Arial surveys conducted in March revealed that the Great Barrier Reef is facing another widespread bleaching event. The last two bleaching events, in 2016 and 2017, decimated about half of the natural wonder's coral reefs.
The March survey shows that all regions of the reef are suffering from the bleaching event, unlike previous events when only the north and central areas were affected. About a quarter of the reef has been severely affected, meaning more than 60 percent of the coral lost its color, and another 35 percent underwent mild bleaching. The news follows a difficult Australian summer of draught, wildfire and flooding.
“They are just getting hammered by these repetitive, destructive heat waves,” says coral reefs expert Australia Ove Hoegh-Guldberg of the University of Queensland in Brisbane to Science News’ John Pickerell. “If this continues over the next 10 years or so, there won’t be much of a Great Barrier Reef left.”
Coral reefs are huge colonies of tiny animals called coral polyps that rely on colorful algae living inside of them for food. The algae photosynthesize nutrients from sunlight and give corals their green, brown, and reddish coloring. But polyps are sensitive to their environments: A heat wave of a few degrees higher than average is enough to make them kick out their algal partners.
When that happens, the reefs turn white, and without algae, the polyps—which are animals—will starve. About half of the corals killed in 2016 died of the heat alone, Terry Hughes, a coral reef expert at James Cook University, told Robinson Meyer at the Atlantic in 2018.
This year’s bleaching event is not as severe as 2016’s, but it is more widespread across the 1,400-mile reef, which is a UNESCO World Heritage Site.
“It’s the first time we’ve seen severely bleached reefs along the whole length of the reef, in particular, the coastal reefs,” Hughes tells Damien Cave at the New York Times. “Those are bleached everywhere.”
A bleached reef is not necessarily a dead one. Given the time and environment, white reefs can recover their algae. But the recovery process can take about a decade, and climate change is making bleaching events like this year’s more frequent.
“Of the five mass bleaching events we’ve seen so far, only 1998 and 2016 occurred during an El Niño – a weather pattern that spurs warmer air temperatures in Australia,” Hughes writes in the Conversation. “But as summers grow hotter under climate change, we no longer need an El Niño to trigger mass bleaching at the scale of the Great Barrier Reef... The gap between recurrent bleaching events is shrinking, hindering a full recovery.”
The bleaching events in 2016 and 2017 were followed by an 89 percent drop in new coral larvae, and a noticeable change in the species of coral that were able to recover. Iconic branching and table corals that were the Great Barrier Reef’s dominant species declined by 93 percent, replaced by hardier brain corals that are less valuable to the quarter of marine species that rely on reefs for food and shelter.
This year’s bleaching is an “absolute tragedy, [but] it’s one we’ve been expecting,” Hoegh-Guldberg tells Science News. He notes that coral reefs that have been less affected by climate change will be a valuable resource for conservation efforts.
But Hughes tells Science News, “The problem with that approach is that we are running out of reefs that haven’t yet bleached.”