(CN) – Scientists say climate change has dramatically increased the frequency of coral bleaching events at reefs throughout the tropics over the past 40 years, threatening the continued existence of these ecosystems and the livelihoods of millions of people.
Their report, published Thursday in the journal Science, examines a transition from local bleaching before the 1980s to mass bleaching events in the 1980s and 1990s – when such incidents were first recorded during warmer than usual El Nino conditions – to the current period of climate-driven bleaching.
“The time between bleaching events at each location has diminished five-fold in the past 3-4 decades, from once every 25-30 years in the early 1980s to an average of just once every six years since 2010,” said lead author Terry Hughes, director of the ARC Center of Excellence for Coral Reef Studies (Coral CoE) in Australia.
“Before the 1980s, mass bleaching of corals was unheard of, even during strong El Nino conditions, 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.”
And tropical sea temperatures during particularly cold La Nina conditions are warmer today than they were 40 years ago during El Nino events.
“Coral bleaching is a stress response caused by exposure of coral reefs to elevated ocean temperatures,” said co-author Andrew Baird, a professor at Coral CoE. “When bleaching is severe and prolonged, many of the corals die. It takes at least a decade to replace even the fastest-growing species.”
Future man-made climate change will further threaten coral reefs.
“Reefs have entered a distinctive human-dominated era – the Anthropocene,” said co-author C. Mark Eakin, a reef expert at the U.S. National Oceanic and Atmospheric Administration. “The climate has warmed rapidly in the past 50 years, first making El Ninos dangerous for corals, and now we’re seeing the emergence of bleaching in every hot summer.”
The Great Barrier Reef has bleached four times since 1998. The iconic reef bleached during back-to-back events for the first time in 2016 and 2017, causing major damage.
“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.