The Great Barrier Reef experienced an extended marine heat wave in 2016 that caused massive coral bleaching and die-off. Most of the impact was along 500 miles of the northern Great Barrier Reef, its most pristine region.
The reef endured coral bleaching in 1998 and 2002, but the northern region sustained only minor damage then. Global heat and coral bleaching began to increase in 2014 and continued through 2017; this event meant that marine heat waves causing bleaching struck three-quarters of the world's coral reefs and that the heat waves that cause corals to die struck almost a third, the researchers said.
The 2016 marine heat wave caused the most severe and catastrophic coral bleaching event the Great Barrier Reef has ever experienced, the study found. Overall, these events have affected every part of the reef.
"We lost 30 percent of the corals in the nine month period between March and November 2016," Terry Hughes, study author and director of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University, said in a statement.
To add insult to injury, another marine heat wave hit the Reef in 2017, with severe heat stress and bleaching striking the central region.
"We've seen half of the corals on the Great Barrier Reef killed by climate change in just two years," Mark Eakin, study author and coordinator for the US National Oceanic and Atmospheric Administration's Coral Reef Watch, wrote in an email. "This study shows that the coral reefs that have been least affected by heat stress in the past are more sensitive to heat stress than we realized. It also shows climate change threatens the diversity that is the hallmark of coral reefs."
Eakin said the increase in marine heat waves is clearly driven by rising temperatures from increasing carbon dioxide and other heat-trapping gases in the atmosphere due to human activity.
The 2016 event compromised 30% of the UNESCO World Heritage Site, which includes 3,863 reefs spanning 1,429 miles of the Queensland coastline.
Eakin said it was surprising how little heat stress was needed to cause the complete collapse of coral reef ecosystems in the northern Great Barrier Reef. Those reefs lost most of their corals at stress levels half of what the researchers would have expected, he said.
This was compounded by the 2017 event, which killed off half of the corals.
"That's like losing half of the trees in the Appalachian or Rocky Mountains in just two years," Eakin said.
It's also incredibly difficult for the coral to recover.
"Under the best conditions, the fastest-growing corals take 10-15 years to come back," Eakin wrote. "Unfortunately, our recent paper in Science
showed that severe coral bleaching events are now happening every six years, on average, across the world's coral reefs. Unless we reduce the carbon dioxide in the atmosphere, increasing marine heatwaves will return far too frequently for reefs to recover."
Although some of the coral has proved resistant to heat waves, those few species won't be able to maintain the diversity that is essential to coral reefs -- a hallmark of the Great Barrier Reef and the vast diversity of marine life it sustains.
"The coral die-off has caused radical changes in the mix of coral species on hundreds of individual reefs, where mature and diverse reef communities are being transformed into more degraded systems, with just a few tough species remaining," Andrew Baird, study co-author at the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University, said in a statement.
Some of the researchers said it's imperative to help the surviving coral.
"That still leaves a billion or so corals alive, and on average, they are tougher than the ones that died," Hughes said. "We need to focus urgently on protecting the glass that's still half full, by helping these survivors to recover."
The Great Barrier Reef is considered to be one of the seven natural wonders of the world and is the largest living structure on the planet. It's even visible from space.
It garners 2 million visitors a year, supports 64,000 jobs and has contributed an estimated $6.4 billion to the Australian economy on an annual basis, according to the Great Barrier Reef Foundation
Although it might seem that damage to the Great Barrier Reef mostly affects Australia, this die-off could affect the entire globe.
"Almost a billion people around the world rely on coral reefs as their main source of food protein," Eakin said. "Coral reefs provide tens of billions of dollars to economies and protect shorelines and infrastructures around the world."
Failure to keep global temperatures from rising more than 2 degrees Celsius from preindustrial levels spells doom for the Great Barrier Reef, the researchers said.
"Unless humans get climate change under control, the increase in the frequency and intensity of marine heatwaves will destroy most of the coral reefs around the world," Eakin said.