Coral bleaching. (Photo: acro_phuket/shutterstock.com)

It’s a beautifully clear day in the tropics. The water is warm and still; there’s no wind, no clouds to block the bright sunshine. It looks like a perfect day in paradise–until you stick your head underwater.

Where coral reefs once flashed magenta and tangerine, mustard and scarlet, there are instead fields of bone white.

Thanks to the warm waters of El Niño, coral reefs are in the midst of the third-ever-recorded global coral bleaching event. Corals are losing much more to El Niño than their color– in fact, bleaching has scientists worried about the future of these tiny castle-building animals on our warming planet.

“We live in world with a shifting baseline of a warmer and more acidic ocean, making it harder for corals to live,” says Andrea Grotolli, an Ohio State University researcher investigating coral resilience. Fifty years ago, she says, bleaching didn’t exist on many reefs, or was a rarity. “Now, bleaching is common, its ubiquitous, it happens regularly, and it’s devastating.”

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Though reef-building corals form structures so enormous they can be spotted from space, most depend on something very tiny: billions of cells of plant-like algae, called zooxanthellae, living within the corals’ tissues.

The coral-algae relationship is an elegant tit-for-tat. The algae find shelter in the coral’s exoskeleton, and use its waste to perform photosynthesis. In exchange, the algae gives the coral oxygen and energy-rich sugars, plus a built-in waste management system. The algae also paint their coral hosts with their famous colors–the riotous colors of a reef are actually a symbol of one of the world’s oldest alliances.

But what happens when these allies split?

When a coral is stressed, it can take drastic measures and expel its photosynthetic roommates. This removes the need to share nutrients, but also takes away the coral’s main food source; according to the National Oceanic and Atmospheric Administration (NOAA), up to 90 percent of the sugars that algae produce are donated to the coral. If the algae don’t return within a few weeks, the coral will die.

And when corals die, they can take an entire section of the ecosystem with them, like a line of dominoes sent tumbling.

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“Coral are the building blocks of reefs,” explains Grottoli. “They provide the texture and framework for worms and crabs and shrimp and small critters; they’re where fish find meeting and reproduction places, hide from storms, where larger fish feed on smaller fish.”

When a reef dies, algae, ironically, often takes over: the animals that keep coral surfaces clear may die off without the reef to support them, allowing the empty coral skeletons to become covered in a thick layer of macroalgae that only a few fish species can eat. “All this connectivity is associated with the reef’s texture,” says Grottoli. “If you lose the coral, you lose all of the organisms using that texture.”

Before the turn of the century, bleaching had primarily been caused by local stressors, like pollution. But in 1998, the world experienced the very first global bleaching event due to abnormally warm ocean temperatures, a sight that has become increasingly common. Most coral species can only withstand a very narrow temperature range; a change of just two degrees can be devastating. Windless, cloudless days, where warm water sits atop reefs, are the deadliest.

Corals can recover from bleaching events if they are able to recapture algae from the water. But the algae’s return is the start of an uphill battle. Corals grow slower, reproduce less, and become more vulnerable to disease for years after a bleaching event. If one bleach-causing stress follows another– such as if temperature spikes become normal, or ocean waters become more acidic as they absorb carbon dioxide– even the strongest of reefs may continually bleach to death.

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“As climate change is intensifying, and there is so much use of near-shore waters, the intensity and frequency of these major disturbances is so rapid there’s no time for recovery,” says Ruth Gates, coral researcher at the University of Hawaii. “It’s a constant insult. Nobody can be hit over and over again with a big stick, and that’s essentially what we’re doing with climate change.”

If you don’t live anywhere near a reef, you may be asking: why should I care?

If you love fish for their beauty or their taste, know that corals support a quarter of the ocean’s fish biodiversity– many species of which end up on dinner plates. Coral fish provide food for over a billion people, 85 percent of whom can’t get their protein anywhere else.

In laboratories and hospitals, scientists are learning how to improve human health with coral compounds. A steroid corals use to fight disease is already used to treat asthma and arthritis. Scientists believe that reefs likely host a wealth of medicines that we haven’t even discovered yet.

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And for island nations, survival by coral is even more personal: reefs form the first buffer against ocean waves, protecting these dots of land from eroding and vanishing into the sea.

Between fisheries, diving and snorkeling tourism, and coastal protection, coral reefs contribute close to $30 billion to the global economy every year.

And in many places, the threat of vanishing corals connects with something deeper.

“The loss could be one of perception; the reef is almost dead, so I am very sad,” says Clive Wilkinson, a researcher at Australia’s Reef and Rainforest Research Center, who has monitored reefs for over 45 years. This year, he watched as huge swaths of Australia’s famous Great Barrier Reef experienced the worst bleaching ever seen.

GREAT BARRIER REEF, AUSTRALIA

The future of coral reefs is not entirely without hope; some reefs appear better equipped to survive the temperature stresses expected from climate change. Corals benefit from large fat stores that they can draw on if they expel their algae. They can also prepare themselves for future stresses if they can take up a more heat-tolerant algae species after bleaching occurs.

However, Grotolli cautions that many corals won’t be able to adapt this way.

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“Most corals will suffer, and some species will suffer more than others,” she says. “And even if some corals are persisting, that doesn’t mean they’re thriving. In many scenarios, the coral still isn’t growing as fast, and so they’re less able to compete for space, or keep up with dramatic sea level rise. The number that will thrive may be very small.”

We are not looking at an ocean without coral reefs entirely, says Wilkinson. It will be an ocean with fewer fish, fewer barrier islands, and many fewer colors–but reefs, in some form, will continue to slowly soldier on.

“It is certain there will be many local extinctions of coral species, and probably some other beasts. But reefs will still be there. We will just have to call it the ‘Just Ordinary Barrier Reef’ or ‘a bit like a barrier reef’ instead.”