Severe bleaching can blot out the color of huge sections of reefs and lead to the death of the constituent corals, destabilizing entire ecosystems. It’s a phenomenon that’s now happening to tropical reefs five times more frequently than it did just a few decades ago, said Terry Hughes, an ecologist at James Cook University in Australia and the study’s lead author, in a statement. Previously, once every two to three decades was typical, and usually only at local scales; now, it’s occurring every six years.
“Before the 1980s, mass bleaching of corals was unheard of, even during strong El Niño conditions,” Hughes added, “but now repeated bouts of regional-scale bleaching and mass mortality of corals has become the new normal around the world as temperatures continue to rise.”
Rising air temperatures, due in large part to the greenhouse gases emitted by human activity, have led to warmer oceans. Now, Hughes and his colleagues have shown that the dangers of bleaching have escalated over that relatively short amount of time by pulling together temperature data and recorded bleaching from across the tropics.
“The climate has warmed rapidly in the past 50 years, first making El Niños dangerous for corals, and now we’re seeing the emergence of bleaching in every hot summer,” said coauthor C. Mark Eakin, an oceanographer at the United States National Oceanic and Atmospheric Administration, in the statement.
In fact, temperatures at the sea surface during the cooler La Niña part of what’s known as the El Niño-Southern Oscillation cycle are on average hotter now than they were during the warmer El Niño part in the 1980s, the authors write.
“When bleaching is severe and prolonged, many of the corals die,” said ecologist Andrew Baird of James Cook University in the statement. “It takes at least a decade to replace even the fastest-growing species.”
It’s taking a devastating toll on reefs around the tropics, including Australia’s Great Barrier Reef. Sections of the reef often referred to as “the world’s largest living organism” have been bleached on four occasions in just the past 20 years. And 2016 and 2017 marked the first-ever consecutive annual bleaching events on the Great Barrier Reef.
“We hope our stark results will help spur on the stronger action needed to reduce greenhouse gases in Australia, the United States and elsewhere,” Hughes said.
As things stand now, the authors predict that we’re headed for a time when bleaching might happen every year.
“The climate has warmed rapidly in the past 50 years, first making El Niños dangerous for corals, and now we’re seeing the emergence of bleaching in every hot summer,” Eakin said. “Reefs have entered a distinctive human-dominated era — the Anthropocene.”
Hughes, T., Anderson, K., Connolly, S., Heron, S., Kerry, J., Lough, J., … & Claar, D. (2017). Spatial and temporal patterns of mass bleaching of corals in the Anthropocene. Science.
Banner image of a turtle by ARC Centre of Excellence for Coral Reef Studies/Kristen Brown.