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To keep the Great Barrier Reef alive, the oceans must be cooler

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Warming waters are the key driver of mass bleaching

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Bleached and fluorescing corals on the northern Great Barrier Reef in April 2017.
Photo by Ed Roberts / Tethys Images

To save coral reefs around the world, global temperatures need to level off or decline, according to a new study of the Great Barrier Reef (GBR) over the past 20 years. Warming waters are the key driver of mass bleaching, the study found. As the world continues to heat up, these bleaching events will become more frequent and more severe, putting the future of coral reefs at risk.

Bleaching occurs when the colorful algae that live inside the corals are expelled. That can happen for a lot of reasons — because the water is too warm or too cold, or because of extreme low tides. But bleaching is really bad for coral reefs, because the algae are essentially its farms, providing about 90 percent of the coral’s energy. Without it, the coral goes white as it starves. Since the 1980s, the world’s coral reefs have experienced three significant mass bleachings. In fact, one is currently ongoing — it started in 2014.

Dead staghorn coral killed by bleaching on the northern Great Barrier Reef, November 2016.
Photo by Greg Torda / ARC Centre of Excellence for Coral Reef Studies

Previous studies have shown that global warming is causing corals to bleach and sometimes die. Last year, record temperatures driven in part by a particularly strong El Niño caused 90 percent of the 1,429-mile reef to suffer from some level of bleaching. Today’s study, published in Nature, is one of the most comprehensive and exhaustive views of the current state of the world's largest barrier reef.

The Great Barrier Reef, which stretches over an area of about 133,000 square miles off the coast of Queensland, Australia, is an important natural habitat as well as a major driver of Australian tourism. The GBR is home to more than 400 kinds of coral, over 1,500 species of tropical fish, more than 200 types of birds, and around 20 types of reptiles, including sea turtles. It was declared a World Heritage Site by UNESCO in 1981.

The researchers conducted air and water surveys of the GBR; they then compared the latest and most severe bleaching event in 2016 with the two other major bleaching events that affected the GBR in 1998 and 2002. They found that the bleaching events are getting worse: only about 9 percent of surveyed reefs escaped with no bleaching in 2016, compared to about 42 percent in 2002 and about 45 percent in 1998.

Aerial view of widespread coral bleaching, northern Great Barrier Reef, March 2016.
Photo by Terry Hughes / ARC Centre of Excellence for Coral Reef Studies

The areas most affected by bleaching had warmer sea surface temperatures for long periods of time. When the sea heated up even just a few degrees Fahrenheit over several consecutive weeks, as many as 70 to 90 percent of corals bleached. So the continuous exposure to warm water is lethal for the corals. “If you think about it, on a really hot day, it’s hot out, it’s not a big deal. But when it goes on and on through time, that’s when people without air conditioning start dying,” study co-author Mark Eakin, a coral reef expert and the coordinator of NOAA’s Coral Reef Watch, tells The Verge. “It’s the same thing here.”

Warming waters are the main thing to worry about when it comes to bleaching, the study says. Even protecting reefs from overfishing and pollution didn’t help the heat-stressed corals. “When you get heat stress at this level, that’s all that matters,” Eakin says.

That doesn’t make protective measures useless, though, Eakin says. A good protective measure, for instance, is a bunch of fish. See, when reefs bleach severely, the corals get covered by a particular type of algae that doesn’t allow new coral larvae to come in and grow. That would strangle the reef — if it isn’t rescued by a large population of plant-eating fish, which eat the algae, letting the coral regrow. As if that weren’t enough, the fishes’ pee also provides nutrients that help the corals rebound, Eakin says.

Scientist Andrew Baird surveying healthy reefs on the Great Barrier Reef in October 2016.
Photo by Tane Sinclair-Taylor

Similarly, less water pollution, sewage, and runoff may also aid coral recovery. But today’s study shows that these efforts aren’t enough, because if the ocean waters keep warming, cleaner waters and healthier fish populations still won’t keep the reefs from dying.

“Global climate change is one of the biggest factors in the decline of the coral reef,” Eakin says. If we want to save the reefs, we need to tackle climate change and reduce global warming. That can be done by committing to climate deals like the Paris agreement and reducing carbon dioxide emissions by switching to renewable energy.

The Queensland government recently approved the development of Australia’s largest coal mine, says Justin Marshall, a marine ecologist who’s been working on the GBR for 30 years, and did not take part in the study. That coal mine will not only pump more greenhouse gases into the atmosphere, it will also expand a local port that’s near the GBR and could damage the ecosystem further. “It’s disgraceful,” Marshall tells The Verge. “We need to do better as planetary guardians.”

At risk are not just natural wonders like the corals reefs around the world, but the lives of thousands of people who depend on them. The Great Barrier Reef provides Australia with nearly $6 billion in revenue from tourism and fishing, including employment for almost 70,000 people. In countries like Indonesia and the Philippines, the reefs provide one of the primary sources of food, Marshall says. “As we burn coal and as we warm up the planet and as we kill their reefs, we’re killing those people,” Marshall says. “Food security has become a great problem.”

The coral reefs aren’t dead yet, but there’s no time to waste. If we want to save the reefs, we need to act on climate change now. “We have to tackle fossil fuels and global climate change much faster,” Marshall says. “This is imperative.”