it’s unlikely they’ll disappear entirely. The key to helping the reefs persist, they add, is a commitment to active management programs, which focus not only on the broad threat posed by climate change, but also on local factors like pollution and overfishing.

Koss agreed that “the next few decades don’t look great for corals,” but there’s reason to believe that with proactive management and conservation efforts, they may persist.

When I talk to folks, whether it’s family or school groups or peers, I try to allay the real panic messaging that’s coming out from some people that all corals are in peril and they’re all going to die,” she said. “But that being said, I think there’s still a lot of need for continued conservation and restoration activity to the extent that we can do those things.”