Global Warming’s Toll on Coral Reefs: As if They’re ‘Ravaged by War’
Posted January 4, 2018 7:07 p.m. EST
Before we call rockfish, shrimp and crab “dinner,” some of these species call coral reefs “home.” But those reefs, home to a quarter of all marine fish species, are now increasingly threatened as rising ocean temperatures accelerate a phenomenon known as coral bleaching.
Large-scale coral bleaching events, in which reefs become extremely fragile, were virtually unheard-of before the 1980s. But in the years since, according to a study published Thursday in the journal Science, the frequency of coral bleaching has increased to the point that reefs no longer have sufficient recovery time between severe episodes.
Jelle Atema, a professor of biology at the Boston University Marine Program who was not involved in the study, said the effects of more frequent bleaching events were very difficult to predict because of the complex networks of dependencies within reefs. But he said they could be devastating.
“When coral dies, it affects the shelter and food that sustain fish, lobsters, shellfish, worms, etc. The same happens in a rain forest. When the trees die, the animals and plants that have developed over millennia die with them,” he said, before adding an analogy. “When a country is ravaged by war, people die and migrate.”
During bleaching events, overheated seawater causes corals to part ways with symbiotic plantlike organisms called zooxanthella that live inside of them. In addition to giving coral reefs their bright colors, zooxanthella also provide corals with oxygen, waste filtration, and up to 90 percent of their energy. Absent zooxanthella, corals not only take on a ghostly pallor, hence the term bleaching, but they are also more susceptible to death.
In theory, coral reefs can recover from even a severe bleaching event. Some of the coral will die off from increased disease susceptibility, but once ocean temperatures drop again, many of the corals will start growing back.
But that’s only if they’re given enough time.
Typically, it takes 10 to 15 years for the fastest-growing corals to recover after a severe bleaching event. Larger corals that provide shelter for bigger fish can take even longer to grow back.
As bleaching events become more frequent, reefs are unlikely to get that needed reprieve. Earth’s average temperature has increased 1 degree Celsius, or 1.8 degrees Fahrenheit, above preindustrial levels, and the median time between severe bleaching events is now just six years, the Science study found.
Case in point: The Scott Reef, 180 miles off the coast of Northwestern Australia, had over the past few years finally begun recovering from a major bleaching event in 1998, with the fastest-growing corals inhabiting much of their earlier territory. But the area was hit by bleaching again in 2016, causing widespread mortality.
Before 1982-83, mass bleaching events across wide areas were nonexistent. That year, reefs across the Tropical Eastern Pacific exposed to warm El Niño year waters bleached. Coral reefs off Costa Rica, Panama and Colombia experienced 70 to 90 percent mortality. Most reefs around the Galápagos Islands, the cradle of Darwin’s theory of evolution, experienced 95 percent mortality.
While many mass bleachings were prompted by El Niño events, which tends to warm Pacific Ocean temperatures, the bleaching event that hit the Great Barrier Reef in 2017 — the reef’s first back-to-back bleaching — occurred at the beginning of a La Niña event, when ocean waters should have been cooler. It’s a sign that global warming is steadily pushing up ocean temperatures even in cooler years.
“La Niña periods today are actually warmer than El Niño periods were 40 years ago,” said Terry Hughes, a senior researcher who specializes in coral reefs at James Cook University in Australia and the lead author of the Science study.
“Coral bleaching is caused by global warming, full stop,” Hughes said. “It’s not due to El Niño. We’ve had thousands of El Niño prior to 1983, none of them caused bleaching. Bleaching is caused by the rising baseline temperatures due to anthropogenic global warming.”
Scientists have long warned that the effects of climate change will not necessarily progress in a linear way as the planet warms. As Earth crosses certain key temperature thresholds, severe and far-reaching changes can unfold relatively rapidly, such as the collapse of ice sheets or the die-off of key ecosystems.
All evidence suggests that bleaching will only get more and more frequent as the Earth continues to warm. By midcentury, climate models suggest, most reefs will experience the sort of heat associated with severe bleaching every year.
If corals can’t adapt quickly enough, “we could be looking at the effective loss of most of the world’s coral reefs,” said Mark Eakin, an oceanographer who is coordinator of the Coral Reef Watch project at the U.S. National Oceanographic and Atmospheric Administration. The Great Barrier Reef had two back-to-back bleaching events that killed just about half the corals along the length of the barrier reef. This means half are still alive. Those corals are the source of larvae that spawn future generations, which means that the reef moving forward will have a distinctly different character than it had two years before the bleaching event.
“The ecological effect of more and more bleachings is that it’s changing the mix of species in favor of the tougher corals that can survive bleaching events and in terms of the corals that bounce back the quickest,” Hughes said. “It’s changing the whole ecology of the reefs.”
There are a few things that can help make reefs more resilient to bleaching. Humans can limit fertilizer and sewage runoff that damage coral. They can avoid overfishing key herbivores like the rabbitfish that nurture the reefs by clearing away excessive algae.
Some researchers are experimenting with even more radical techniques, such as trying to breed coral that can thrive in warmer temperatures, or looking at ways to pump cooler water into reefs to protect the coral from overheating, or even placing giant “shade cloths” over reefs.
Some of these ideas are admittedly wild, Eakin said, and none of them can ever be a substitute for reducing greenhouse gas emissions. “We can’t act as if we can keep emitting carbon dioxide into the atmosphere and just by tinkering around with corals in a lab we’re going to solve the problem,” he said.
But given that frequent bleaching is already underway, and given that at least half a degree of additional global warming appears inevitable, coral researchers are desperate for new ideas.
“We’ve got to start taking steps that we haven’t thought about before — even if they sound absolutely crazy,” Eakin said. “Because the stuff we thought made sense will no longer work.”