Coral Could Be Adapting to Rising Water Temperatures, but Is It Enough?

Coral appears to be fighting for its own survival by learning to tolerate the warmer waters caused by climate change, according to a new study, but the rising temperatures may outpace its ability to adapt.

In a study published Tuesday in the journal PeerJ, researchers from the Bernice Pauahi Bishop Museum in Honolulu and the University of Hawaii at Manoa replicated experiments from the 1970s and found that coral's resilience to warmer ocean waters may have increased dramatically.

When the original study was conducted, the scientists simulated rising temperatures and found that the coral had a very low tolerance. It could handle only about 1 to 2 degrees Celsius above the normal maximum. Although temperatures vary across the oceans, most coral species would begin to bleach with that small increase in temperature. Only 0 to 40 percent of the coral in the experiments was able to withstand the warmer waters.

About five decades later, the researchers in the new study exposed the coral to similar temperatures as the original for one month, then gave the coral 28 days to recover. An unexpected 60 to 90 percent of the coral survived.

Coral Reef Israel
A scuba diver checks coral reefs in the Red Sea, off the southern Israeli resort city of Eilat, on June 12, 2017. A new study shows that coral may have increased its tolerance to the warmer waters caused by climate change. MENAHEM KAHANA/AFP/GETTY IMAGES

"Most studies compare corals from different reef locations, whereas this is the first study to compare the same coral species from the same location over time," Keisha Bahr, a co-author on the study, said in a press release. "Re-running a 48-year-old experiment using the same coral species, same experimental setup, and the same observer allows us to directly test changes in coral temperature tolerance over the last half century."

The coral was able to improve its ability to withstand warmer waters either because the algae that live inside it changed or because natural selection has allowed only the most temperature-tolerant corals to survive over the past half-century, according to the researchers.

Warmer waters threaten coral by causing bleaching. Coral bleaching is when algae, which has a symbiotic relationship with the coral and lives in it, leaves because the water is too warm, according to the National Ocean Service. The coral will then turn completely white. This doesn't mean the coral is dead, but it is subject to disease and is under stress since algae are coral's main source of food.

A study published in Science in January found that 31 percent of coral reefs were affected by bleaching in 2016, and the risk of bleaching has increased by 4 percent every year since the 1980s. The study also found that the time between bleaching events is less than half than before, which leaves less opportunity for the coral to recover.

Despite the results of the new study, the scientists say it's not enough to ensure the survival of coral reefs, which support more species of animals than any other marine environment, including an estimated 1 to 8 million undiscovered species. "Although these results are encouraging," Ku'ulei Rogers, an author on the study, said in the release, "increased bleaching tolerance may not be enough for widespread coral survival."