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The unprecedented coral bleaching disaster at the Great Barrier Reef, explained

The scientists who study the Great Barrier Reef are shocked and horrified by what they're seeing right now.

Record hot temperatures in the Pacific Ocean — driven by global warming and a powerful El Niño — have fueled the worst coral bleaching event ever seen along the northern third of Australia's famed reef. Researchers who have recently ventured into this region say the once-vibrant ecosystem is now a ghastly tableau, filled with pale-white corals that are at risk of dying off:

(XL Caitlin Survey)

Coral reefs are often thought of as the rain forests of the ocean — they cover just 0.1 percent of the world's sea floor, but they're home to 25 percent of marine fish species. They're popular spots for divers and tourists, but they also sustain food for half a billion people and protect shorelines from storms. And they're just plain lovely.

But coral reefs are also extremely vulnerable to soaring temperatures. In normal times, the living coral form a symbiotic relationship with zooxanthellae, a colorful type of algae that provides the reef with oxygen and nutrients. But this symbiosis only thrives within a fairly narrow temperature range. If the water in the reef gets too warm (or too polluted), the coral will expel the algae from their tissue, leaving the coral with a ghastly "bleached" appearance. At that point, the coral lose a key source of food and become more susceptible to deadly diseases. Often many coral will then die off, which in turn can adversely affect the fish that rely on the reefs.

And right now, scientists say, Australia is suffering one of the worst mass bleaching events in recorded history.

How bleaching got so bad in the northern Great Barrier Reef

The Great Barrier Reef stretches for 1,000 miles along Australia's northeastern coast, one of the world's great natural wonders. It consists of 3,000 individual reefs and is home to 1,500 species of fish.

The southern part has long sustained heavy damage from tourism, pollution, and invasive species. But the northern third has always been more pristine, located far from (most) human activity. It's usually a bright feeding ground for dugongs, sea turtles, and other marine life.

Now THIS is more like it. (Shutterstock) (Shutterstock)

Now THIS is more like it. (Shutterstock)

But ever since December 2015, as the Australian summer got underway, ocean temperatures around the reef have surged to record highs, aided by global warming and a powerful El Niño currently raging in the Pacific:

Sea surface temperatures across northern and Coral Sea areas of Australia, from December 2015 to February 2016 (Bureau of Meteorology)

Sea surface temperatures across northern and Coral Sea areas of Australia, from December 2015 to February 2016 (Bureau of Meteorology)

That's helped trigger severe coral bleaching in the northern third of the Great Barrier Reef — an area that had also been hit by several cyclones as well as a series of unusually hot days during low tide.

The damage has been horrifying. This week, Terry Hughes, head of the Australian Research Council Centre of Excellence for Coral Reef Studies, returned from an aerial survey of the reef's northern section and announced that the bleaching was unlike anything he'd seen before.

Of 520 reefs surveyed north of Cairns, a staggering 95 percent were "severely" bleached. Only four of the 520 were healthy. "This will change the Great Barrier Reef forever," Hughes told Australia's ABC.

Aerial survey of coral bleaching in the Great Barrier Reef. (Terry Hughes/ARC Centre of Excellence for Coral Reef Studies)

Aerial survey of coral bleaching in the Great Barrier Reef. (Terry Hughes/ARC Centre of Excellence for Coral Reef Studies)

Jodie Rummer, another scientist at the ARC Centre, said the view was equally grim from underwater, as the hot water had ravaged corals, anemones, and even giant clams. While the local fish populations were still abundant for now, she worried that the loss of coral and rising temperatures could soon take a toll there, too.

Rummer's statement was dire: "I witnessed a sight underwater that no marine biologist, and no person with a love and appreciation for the natural world for that matter, wants to see."

Here's a short underwater video from WWF Australia of the eerie scene around Lizard Island:

The Great Barrier Reef Marine Park Authority has raised its bleaching warning to the highest alert and will begin to put in place limits on water pollution — to avoid further weakening the reefs — while monitoring the corals closely going forward. Simply put, it's a disaster.

Bleached coral reefs can recover — but only if they're given a chance

In theory, coral reefs can recover from a severe bleaching event, says Mark Eakin, who runs NOAA's Coral Reef Watch program. But the big question is whether these reefs actually will get a chance to heal.

