An aerial view of the Great Barrier Reef off the coast of the Whitsunday Islands, along the central coast of Queensland, in November 2014. (Sarah Lai/AFP/Getty Images)

Nearly three-quarters of the world’s most majestic coral reefs have suffered severe and repeated heat stress in the past three years, a United Nations report found Friday.

The report, prepared for the U.N. Educational, Scientific and Cultural Organization’s (UNESCO) World Heritage Centre by a group of coral scientists, found damage in 21 of 29 reefs on the organization’s list of World Heritage sites, or 72 percent, including Australia’s Great Barrier Reef.

That is the broad time span of a global bleaching event, which ran from 2014 through 2017 and hit the Great Barrier Reef in two successive years, causing devastating levels of coral death.

“Coral mortality during the third global bleaching event has been among the worst ever observed, including at World Heritage reefs; e.g., Great Barrier Reef (Australia), Papahānaumokuākea (USA) and Aldabra Atoll (Seychelles),” noted the report.

World Heritage sites are considered “of outstanding value to humanity,” according to UNESCO. They range from Stonehenge to the pyramids to Greenland’s most dramatic fjord, but also include many marine areas containing particularly extensive coral reefs.

UNESCO warned that without action to stop climate change, the reefs could “disappear” by the end of the century.

The document finds that only if the world somehow hits the most ambitious target in the Paris climate agreement — holding warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial temperatures — will there be “a chance of retaining coral-dominated communities for many reef locations around the globe.”

But scientists have agreed that 1.5 C is a near-impossible target, given current carbon dioxide emissions. That means, in effect, that coral reefs could be some of the first ecosystems to become casualties of rapid climate change.

Lowering emissions could help many reefs, putting off a time when coral bleaching could be an almost annual event.

That in turn would buy time to find out a way to save them.

On a business as usual scenario, in contrast, annual bleaching could hit some key reefs as soon as the 2030s.

“Drastic reductions in CO2 emissions are essential — and the only real solution — to giving coral reefs on the World Heritage List a chance to survive climate change,” the report said.