SYDNEY — The Great Barrier Reef has long been one of the world’s most magnificent natural wonders, so enormous it can be seen from space, so beautiful it can move visitors to tears.

But the reef and the profusion of sea creatures living near it are in profound trouble.

Huge sections of the Great Barrier Reef, stretching across hundreds of miles of its most pristine northern sector, were recently found to be dead, killed last year by overheated seawater. More southerly sections around the middle of the reef that barely escaped then are bleaching now, a potential precursor to another die-off that could rob some of the reef’s most visited areas of color and life.

“We didn’t expect to see this level of destruction to the Great Barrier Reef for another 30 years,’’ said Terry P. Hughes, director of a government-funded center for coral reef studies at James Cook University in Australia and the lead author of a paper on the reef that is being published Thursday as the cover article of the journal Nature. “In the north, I saw hundreds of reefs — literally two-thirds of the reefs were dying and are now dead.’’

The damage to the Great Barrier Reef, one of the world’s largest living organisms, is part of a global calamity that has been unfolding intermittently for nearly two decades and seems to be intensifying. In the paper, dozens of scientists described the recent disaster as the third worldwide mass bleaching of coral reefs since 1998, but by far the most widespread and damaging.

The state of coral reefs is a telling sign of the health of the seas. Their distress and death are yet another marker of the ravages of global climate change.

If most of the world’s coral reefs die, as scientists fear is increasingly likely, some of the richest and most colorful life in the ocean could be lost, along with huge sums from reef tourism. In poorer countries, lives are at stake: Hundreds of millions of people get their protein primarily from reef fish, and the loss of that food supply could become a humanitarian crisis.

With this latest global bleaching in its third year, reef scientists say they have no doubt as to the responsible party.

They warned decades ago that the coral reefs would be at risk if human society kept burning fossil fuels at a runaway pace, releasing greenhouse gases that warm the ocean. Emissions continued to rise, and now the background ocean temperature is high enough that any temporary spike poses a critical risk to reefs.

“Climate change is not a future threat,’’ Hughes said. “On the Great Barrier Reef, it’s been happening for 18 years.’’

Corals require warm water to thrive, but they are exquisitely sensitive to extra heat. Just two or three degrees Fahrenheit of excess warming can sometimes kill the tiny creatures.

Globally, the ocean has warmed by about 1.5 degrees Fahrenheit since the late 19th century, by a conservative calculation, and a bit more in the tropics, home to many reefs. An additional kick was supplied by an El Niño weather pattern that peaked in 2016 and temporarily warmed much of the surface of the planet, causing the hottest year in a historical record dating to 1880.

It was obvious last year that the corals on many reefs were likely to die, but now formal scientific assessments are coming in. The paper in Nature documents vast coral bleaching in 2016 along a 500-mile section of the reef north of Cairns, a city on Australia’s eastern coast.

Bleaching indicates that corals are under heat stress, but they do not always die and cooler water can help them recover. Subsequent surveys of the Great Barrier Reef, conducted late last year after the deadline for inclusion in the Nature paper, documented that extensive patches of reef had in fact died, and would not be likely to recover soon, if at all.

Hughes led those surveys. He said that he and his students cried when he showed them maps of the damage, which he had calculated in part by flying low in small planes and helicopters.

Hughes said he hoped the die-off this time would not be as serious as last year’s, but “back-to-back bleaching is unheard-of in Australia.’’ The central and southern part of the reef had already been badly damaged by human activities like dredging and pollution.

The Australian government has tried to combat these local threats with its Reef 2050 plan, restricting port development, dredging and agricultural runoff, among other risks. But Hughes’s research found that, given the high temperatures, these national efforts to improve water quality were not enough.