After a bleaching event occurs, a certain fraction of coral are likely to die off from disease. (It's still unknown how many of the northern Great Barrier Reef's coral will perish, although an early survey off Cape York found 50 percent mortality.) Eventually, however, when temperatures return to normal, the coral start growing back.

The hitch is that recovery takes time. Lots of time. In places like the Seychelles — where reefs are mostly sheltered from pollution, tourism, and heavy fishing — it has taken at least 15 years for damaged reefs to come back. In areas stressed by human activity, the process can take much longer.

What's more, recovery is often uneven. The fast-growing "branching" corals bounce back first. But there are also older, massive corals that are centuries old and provide valuable shelter for bigger fish. When those die off, they don't return overnight.

The first step is admitting we have a problem.

And here's the catch: the current pace of global warming may not give these damaged reefs sufficient time to bounce back fully. Before the 1980s, mass bleaching events were virtually unheard of. Now they're becoming more and more frequent, particularly every time there's an El Niño, as ocean temperatures spike.

Another complication: as we pump more carbon dioxide into the atmosphere, the oceans are becoming more acidic. In some cases, acidification can make corals more sensitive to bleaching at lower temperatures. It can also make it harder for the corals to build their protective skeletons and recover from events like this.

Now, there are some things that Australia (and other countries) can do to help make reefs more resilient to bleaching. Humans can limit fertilizer and sewage runoff that further damage the coral. We can avoid overfishing key herbivores like the rabbitfish that nurture the reefs by clearing away excessive algae. We can also avoid wreaking havoc on reefs with boats and construction. (Australia is on the wrong track here: in 2015, the government approved plans to expand coal exports via ship in the southern part of the Great Barrier Reef.)

But ultimately, Eakin points out, reducing our CO2 emissions is the crucial step. He argues that we'd likely need to keep total global warming to below 1.5 degrees Celsius for coral reefs to continue thriving. Right now, we're on course to blow past 2 degrees Celsius, which could doom recovery efforts.

"At 2°C," Eakin says bluntly, "we are likely to lose numerous species of coral and well over half of the world's coral reefs."

It's not just Australia: Bleaching is threatening corals worldwide

The Great Barrier Reef is getting all the attention right now because it's big and famous and hugely important. But large swaths of the Pacific have been experiencing severe coral bleaching ever since El Niño began kinda-sorta poking its head up in June 2014. Different spots get hit as summer descends on different parts of the globe.

"We've seen bleaching as far west as Tanzania, and as far east as French Polynesia," says Eakin. "There's severe bleaching in Fiji and New Caldenoia and the northern third of the Great Barrier Reef. Severe bleaching in the island of Reunion. Bleaching in Seychelles. We might be about to see more bleaching in Galapagos and the Pacific side of Panama, though we're not sure about that just yet."

Here's NOAA's forecast of places with a 60 percent chance of coral bleaching between April and July of 2016:

Areas where NOAA currently predicts a 60 percent chance of coral bleaching thermal stress in April through July of 2016.
(NOAA Coral Reef Watch)

The last time we saw such widespread bleaching was during the record-setting El Niño of 1997 to '98, when the world functionally lost 15 to 20 percent of its coral reefs. Eakin points out that this current event is still ongoing, so it's difficult to say what the precise damage will be. There are nuances and quirks with every El Niño. But this one is also occurring in the context of warmer ocean temperatures overall, due to climate change.

"Hawaii, for instance, does not normally have El Niño bleaching," Eakin notes. But last year, a huge coral colony in the Olowalu reef in Maui was hit. "This is only the third time they've had bleaching — the other two were 2014 and 1996. So we're seeing signs where the signal of climate change is very strong in this global event."

And further shocks may still be in store, he adds. "We may next see bleaching in the Indian Ocean, in the coral triangle [in the Western Pacific Ocean]. It may spread into Southeast Asia. And then come this summer or fall, we may see further bleaching in the Caribbean" — a place where coral cover has declined by nearly half since the 1970s, mainly due to invasive pathogens, overfishing, coastal pollution, and tourism.

"So keep your eyes open," Eakin says. "This current story is nowhere near over."

Correction: This piece originally said coral reefs cover 0.01 percent of the ocean floor. It is 0.1 percent. Always watch those decimals...

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