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2016 Ocean Heatwave Killed 30 Percent of the Great Barrier Reef

Combined with a 2017 temperature spike, half of the 2 billion corals on the reef have died since 2015

Three color morphs of bleached corals on the Great Barrier Reef (ARC Centre of Excellence for Coral Reef Studies/ GergelyTorda )
smithsonian.com

The Great Barrier Reef is still one of the natural wonders of the world, but the beautiful mass of colorful coral is losing its greatness. As Jacqueline Williams reports for The New York Times, a 2016 underwater heatwave severely impacted the reef, causing large-scale coral die-offs.

A new study published in the journal Nature shows that over a nine-month period, elevated ocean temperatures killed 30 percent of the corals on the reef, and have likely caused a long-term change in the mix of coral species living throughout the World Heritage Site.

In March of 2016, water temperatures along the eastern coast of Australia spiked much higher than normal with the bump lasting until November. At the time, researchers conducted surveys of the reefs, noting the areas that had experienced bleaching. Nine months later, they assessed the reef again via satellite and in the water to see how many of the corals had survived and regained their color after the heatwave. According to a press release, what they found is that 29 percent of the 3,863 reefs that make up the GBR lost two-thirds of their coral, mostly in the north of the reef. Averaged over the entire 1,400 mile-long GBR, about 30 percent of the corals died off in the 2016 event.

Bleaching and dying are not synonymous. Coral polyps depend on a symbiotic relationship with a type of algae called zooxanthellae, which give coral their vibrant colors and provide food. When coral experiences stress, like warmer water temperatures or pollution, it expels the zooanthellae, leaving the reefs bleached bone white. If conditions return to normal, the algae can recolonize the coral and the whole system can recover. If the stress is too extreme, however, the algae stays away and the coral slowly starves.

Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies and lead author of the study tells Ben Smee at The Guardian that the 2016 heatwave did not follow the normal bleaching patterns. “The conventional thinking is that after bleaching corals died slowly of ... starvation. That’s not what we found. We were surprised that about half of the mortality we measured occurred very quickly,” he says. In fact, many of the corals died within 2 to 3 weeks, essentially cooking to death. “These widespread losses were not due to the attrition of corals that slowly starved because they failed to regain their symbionts. Rather, temperature-sensitive species of corals began to die almost immediately in locations that were exposed to heat stress,” he says.

While it’s unlikely that the GBR will completely die off, the study suggests that it is quickly transforming into a different type of coral ecosystem. “The coral die-off has caused radical changes in the mix of coral species on hundreds of individual reefs, where mature and diverse reef communities are being transformed into more degraded systems, with just a few tough species remaining,” co-author Andrew Baird of James Cook University says in the press release.

While there are some natural fluctuation of ocean temperatures, the bleaching of the GBR is almost certainly linked to climate change. In fact, it was part of a global bleaching event that struck between 2014 and 2017, which impacted every major reef on Earth.

While localized bleaching is a naturally occurring event, mass bleachings that cover hundreds or thousands of miles are a recent phenomenon. Earlier this year, Hughes and his colleagues published a study showing that these large-scale bleaching events are becoming more frequent. The first recorded mass bleaching event happened in 1982. Before that, Hughes says that several centuries of coral growth bands, which are similar to tree rings, do not suggest that coral reefs anywhere experienced mass bleachings.

Hughes directly attributes the bleaching to human-caused climate change. “Our study shows that the transition of the GBR to a new system is already underway, due to global warming,” he tells Andrew Freedman at Mashable. “It’s here and now, and it’s happening faster than we expected.”

The die-off in 2016, however, was just the beginning. Another major heatwave in 2017 also struck other parts of the reef. As Robinson Meyer at The Atlantic reports, since 2015, half of the two-billion corals that make up the reef have died. If global climate change isn’t halted, the prognosis for the survival of coral reefs in the long term is grim.

But Hughes says there is still some hope for the GBR if immediate action is taken. “[T]hat still leaves a billion or so corals alive, and on average, they are tougher than the ones that died,” he says in the release. “We need to focus urgently on protecting the glass that's still half full, by helping these survivors to recover.”

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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