Worldwide coral bleaching has sped up dramatically in 30 years

The time between coral bleaching events at multiple reef locations has decreased five-fold in the past four decades, new research has found.

A study in the journal Science reports that time elapsed between bleaching events in the tropics has contracted from 25-to-30 years in the early 1980s to just six years by 2010.

“Before the 1980s, mass bleaching of corals was unheard of, even during strong El Niño conditions, but now repeated bouts of regional-scale bleaching and mass mortality of corals have become the new normal around the world as temperatures continue to rise,” says lead author Terry Hughes of the ARC Centre of Excellence for Coral Reef Studies based at James Cook University in Queensland, Australia.

Using data from 100 reef sites around the world, Hughes and colleagues from Australia, Saudi Arabia, Canada and the US demonstrate that tropical sea temperatures are warmer today during cooler-than-average La Niña conditions than they were 40 years ago during El Niño periods.

They find that the frequency of the bleaching events is having dire consequences for the complex ecosystems of coral reefs, because six years is insufficient time for the mature assemblages of the reef to recover. Even the fastest growing coral communities take approximately 10 to 15 years to recover after a bleaching event. 

The researchers fear that annual bleaching could soon occur. 

“Reefs have entered a distinctive human-dominated era – the Anthropocene,” says co-author Mark Eakin of the US National Oceanic & Atmospheric Administration. 

“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.”

The timing and severity of mass bleaching events has varied across geographic regions. In the 1980s, the Western Atlantic and Pacific regions were at highest risk. More recently, bleaching risk has increased only slowly in the Western Atlantic, at an intermediate rate in the Pacific and very strongly in the Middle East and Australasian regions. 

The study highlights the Great Barrier Reef, which has bleached four times since 1998, including unprecedented back-to-back events in 2016 and 2017. 

The researchers conclude that the future conditions of reefs, and the ecosystem services they provide to people, will depend critically on the trajectory of global emissions. 

“We hope our stark results will help spur on the stronger action needed to reduce greenhouse gases in Australia, the United States and elsewhere,” says Hughes.