GOOD Science: What Is “Coral Bleaching” And Just How Bad Is It?

Last week, scientists delivered the news that more than 90% of Australia's Great Barrier Reef is suffering from bleaching. Researchers flew in helicopters and small planes over 911 of the individual reefs that comprise the Great Barrier Reef and found that only 68 of them had escaped bleaching entirely. Many of the rest have turned a ghostly white. "It's like 10 cyclones have come ashore all at once," said Terry Hughes, a researcher at the ARC Centre of Excellence for Coral Reef Studies at James Cook University and head of Australia's National Coral Bleaching Task Force.

The problem is not limited to Australia's coastal waters, either. Reefs in the Caribbean bleached last summer, with most of the damage being observed around South Florida and into the Keys. Last fall, reefs around the Hawaiian archipelago suffered a similar fate. But what is actually going on with these oceanic habitats at a cellular level to make them turn stark white? We’ve heard a lot about rising water temperatures stemming from climate change, and Hughes was one of the scientists who co-signed a full-page newspaper ad in Queensland, Australia calling for immediate government action to protect the Reef.

But what is the specific problem with the coral, and is there any hope of reversing this terrible trend? We’ve got you covered with the science behind the issue.

First of all, coral polyps are tiny organisms related to jellyfish and sea anemones. Lots of corals are perfectly happy living alone, but some live in large colonies, or reefs. The hard, rocky surface with which we're all familiar is actually their exoskeleton, while the animals themselves are quite squishy. The beautiful colors typical of tropical reefs are produced not by corals but by the algae that live with them in symbiotic harmony.

Symbiotic relationships abound in nature, partnerships between different organisms that are mutually beneficial. The symbiotic relationship between bees and flowers is probably one of the most common we see in the world. Bees get food while aiding the flowers in reproduction. Fish called remoras keep whales and sharks free of skin parasites by hovering below the massive sea creatures and feasting on the tiny destructive organisms.

Corals have such a partnership with a group of algae called zooxanthellae, which are responsible for providing reefs with their stunning pigmentation. There are lots of species of both corals and of zooxanthellae, but all reefs are built upon this ancient agreement. The algae provide the coral with up to ninety percent of their nutrition, while the coral provide the algae with a permanent, sunny homestead in which to carry out their photosynthesis without fear of predation.

So here’s where the bleaching comes in. If the surrounding seawater becomes too hot or if there's too much sunlight, the algae start producing harmful toxins rather than tasty sugars. That triggers the coral's immune system, and the once beneficial partnership breaks down. The corals give their algal partners the metaphorical boot, and the bright white exoskeleton is what remains. Regardless of the term, "There is no bleach involved," says Scott Heron, a physicist with NOAA's Coral Reef Watch.

If the environmental circumstances that lead to bleaching are brief and infrequent enough, then the corals can rebuild their relationship with the algae once the waters cool and everything can proceed swimmingly.

Indeed, a study published earlier this month in the journal Science reports that corals were generally able to survive historic bleaching events perhaps because they could practice for it in the days leading up. "We found that if there's a short period of warm-but-not-too-warm temperatures, followed by a little cooling off, it primes the corals. It gets them ready for [the] subsequent stress," says Heron, who led the survey with Tracy D. Ainsworth of James Cook University's ARC Centre of Excellence for Coral Reef Studies.

The problem is that climate change is making for more intense and more frequent bleaching events, which do not provide the reefs with the pre-bleaching practice period or a long enough cooling-off period. And it's happening on a global scale.

[quote position="full" is_quote="true"]Many open ocean fish—big fish that are fundamental to the food supply of many, many nations—[have] larvae that hang out on coral reefs. Once that habitat is gone, those fish are not going to make it.[/quote]

This should worry you. "The benefits of coral reef ecosystems for people are dramatic, and it's not just people who live in coastal locations," says Heron.

Reefs provide us not just with beautiful scenery, though that shouldn't be discounted either. There's good evidence that exposure to the natural world provides a host of benefits for our mental and physical well being, from reducing stress to lowering blood glucose. But there's a host of more concrete services provided to us by these marine habitats.

Reef-related tourism and recreation as an industry would simply not be viable in a world of boring, bland, bleached coral. Millions of divers and snorkelers visit reefs throughout the world specifically because of the beautiful ecosystems they support, spending some $9.6 billion dollars annually to do so. Fifteen percent of Belize's economy is derived from tourism, and in Southeast Asia each square kilometer of healthy reef within tourism-appropriate areas can draw between $23,000 and a $270,000 each year.

Reefs also ensure the security of global food supplies. Reefs are incredibly diverse and rich ecosystems, and one of the services they provide is acting as fish nurseries. “Many open ocean fish—big fish that are fundamental to the food supply of many, many nations—[have] larvae that hang out on coral reefs," says Oregon State University biologist Virginia M. Weis. "Once that habitat is gone, those fish are not going to make it." And as several researchers have recently argued, food insecurity can have dire repercussions. It has been directly linked to organized crime, human trafficking and global terrorism in some of the poorest corners of the world.

Healthy coral reefs also provide buffers against extreme weather conditions, guarding coastlines from devastating storm surges. If climate change proceeds unimpeded then reefs will be unable to provide that protection against the increasingly powerful storms that are predicted. The bleaching of reefs is terrible, but it’s only a symptom of a much larger, much more terrifying problem. By virtue of our own actions, the planet is turning against the way of life we’ve built on top of it.

And if that's not enough, says Weis, coral reefs provide a biological bounty for pharmaceutical discoveries. Ziconotide, for example, is a drug used to treat chronic pain. The active ingredient, now synthesized, was first discovered in a toxin produced by a reef-dwelling snail called Conus magus.

But now the big question: Is it too late? Perhaps not. "I would hope that not only would 2°C be achieved, but in fact that it could be bettered by bringing that down to the lower goal of 1.5°C warming, or even less," says Heron of the Paris agreement hammered out at the COP21 meeting in France last December, and signed by almost 200 countries last Friday. If the world can come together and limit the amount of warming that occurs, reefs might yet stand a chance.

And, of course, corals could yet surprise us and find a way of adapting. They could develop abilities to better cope with warming events when they do occur. Just last week scientists announced the discovery of a previously unknown 600 mile-long coral reef system in the Atlantic Ocean at the mouth of the Amazon River. It’s remained hidden thanks to murky water, challenging the commonly held belief that corals require clear water to get enough sunlight to photosynthesize. What other secrets are tropical corals hiding from us?

As for Weis, she refuses to acquiesce to pessimism. "You can't give up hope, because hope is the currency [of] change," she says.