Scientists Predict Most Extensive Coral Bleaching Event on Record

The world’s coral reefs are in the throes of a global bleaching event caused by extraordinary ocean temperatures, the National Oceanic and Atmospheric Administration and international partners announced Monday.

It is the fourth such global event on record and is expected to affect more reefs than any other. Bleaching occurs when corals become so stressed that they lose the symbiotic algae they need to survive. Bleached corals can recover, but if the water surrounding them is too hot for too long, they die.

Coral reefs are vital ecosystems: limestone cradles of marine life that nurture an estimated quarter of ocean species at some point during their life cycles, support fish that provide protein for millions of people and protect coasts from storms. The economic value of the world’s coral reefs has been estimated at $2.7 trillion annually.

“This is scary, because coral reefs are so important,” said Derek Manzello, the coordinator of NOAA’s Coral Reef Watch program, which monitors and predicts bleaching events.

The news is the latest example of climate scientists’ alarming predictions coming to pass as the planet heats. Despite decades of warnings from scientists and pledges from leaders, nations are burning more fossil fuels than ever and greenhouse gas emissions continue to rise.

Substantial coral death has been confirmed around Florida and the Caribbean, particularly among staghorn and elk horn species, but scientists say it’s too soon to estimate what the extent of global mortality will be.

To determine a global bleaching event, NOAA and the group of global partners, the International Coral Reef Initiative, use a combination of sea surface temperatures and evidence from reefs. By their criteria, all three ocean basins that host coral reefs — the Pacific, Indian and Atlantic — must experience bleaching within 365 days, and at least 12 percent of the reefs in each basin must be subjected to temperatures that cause bleaching.

Currently, more than 54 percent of the world’s coral area has experienced bleaching-level heat stress in the past year, and that number is increasing by about 1 percent per week, Dr. Manzello said.

He added that within a week of two, “this event is likely to be the most spatially extensive global bleaching event on record.”

Each of the three previous global bleaching events has been worse than the last. During the first, in 1998, 20 percent of the world’s reef areas suffered bleaching-level heat stress. In 2010, it was 35 percent. The third spanned 2014 to 2017 and affected 56 percent of reefs.

The current event is expected to be shorter-lived, Dr. Manzello said, because El Niño, a natural climate pattern associated with warmer oceans, is weakening and forecasters predict a cooler La Niña period to take hold by the end of the year.

Bleaching has been confirmed in 54 countries, territories and local economies, as far apart as Florida, Saudi Arabia and Fiji. The Great Barrier Reef in Australia is suffering what appears to be its most severe bleaching event; about a third of the reefs surveyed by air showed prevalence of very high or extreme bleaching, and at least three quarters showed some bleaching.

“I do get depressed sometimes, because the feeling is like, ‘My God, this is happening,’” said Ove Hoegh-Guldberg, a professor of marine studies at the University of Queensland who published early predictions about how global warming would be catastrophic for coral reefs.

“Now we’re at the point where we’re in the disaster movie,” he said.

The most recent confirmation of widespread bleaching, prompting Monday’s announcement, came from the Western Indian Ocean, including Tanzania, Kenya, Mauritius, the Seychelles and off the western coast of Indonesia.

Swaleh Aboud, a coral reef scientist at CORDIO East Africa, a research and conservation nonprofit group based in Kenya and focused on the Indian Ocean, said coral species that are known to be thermally resistant are bleaching, as are reefs in a cooler area considered to be a climate refuge.

Recently he visited a fishing community in Kenya called Kuruwitu that has worked to revive its reef. Many of the restored coral colonies had turned ghostly white. Others were pale, apparently on their way.

“Urgent global action is necessary to reduce future bleaching events, primarily driven by carbon emissions,” Mr. Aboud said.

Scientists are still learning about corals’ ability to adapt to climate change. Efforts are underway to breed coral that tolerate higher temperatures. In a few places, including Australia and Japan, coral appear to be migrating poleward, beginning to occupy new places. But scientists say a variety of factors, such as how much light penetrates the water and the topography of the sea floor, make such migration limited or unlikely in much of the world. Plus there’s the problem of ocean acidification; as seawater absorbs carbon dioxide from the atmosphere, it becomes more acidic, making it harder for coral to build and maintain reefs.

Dr. Hoegh-Guldberg, who has studied the impact of climate change on coral reefs for more than three decades, was an author of a 2018 report from the Intergovernmental Panel on Climate Change that found the world would lose the vast majority of its coral reefs at 1.5 degrees Celsius of warming, and virtually all at 2 degrees. Current pledges by nations put the Earth on track for about 2.5 degrees by 2100. Still, he has not lost hope.

“I think we will solve the problem if we get up and fight to solve the problem,” Dr. Hoegh-Guldberg said. “If we continue to pay lip service but not get on with the solutions, then we’re kidding ourselves.”