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Coral Countdown: Groundbreaking Study Finds Time’s Up to Save Reefs

A new analysis shows a dramatic increase since 1980 in the rate of catastrophic coral bleaching events striking ecologically and economically crucial reef systems worldwide. That leaves little opportunity for damaged corals to rebound.

Written by Todd Woody Published on Read time Approx. 5 minutes
The Great Barrier Reef, Australia.Reinhard Dirscherl\ullstein bild via Getty Images

Remember the Cold War-era Doomsday Clock that counts down the minutes to midnight and nuclear Armageddon?

It’s time for a Coral Clock.

A first-of-its-kind study published Thursday has found that the coral bleaching phenomenon that devastated reefs worldwide in 2015–2016 is now the new normal, accelerating at a rate that prevents the recovery of unique ecosystems before the next heat wave hits. Ultimately, few if any reefs will be left untouched.

Twenty-five coral scientists collaborated on the study, published in Science, assembling and analyzing records of 612 bleaching events that struck at 100 reef locations around the world between 1980 and 2016. Virtually unknown before 1980, bleaching occurs when water temperatures exceed corals’ tolerance and the organisms expel now-toxic algae that supply their nutrition and palette of eye-popping colors in exchange for shelter. The symbiotic bond severed, reefs – which harbor a quarter of marine species and are the source of food and livelihoods for hundreds of millions of people – turn a ghostly white. Deprived of food, corals can die unless the ocean cools. In recent decades, as climate change has raised water temperatures, the El Niño weather pattern that periodically warms the ocean has intensified the impact and reach of bleaching events.

But soon, if not now, the researchers concluded, there will likely be little respite for reefs, even when El Niño is replaced by its cooler sister, El Niña. In fact, the scientists found, even cooling El Niña’s today are now warmer than the hot El Niño’s of 30 years ago.

“Our analysis indicates that we are already approaching a scenario in which every hot summer, with or without an El Niño event, has the potential to cause bleaching and mortality at a regional scale,” the scientists wrote. “The time between recurrent events is increasingly too short to allow a full recovery of mature coral assemblages, which generally takes from 10 to 15 years for the fastest growing species and far longer for the full complement of life histories and morphologies of older assemblages.”

“Areas that have so far escaped severe bleaching are likely to decline further in number and the size of spatial refuges will diminish,” they added.

Julia Baum, an assistant professor of biology at the University of Victoria in British Columbia and a coauthor of the study, has seen the future first-hand and it’s grim. Baum has long studied the corals hit hardest by bleaching, those at the Kiritimati atoll – also known as Christmas Island – in the South Pacific.

Documenting the dead coral after the bleaching event at Lizard Island on the Great Barrier Reef in 2016. (The Ocean Agency/XL Catlin Seaview Survey/Richard Vevers.)

“Corals are very, very precisely attuned to the temperature in their specific location and when water temperature rises even as little as 1 degree Celsius above their preferred temperature, they can bleach,” said Baum. “Four weeks can cause corals to bleach and after a couple of months of high temperatures, corals will die.”

In the waters surrounding Christmas Island, high temperatures persisted for more than 40 consecutive weeks in 2015–2016.

“High temperatures have never before been measured in months – they’re measured in weeks,” said Baum. “Suddenly we were looking at months and months and it was just seemingly unending. And it went on for 10 months and what we found at that particular location was that pretty much all of the corals bleached and in the end 90 percent of corals died.”

Globally, the scientists’ analysis discovered that as of 2016, severe bleaching events are occurring every 5.9 years, compared to every 27 years in the early 1980s. Since 2000, a third of bleaching events have reoccurred in just one, two or three years.

That simply is not enough time for corals to recover.

“One of the things that this paper hopefully really draws home is that it is not the gradual ocean warming that is going to get coral reefs – I mean that will get them eventually – but that the much more immediate threat is these pulse warming events,” said Baum. “In certain locations, the ocean heats up for a short period of time, and for that short period of time, temperatures go up, they stress the corals out and they bleach. What we’re seeing is that more and more locations around the world are experiencing these pulse events.”

“It has been a very popular idea that there will be spatial refuges from warming and those spatial refuges will be the special places where remnant coral reefs manage to survive,” she added. “I think that’s a fallacy. Currently, there are some of those spaces, but I think that the window is closing quickly.”

In the western Atlantic Ocean, for instance, widespread bleaching – defined as affecting 50 percent of reef locations – has struck seven times since 1980, according to the study. That’s compared to three times in Australasia and the Indian Ocean and twice in the Pacific.


A turtle swimming over a reef destroyed by the 2016 bleaching event in the Indian Ocean. (ARC CoE for Coral Reef Studies/Kristen Brow)

“There has been a clear variability in the spatial pattern of bleaching in the Caribbean as you look from one severe event to the next, i.e., the pattern of bleaching in the Caribbean has been different for each event,” C. Mark Eakin, a study coauthor and coordinator of Coral Reef Watch at the National Oceanic and Atmospheric Administration, said in an email. “However, if you look at the bleaching through time you will see that nowhere in the region has been spared from severe bleaching.”

“This has had significant impact on the community of corals found in reefs across the western Atlantic and has been a significant driver of the decline in coral cover and the loss of structure in Caribbean coral reefs,” he added.

The study is another wake-up call for what coral scientists say is the only solution to the coral crisis: a dramatic reduction of greenhouse gas emissions. But it’s also likely to ratchet up the debate over efforts to stem the tide of coral bleaching by repopulating decimated reefs with corals bred in the laboratory to be resilient to higher ocean temperatures.

Some scientists, most notably study lead author Terry Hughes, director of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University, have been vocally skeptical about such strategies, arguing they will have negligible impact in a rapidly warming world.

“We definitely have a diversity of opinions about this among the coauthors of this very paper,” Baum said. “There is no magical technological fix that is going to save coral reefs if we do not also drastically reduce greenhouse gas emissions.”

But she said it would be dangerous not to try.

“Even if we reduced greenhouse gas emissions dramatically today, we’re still left with residual heat in the system,” said Baum. “With corals, we are in a triage situation where we need all hands on deck and we need all approaches put on the table, discussed, evaluated and attempted. I think if we don’t try all potential measures, we’re not going to succeed with the same types of solutions that we currently have. We need to keep moving forward.”

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