Bleaching in the Great Barrier Reef, photographed during a survey in March 2016. Image: ARC Centre of Excellence for Coral Reef Studies.

The Great Barrier Reef is one of the most celebrated ecosystems on Earth—and it’s dying. Months of extreme heat have turned thousands of miles of pristine habitat into an endless watery graveyard. This year’s coral bleaching event comes as a warning. If we don’t bring carbon emissions down fast, the Great Barrier Reef will not survive the century.

That is the stark conclusion of a new study led by scientists at James Cook University and the National Oceanic and Atmospheric Administration. The research, published in Science today, reveals a protective mechanism by which reefs have weathered heatwaves for thousands of years. But that protection is now being eroded by climate change, leaving corals more susceptible to the ravages of bleaching. Within a few decades, we could witness widespread ecological collapse—not just in the Great Barrier Reef, but on a global scale.

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“This is new territory we’re marching into,” Scott Heron, an oceanographer at NOAA and co-author on the new study, told Gizmodo. “We have not seen temperatures and thermal stress of this magnitude historically.”

Corals are fascinating organisms. A squishy animal wrapped in a crunchy skeleton infested with microscopic plants, their unique symbiosis has existed for hundreds of millions of years, building vast reefs that support roughly a quarter of all marine species today. Unfortunately, corals are also extremely sensitive to environmental disturbance, which has made them become a poster-child for the impacts of industrial society.

One of the most dramatic indicators that corals will not fare well in the warmer future is bleaching. When the water gets a few degrees too toasty, corals expel their zooxanthellae, the symbiotic algae that provide the animal with a regular meal. If the water stays too hot for too long, the algae won’t return, and the coral will starve. Even if corals do recover, a bad bleaching event leaves them more susceptible to disease, predation, ocean acidification, and pollution.

Coral bleaching has been documented for decades, these events have become more frequent, widespread, and destructive. The first global bleaching, which occurred during the 1997-98 El Niño year, killed some 18 percent of all reefs worldwide. We’re currently in the midst of another mass die-off, and this one’s shaping up to be far worse. By the end of last year, 30 percent of the planet’s coral reefs had been exposed to thermal stress; today, we’re pushing 40 percent. And according to a recent NOAA analysis, the bleaching could continue through early 2017.

If these numbers feel a bit abstract, their meaning became painfully apparent last month, when an aerial survey of the Great Barrier Reef revealed a nightmarish scene of death and destruction. Terry Hughes, the oceanographer at James Cook University who led the survey, estimates that 95 percent of the northern Great Barrier Reef—the most pristine section of the 133,000 square-mile ecosystem—is now “severely bleached,” marking the worst such event on record.

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“It’s much worse than we knew in February,” Mark Eakin, a coral reef specialist at NOAA and co-author on the new study told Gizmodo. “The reports are not all in yet, but I’ve heard numbers in the range of 30 to 50 percent mortality.”

A severely bleached patch of coral. Image: Peter Mumby

The timing of the new Science paper is coincidental—but the sorry state of the Great Barrier Reef makes its conclusions all the more salient. Pulling together historic records, laboratory experiments, and climate models, the study offers a detailed look at how corals respond to heat stress, and what that will mean for their future.

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To start, the researchers examined 27 years of satellite-based sea surface temperature records for the Great Barrier Reef. From these records, the team identified 372 “thermal stress events”—periods of anomalously warm water that could trigger bleaching.

372 hot spells over thirty years sounds like a lot. But on closer inspection, many of these events turned out to be quite tolerable to reefs, because of the nature of their onset. Three quarters of all thermal stress events, the researchers found, were preceded by a period of slightly elevated temperatures—a so-called “protective trajectory.” Another 20 percent were marked by a single continuous rise in temperature (a “single bleaching trajectory”), while five percent featured back-to-back heat waves that exceeded the bleaching threshold (a “repetitive bleaching trajectory”).

To learn how the type of heat exposure impacts corals, the researchers ran laboratory experiments, subjecting the model species Acropora aspera to the three different trajectories (protective, single, and repetitive) they identified. The results were dramatic.

Corals exposed to the protective trajectory exhibited greater thermal tolerance, less bleaching, and less cell death when the heat was cranked up. No such tolerance was seen in corals that experienced single or repetitive bleaching. These physiological differences were paralleled by genetic changes, with corals under the single and repetitive bleaching scenarios showing gene expression profiles consistent with heat stress.

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“Symbiont density, cell death, and genetic switches are all showing the same benefit coming from that pre-conditioning pulse,” Heron said. “That was really remarkable to see.”

Conceptually, it’s similar to how a vaccine primes the immune system to cope with disease. Unfortunately, for much of the Great Barrier Reef, this protective mechanism may not last long. The researchers’ models show that a two degree Celsius rise in global temperatures will cause the proportion of protective bleaching events to fall from 75 percent to less than a quarter.

“It’s not just that we’re going to see more [bleaching] events,” lead study author Tracy Ainsworth told Gizmodo. “They’re going to be more frequent and more severe.”

Bleaching left devastation at the Airport Reef in American Samoa last summer. Image: XL Catlin Seaview Survey

And that is going to make it very hard for the reef to survive. If global carbon emissions continue to grow unchecked, the authors’ models predict coral cover in the Great Barrier Reef crashing by mid-century. “The sad part is, in looking toward the future, this [protective mechanism] is likely to evaporate,” Eakin said. “It’s really sad to find that something helping to protect these corals might go away.”

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On the other hand, under an aggressive emissions reductions scenario where global warming is limited to less than two degrees Celsius, many regions of the Great Barrier Reef retain their heat protective mechanism, and show no long-term decline.

“Our hope is that studies like ours will put the rubber on the road for policy makers,” Heron said, adding that the Paris climate accord was an important first step, but that we need to ratchet up the ambition even further.

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“The two degree warming limit is not enough to protect corals,” Eakin said. “We really have to be looking at holding temperatures to a 1.5 degree increase, or less.”

If we don’t, he added, the consequences are clear. “We are going to lose most of our coral reefs.”