SYDNEY, Australia — When Terry Hughes surveyed the Great Barrier Reef four years ago from a small plane, mapping the bleaching and death of corals from water warmed by climate change, he hoped such a rare and heartbreaking scene would not be repeated anytime soon.
But rising temperatures sent him back to the air in 2017, when the reef bleached again. Then he returned last month, leading to another devastating conclusion: The reef was being ravaged by bleaching yet again, this time across an even wider area.
“It’s the first time we’ve seen severely bleached reefs along the whole length of the reef, in particular, the coastal reefs,” said Professor Hughes, the director of the ARC Center of Excellence for Coral Reef Studies at James Cook University. “Those are bleached everywhere.”
New aerial data from Professor Hughes and other scientists released on Monday shows example after example of overheating and damage along the reef, a 1,500-mile natural wonder. The survey amounts to an updated X-ray for a dying patient, with the markers of illness being the telltale white of coral that has lost its color, visible from the air and in the water.
The mass bleaching indicates that corals are under intense stress from the waters around them, which have been growing increasingly hotter.
The world’s oceans, which absorb 93 percent of the heat trapped by the greenhouse gases that humans send into the atmosphere by burning fossil fuels, are warming up 40 percent faster on average than scientists estimated six years ago.
According to one recent study, 2019 was the hottest year on record for the world’s oceans.
For corals, the stress from these ever-warming waters usually leads to death. In the past, they might have recovered after the water cooled, a process that could take 10 to 15 years. Now, though, a wide range of species of coral reefs all over the world are experiencing mass die-offs.
Roughly 30 percent of the corals on the Great Barrier Reef died after the 2016 bleaching, which was the worst of five separate bleaching events since 1998.
This year’s bleaching — which was documented by surveying 1,036 sections of the reef — appears to rank second only to 2016, according to Professor Hughes.
“We’re surprised by the pace of this acceleration in bleaching,” he said. “We had a 14-year gap between 2002 and 2016, and now in five years we’ve had three severe events.”
The Great Barrier Reef is not entirely dead — it is large enough to support swaths of healthy coral. But in many areas of the reef, for miles and miles, corals that were once colorful are now white, brittle and broken, or gray and covered with unsightly bacteria.
They’re sensitive creatures. Corals are tiny polyps that gather algae that convert sunlight into food, forming colorful colonies that build a limestone structure — a reef — on which to live. They thrive in warm water, but only up to a point: Just two or three degrees Fahrenheit of excess warming can kill the tiny creatures.
Temperatures in February, during the Southern Hemisphere summer, were far above that. It was the warmest month on record for water temperatures near the reef, with readings in some places peaking at more than 5 degrees Fahrenheit above average for the time of year.
Professor Hughes said the heat waves of recent years were creating a cumulative effect that was drastically altering the makeup of the reef.
The species most likely to bleach and die are staghorn coral and other root and branch corals with spaces that allow many kinds of fish to swim and gain protection. The species that seem hearty enough to survive tend to be dome-like corals, known as brain corals, which play a role in protecting against coastal erosion but are less valuable to fish and other wildlife.
“This is a transition from high diversity and lots of species, to lower diversity, with fewer tougher species,” Professor Hughes said.
The ripple effect could be significant. Hundreds of millions of people get their protein primarily from reef fish like the coral trout, which is already being affected by the bleaching events on the Great Barrier Reef. Many scientists worry that the loss of that food supply could become a humanitarian crisis.
In the meantime, scientists who were conducting research in March before the coronavirus outbreak forced lockdowns said they had been saddened and frustrated by what they witnessed at sea.
Tracy Ainsworth, a marine biologist at the University of New South Wales, was working with a team on Heron Island, in the southern section of the Great Barrier Reef that bleached this year for the first time.
The corals started to show signs of stress in January, she said, around the same time that Australia’s bush-fire crisis reached its apex.
After a stretch of cooler weather in February, the overheating continued from late February through March, with scientists finding dead corals in shallow reef habitats, and bleaching in deeper reef slopes that were believed to be better protected.
It was in many ways what Professor Ainsworth expected.
“It’s frustrating to still see reports of bleaching as a surprise,” she said.
She noted that scientists had warned 20 years ago that coral reefs would be at risk if humans did not address climate change, adding that many people who lived near them believed the science back then and are now demanding to know why more isn’t being done.
“We are going to keep reporting decline and recording change on reefs and telling people what they already know about how corals die,” Professor Ainsworth said. “Isn’t it time we started telling them something more than that?”