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 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 last week 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 Programme 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 in reefs, but 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 rainforest. When the trees die, the animals and plants that have developed over the 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 them. In addition to giving coral reefs their bright colours, zooxanthella also provide corals with oxygen, waste filtration and up to 90 per cent of their energy. Without 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 a severe bleaching event. Some coral will die from increased disease susceptibility, but once ocean temperatures drop again, many 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 severe bleaching. As bleaching events become more frequent, reefs are unlikely to get that needed reprieve. The median time between severe bleaching events is now just six years, the Science study found.
Case in point: The Scott Reef, 290km off the coast of northwestern Australia, had finally begun recovering from a major 1998 bleaching event. 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 Nino year waters bleached. Coral reefs off Costa Rica, Panama and Colombia experienced 70 to 90 per cent mortality. Most reefs around the Galapagos Islands, the cradle of Darwin’s theory of evolution, experienced 95 per cent mortality.
While many mass bleaching were prompted by El Nino events, which tend to warm Pacific Ocean temperatures, the bleaching event that hit the Great Barrier Reef last year, the reef’s first back-to-back bleaching, occurred at the beginning of a La Nina event, when ocean waters should have been cooler. It’s a sign global warming is pushing up ocean temperatures in cooler years.
“La Niña periods today are actually warmer than El Niño periods were 40 years ago,” said Terry Hughes, a senior researcher in coral reefs at James Cook University in Australia and lead author of the Science study.
“Coral bleaching is caused by global warming, full stop,” he said. “It’s not due to El Niño. We’ve had thousands of El Niño prior to 1983, none of them caused bleaching.”
Scientists have long warned that the effects of climate change may not progress linearly. As Earth crosses key temperature thresholds, severe and far-reaching changes can unfold rapidly, such as the collapse of ice sheets or the die-off of key ecosystems.
Evidence suggests that bleaching will get more frequent as earth warms. By mid-century, climate models suggest that most reefs will see the sort of heat associated with severe bleaching every year.
“We could be looking at the effective loss of most of the world’s coral reefs,” said Mark Eakin, coordinator of the Coral Reef Watch project at the US National Oceanographic and Atmospheric Administration.
A few things can make reefs more resilient to bleaching. Humans can limit fertiliser and sewage runoff that damage coral, and avoid overfishing key herbivores like rabbitfish that nurture reefs by clearing excessive algae.
Researchers are experimenting with 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 coral from overheating, or placing giant “shade cloths” over reefs.
“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.” NYT