Coral bleaching has officially killed large swaths of the Great Barrier Reef

It's official: Large chunks of the Great Barrier Reef are dying. The big question now is whether they'll get a chance to recover.

For months, the northern half of Australia's famed coral reef has been ravaged by an unprecedented mass bleaching event. Record high temperatures in the Pacific Ocean, fueled by global warming and a powerful El Niño, have turned this once vibrant ecosystem into a ghastly pale white tableau.

Now scientists are assessing the damage — and it's grim. On Sunday, Australian researchers reported that 35 percent of the corals in the northern and central part of the reef are either dead or dying. These are among the most pristine parts of the 1,000-mile-long reef, one of the great natural wonders of the world:

Map of mortality estimates on coral reefs along 1,100 km of the Great Barrier Reef.

"We found on average, that 35 percent of the corals are now dead or dying on 84 reefs that we surveyed along the northern and central sections of the Great Barrier Reef, between Townsville and Papua New Guinea," said Terry Hughes, director of the Australian Research Council (ARC) Centre of Excellence for Coral Reef Studies, in a statement.

The picture was less bleak south of Cairns, Hughes added, "where the average mortality is estimated at only 5 percent." The researchers will revisit the reefs in the coming months to record the final damage as El Niño winds down.

How mass bleaching can ravage coral reefs, in three images

Coral reefs are often dubbed the rain forests of the ocean. Anchored by millions of coral polyps — tiny, soft-bodied animals that create elaborate calcium carbonate skeletons that shelter fish — these reefs cover just 0.1 percent of the sea floor but are home to 25 percent of marine fish species. They're popular spots for divers and tourists. They protect coasts from storms. They sustain food for half a billion people. And they're just plain lovely. Here's what they usually look like:

Coral reefs are, however, extremely vulnerable to soaring temperatures. In normal times, the living coral polyps form a symbiotic relationship with zooxanthellae, a colorful type of algae that synthesizes sunlight and CO2 to create nutrients for the reef. This algae gives the coral its purple/gold color.

But this symbiosis only thrives within a fairly narrow temperature band. If the water in the reef gets too warm, the zooxanthellae's metabolism goes into overdrive and starts producing toxins. The polyps recoil and expel the algae from their tissue, leaving the coral with a ghastly "bleached" appearance. At that point, the coral loses a key source of food and becomes more susceptible to deadly diseases.

Here's a shot of bleached staghorn coral at Lizard Island on the Great Barrier Reef, taken February 2016:

Bleaching doesn't kill the coral right away; if temperatures drop again, the zooxanthelle will come back. But if temperatures stay high for a long period and the bleaching gets really severe, as is the case in the Great Barrier Reef, then a lot of coral will start to die of malnutrition or disease.

Here's another picture of Lizard Island taken two months later, in April 2016 — the staghorn coral is completely dead and smothered in algae:

Once the coral dies off, it can adversely affect the fish that rely on the reefs. The entire ecosystem suffers.

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

Now, the good news is that coral reefs can recover from these mass die-offs. If temperatures fall back down — as they're expected to in Australia later this year, as El Niño subsides — new polyps will return and start building skeletal structures to replace the dead coral.

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.

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. In April, a paper in Science warned that the Great Barrier Reef may lose its ability to bounce back as global warming continues.

"This year is the third time in 18 years that the Great Barrier Reef has experienced mass bleaching due to global warming, and the current event is much more extreme than we’ve measured before," Hughes said.

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.

How to give the reefs time and space to recover

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 by rerouting boats around them and restricting construction in the coastal areas near them. 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, reducing our CO2 emissions is the crucial step. Mark Eakin, who runs who runs NOAA's Coral Reef Watch program, told me 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."

How bleaching got so bad in the northern Great Barrier Reef

Some background on the current bleaching event: The Great Barrier Reef stretches for 1,000 miles along Australia's northeastern coast. 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.

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:

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. In March, Hughes of the ARC 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.

Jodie Rummer, another scientist at the ARC Centre, said the view in March 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.

"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," Rummer said in a statement.

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 poking up its head 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," Eakin told me in late March. "There's severe bleaching in Fiji and New Caledonia and the northern third of the Great Barrier Reef. Severe bleaching in the island of Reunion. Bleaching in Seychelles."

One of the worst bleaching events this year has occurred in Kiritimati, the largest coral atoll in the world, some 1,300 miles south of Hawaii. In April, scientists announced that 80 percent of coral colonies were dead and another 15 percent could soon die. "Overnight," wrote Eric Holthaus in Climate Progress, "an entire ecosystem has essentially blinked out of existence."

Although El Niño is starting to fade, it has already caused considerable damage, and it's not over yet. Here's NOAA's forecast of places with a 60 percent chance of coral bleaching between June and September 2016:

The last time we saw such widespread bleaching was during the record-setting El Niño of 1997-'98, when the world functionally lost 15 to 20 percent of its coral reefs. Eakin points out that the 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."

Further reading:

• Back in April, Elizabeth Kolbert in the New Yorker about scientists who are trying to breed hardier coral that can survive bleaching and acidification.
• A look at how can help the Great Barrier Reef stay resilient.
• Scientists that the bleaching in the Great Barrier Reef would be impossible without man-made global warming.