Corals in warm tropical waters and waters with high temperature variability could weather climate change better.
Most corals are highly sensitive to changes in water temperatures and so environmentalists have sounded the alarm for the planet’s reefs. Thermal stress can cause fragile corals to bleach and so temperatures rises owing to climate change could spell the end of most reefs, including the famed Great Barrier Reef in Australia.
Yet not all reefs may be at the same risk, at least for the time being.
A team of researchers has found that corals that inhabit warm tropical waters all year round are naturally better adapted to withstand slight rises in temperatures. Meanwhile, corals that live in waters with more widely fluctuating temperatures also seem able to cope better with warming waters thanks to their ability to handle thermal variability.
“I think most people probably think that you would see more bleaching in places where it’s warmer year-round, and that was one of my assumptions as well,” explains Deron Burkepile, a biologist at the University of California in Santa Barbara, who was a coauthor of a new study in the journal Nature Communications.
Yet that’s not what the scientists have found. Burkepile and his colleague examined records with field observations of coral bleaching at 3,351 sites from across 81 countries between 1998 through 2017. They found that mass bleaching episodes tended to be less severe in tropical waters close to the equator and in regions with naturally high surface temperature variability.
Corals bleach when thermal or another environmental stress causes them to shed the symbiotic, colorful algae that provide them with extra energy through photosynthesis. Once the algae are expelled, what remains are white and brittle skeletal coral structures that are highly sensitive to further stress.
Yet it appears that corals living in habitually warm waters and those inhabiting waters with high temperature variability are naturally better able to handle heat stress and may not bleach as easily.
“Think of someone from New York, where it gets really cold in the winter but it’s also really hot in the summer. They have different wardrobes — coats, boots hats, shorts, sandals — and they can adapt to the changing weather,” explains Mary Donovan, a postdoctoral researcher who was a coauthor of the study. “Compare this to someone who lives in the Caribbean and only has shorts; if it got cold they’d be out of luck.”
Not only that but many corals seem to have adopted fairly well to consistently higher temperatures over the past years so that they may be able to withstand them better before bleaching. It’s possible that natural selection has been having an effect on corals so that those that are better at enduring thermal stress have taken the place of those that are less so on many reefs.
Even if this is the case, however, it does not mean that corals will simply survive rising water temperatures unscathed in coming years and decades. “[C]oral reefs are not out of the woods,” Burkepile cautions. “They are under extreme threat in the near-term — the next few decades to century — from climate change.”
Experts are trying to forestall a mass death of corals, which would be an ecological catastrophe, by growing climate-resilient corals in labs and seeding reefs with heat-resistant corals. Yet their job isn’t made any easier by a perfect storm of manmade environmental stressors. In addition rising ocean temperatures, many coral reefs are also facing threats from ocean acidification, plastic pollution, overfishing and mass tourism.