The Great Barrier Reef has suffered the worst coral die-off on record

Earlier this year, the Great Barrier Reef was devastated by the largest mass bleaching event ever seen — as record-warm ocean temperatures turned large swaths of this vibrant 1,400-mile habitat into a ghastly white boneyard.

Now scientists have finally tallied up the damage. Data released Monday by Australian researchers shows that an unprecedented fraction of the shallow-water coral in the pristine northern part of the reef has died, with average mortality rates of 67 percent.

The brighter news is that the southern sections fared much better, with just 6 percent of coral dead in the central section and 1 percent dead in the south. “The[se] corals have now regained their vibrant color, and these reefs are in good condition,” said Professor Andrew Baird of the Australian Research Council Centre of Excellence for Coral Reef Studies in a release. Here’s a map showing the damage:

(ARC Centre of Excellence for Coral Reef Studies)

The scientists note that it could take 10 to 15 years for the worst-hit sections of the reef recover — but the real fear is that, thanks to global warming, another mass bleaching event will come along very soon and make the situation even worse.

How mass bleaching ravaged the Great Barrier Reef this year

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 a healthy reef looks like:

(Shutterstock)

But coral reefs are also 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 carbon dioxide to create nutrients for the reef. This algae gives the coral its purple and gold colors.

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.

That’s what happened in January through March of this year. Record high temperatures in the Pacific Ocean, fueled by global warming and a powerful El Niño, caused mass bleaching throughout the Great Barrier Reef. Here's a shot of bleached staghorn coral at Lizard Island, taken February 2016:

(XL Caitlin Survey)

Bleaching doesn't kill the coral right away; if ocean temperatures drop again, the zooxanthellae will come back. But if temperatures stay high for a long period and the bleaching gets really severe, as was the case in the Great Barrier Reef, then a lot of coral polyps 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:

(ARC Centre of Excellence for Coral Reef Studies)

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. Now that El Niño is gone, ocean temperatures have fallen around Australia. New polyps are returning and starting to build new 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," Terry Hughes of the ARC Centre said back in May.

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.

(Shutterstock)

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 carbon-dioxide emissions is the crucial step. Mark Eakin, who runs who runs NOAA's Coral Reef Watch program, told me back in March 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 degrees Celsius," Eakin said bluntly, "we are likely to lose numerous species of coral and well over half of the world's coral reefs."

Further reading:

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

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