Scientists Find Some Hope for Coral Reefs: The Strong May Survive

Among the threatened corals of Australia’s Great Barrier Reef, one of the natural wonders of the world that has been ravaged by global warming, researchers have found a reason for optimism — or at least a reason not to despair completely.

Coral reefs, which by some estimates support a quarter of all ocean life, are harmed by warming oceans. The effects can be seen in the loss of their vibrant colors, a phenomenon known as bleaching. But after ocean temperatures surged in 2016 around the Great Barrier Reef, causing severe damage, researchers found that the corals that survived were more resistant to another period of extreme warmth the following year.

“It’s one enormous natural selection event,” said Terry Hughes, an expert on coral reefs at James Cook University in Australia and the lead author of a study published Monday in the journal Nature Climate Change. In effect, the 2016 heat wave killed off many of the most heat-sensitive corals and selected for the corals that could handle higher ocean temperatures.

“So when the heat returned in 2017, the susceptible corals had been substantially depleted,” Dr. Hughes said. “The new coral assemblage, if you like, at the beginning of the second heat waves, was made up predominantly of the more heat-tolerant species, the more robust ones.”

The study found that in 2016, when parts of the reef were exposed to ocean temperatures seven to 14 degrees Fahrenheit higher than normal, roughly half the coral exhibited bleaching within four to five weeks. But in 2017, the coral had to be exposed to those temperatures for eight to nine weeks before it suffered the same level of bleaching.

“Despite the fact that Year 2 was hotter, we saw less bleaching over all across the whole reef,” Dr. Hughes said. The reason, he said, was a novel concept scientists call ecological memory: the idea that the past experience of a biological community can influence its ecological response today or in the future.

Coral reefs are vast colonies of hard coral, a type of coral that extracts calcium carbonate from seawater to construct a limestone structure for protection. The tiny animals, which resemble sea anemones, are clear. They get their bright reds and vivid purples from colorful algae called zooxanthellae that live in their cells, providing the oxygen that the corals need to grow — albeit slowly. Some species of coral grow as little as a tenth of an inch a year.

The corals are highly sensitive to water temperature. Too cold and the corals suffer; too warm and the algae and corals separate, leaving the corals stripped of their color. The resulting ghostly pallor is the reason scientists call this kind of event bleaching.

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