SOUTH ARI ATOLL, Maldives — There were startling colors here just a year ago, a dazzling array of life beneath the waves. Now this Maldivian reef is dead, killed by the stress of rising ocean temperatures. What's left is a haunting expanse of gray, a scene repeated in reefs across the globe in what has fast become a full-blown ecological catastrophe.

The world has lost roughly half its coral reefs in the last 30 years. Scientists are now scrambling to ensure that at least a fraction of these unique ecosystems survives beyond the next three decades. The health of the planet depends on it: Coral reefs support a quarter of all marine species, as well as half a billion people around the world.

"This isn't something that's going to happen 100 years from now. We're losing them right now," said marine biologist Julia Baum of Canada's University of Victoria. "We're losing them really quickly, much more quickly than I think any of us ever could have imagined."

Even if the world could halt global warming now, scientists still expect that more than 90 percent of corals will die by 2050. Without drastic intervention, we risk losing them all.

"To lose coral reefs is to fundamentally undermine the health of a very large proportion of the human race," said Ruth Gates, director of the Hawaii Institute of Marine Biology.

Coral reefs produce some of the oxygen we breathe. Often described as underwater rainforests, they populate a tiny fraction of the ocean but provide habitats for one in four marine species. Reefs also form crucial barriers protecting coastlines from the full force of storms.

They provide billions of dollars in revenue from tourism, fishing and other commerce, and are used in medical research for cures to diseases including cancer, arthritis and bacterial or viral infections.

"Whether you're living in North America or Europe or Australia, you should be concerned," said biologist Ove Hoegh-Guldberg, director of the Global Change Institute at Australia's University of Queensland. "This is not just some distant dive destination, a holiday destination. This is the fabric of the ecosystem that supports us."

And that fabric is being torn apart.

Corals are sensitive to temperature fluctuations, and are suffering from rising ocean temperatures and acidification, as well as from overfishing, pollution, coastal development and agricultural runoff.

A temperature change of 1.8 to 3.6 degrees Fahrenheit can force coral to expel the algae, leaving their white skeletons visible in a process known as "bleaching."

Bleached coral can recover if the water cools, but if high temperatures persist for months, the coral will die. Eventually the reef will degrade, leaving fish without habitats and coastlines less protected from storm surges.

The first global bleaching event occurred in 1998, when 16 percent of corals died. The problem spiraled dramatically in 2015-2016 amid an extended El Nino natural weather phenomenon that warmed Pacific waters near the equator and triggered the most widespread bleaching ever documented. This third global bleaching event, as it is known, continues today even after El Nino ended.

Headlines have focused on damage to Australia's famed Great Barrier Reef, but other reefs have fared just as badly or worse across the world, from Japan to Hawaii to Florida.

Around the islands of the Maldives, an idyllic Indian Ocean tourism destination, some 73 percent of surveyed reefs suffered bleaching between March and May 2016, according to the country's Marine Research Center.

"This bleaching episode seems to have impacted the entire Maldives, but the severity of bleaching varies" between reefs, according to local conditions, said Nizam Ibrahim, the center's senior research officer.

Worst hit have been areas in the central Pacific, where the University of Victoria's Baum has been conducting research on Kiritimati, or Christmas Island, in the Republic of Kiribati. Warmer water temperatures lasted there for 10 months in 2015-2016, killing a staggering 90 percent of the reef.

Baum had never seen anything like it.

"As scientists, we were all on brand new territory," Baum said, "as were the corals in terms of the thermal stress they were subjected to."