Coral reef bleaching has become more widespread, frequent and lethal in the last two decades, draining the color and life out of underwater coral gardens around the planet and leaving behind huge swaths of sea bottom spiked with ghostly reef skeletons.

Throughout the 2000s, grim reports of crumbling, pale corals multiplied, arriving from remote South Pacific atolls, the Indian Ocean, the Red Sea and, once again this year, from Australia's Great Barrier Reef, where the overheated Pacific Ocean scalded corals for the third time in five years. The renewed bleaching of the world's largest reef confirms that coral reefs globally are in big trouble, said scientists, many of whom reported that their dismay has been compounded by government failures to protect collapsing coral-based ecosystems despite decades of warnings.

In the Caribbean, global warming has already pushed some reef ecosystems over a climate cliff. Today's underwater landscapes are nearly unrecognizable for scientists and divers who started visiting them 50 years ago, said University of Windsor coral reef ecologist Peter Sale.

"What we are doing to coral reefs is akin to eliminating all the rainforests on the planet," he said. "Coral reefs are probably going to be the first globally distributed ecosystem wiped off the face of the Earth by humans."

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Corals bleach when the water they inhabit gets too warm, and they shed the pigmented algaes that provide them with food through photosynthesis. The affliction isn't always fatal. Depending on the intensity of the event, reefs can partly recover in a decade, but lately, the waves of bleaching have come so fast that there's no time for recovery, Sale said. Mass coral reef bleaching has become five times more common in the past 40 years, research shows.

Since the first global bleaching event in 1998 shocked coral scientists with its reach and intensity, subsequent die-offs affected reefs in regions with no known history of bleaching, including some in Hawaii. Sale said that, even if global warming is capped at the limit set by the Paris climate agreement, 90 percent of the planet's reefs will disappear.

"Reefs are toast if we let warming get much above 1.5 degrees Celsius (2.7 degrees Fahrenheit)," he said. "I am surprised at the fact that the bleaching of coral reefs has not evoked the response from society that I would have thought. Since 1998, scientists all assumed it would be a huge wakeup call to do something. We've been surprised and disappointed."

It's not for lack of trying. The most recent key reports from the International Panel on Climate Change included stark warnings about the reef death spiral and urged immediate, significant cuts of greenhouse gas pollution to avert their annihilation.

The world's failure to act decisively to avoid mass global coral extinctions is anguishing for Australian coral reef scientist Terry Hughes. His annual surveys of the decline of the Great Barrier Reef have led him to publicly spar with his own government and with Rupert Murdoch-owned Australian media when they downplay the damage caused by global warming or promote reef-threatening projects like the expansion of a coal-shipping terminal in North Queensland, where dredging would harm corals.

"I'm not sure I have the fortitude to do this again. It's heartbreaking to see the #GreatBarrierReef decline so fast," he wrote on Twitter, in between posting coral bleaching maps and photos of ghostly reef graveyards.

The Great Barrier Reef is a sentinel for global coral reef health simply because of its sheer size, which accounts for 10 percent of total reef area globally, and because it's made up of more than 600 hard and soft coral species, living in habitats that range from coastal shallows to deep water. It's home to 100 species of jellyfish, 3,000 varieties of molluscs, 133 varieties of sharks and rays and more than 30 species of whales and dolphins.

On the other side of the world, in Maryland, Mark Eakin has warned about coral bleaching for the last 15 years as head of the National Oceanic and Atmospheric Administration's Coral Reef Watch program. The current outlook from the program warns that bleaching conditions will shift north of the equator during the coming summer, with reefs in the far Western Pacific and Indian oceans particularly vulnerable.

A recent study posted by NOAA concluded that, even absent bleaching, marine heat waves can be very damaging to reefs, "decaying their skeletons within weeks." Other research shows the frequency of marine heat waves has increased by 34 percent in the last century, and that they are a primary driver of bleaching.

Coral Arks?

Despite the grim projections for reefs, Eakin said he's not giving up hope. While the program's coral bleaching forecasts haven't moved the climate policy needle, the information was useful for reef conservationists in Hawaii in 2015, as the global bleaching wave swept toward the islands.

NOAA issued a warning for Hawaii, and the initial reaction was disbelief, Eakin said, because the threatened zones covered areas with no history of bleaching. But local ocean managers took the warning seriously and, with the help of citizen scientists, collected samples of rare and highly endangered corals before the bleaching hit.

By the time the episode subsided, some of the corals had vanished from the wild, and the collected specimens were all that remained, he said. Micro-fragments of some of the rescued coral were propagated at a nursery and, within four years of the bleaching episode, replanted at their original location. They still appear to be doing well, showing that restoration efforts can have significant local benefits, like maintaining reef diversity.

But rescue and restoration projects aren't going to save coral reefs on a global scale. Reefs globally cover an area the size of Wyoming, Eakin estimated, while the total area of restoration projects might be comparable to the size of a backyard garden.

Another example of reef rescue is the philanthropically-funded 50 Reefs Initiative, an $86 million project to identify and protect coral communities that can survive climate change and help them repopulate nearby areas.

Tracking reef conditions in real time is the key to successful adaptive management, Eakin said. Data from the reef watch program can help identify suitable areas for future restoration efforts and protect vulnerable, heat-stressed reefs from other impacts that magnify bleaching, such as pollution and recreation. He said that, in 2016, Indonesia proactively closed many sport diving sites based on coral bleaching warnings.

But coral reefs are so spread out and fragmented that it's surprisingly hard to accurately measure their total size, much less quantify the damage from the last few decades of repeated bleaching. The best recent estimates identify about 98,000 square miles of coral reefs globally, Eakin said, and between 2014 and 2017 about a quarter of that—an area the size of West Virginia—experienced bleaching conditions.

"There are problems with any calculation like this," he said. "We don't really even know the total area of all coral reefs. We have multiple estimates. Most coral reefs around the world are not regularly monitored as they are too remote and in countries with limited monitoring programs."

An international reef conservation program, the Global Coral Reef Monitoring Network, has been stalled for lack of funds and governance since 2009, hindering efforts to compile a global database. There are no good estimates for total reef damage, or the rate of change over the last 10 years, partly because of the complexity of calculating the cumulative impacts of multiple bleaching events on a reef. The waves of bleaching are intensifying so fast that researchers can barely keep up.

Ecosystems in the Sea and Communities on Land at Risk

Healthy corals at the north end of the Hawaiian archipelago highlight what's at risk. Robust coral reefs protect the shorelines behind them from waves and storm surges that are being intensified by rising sea levels. The reefs also sustain populations of fish that are important local sources of food.

When waves break over the corals, algae and plankton are stirred up toward swarms of small fish near the surface. One of the world's biggest schools of shadowy manta rays glides just beneath the waves to feed. From above, fairy terns, known locally as manu-o-Kū, plunge into the roiling surf to catch the small fish for their chicks. The bright white birds long served as navigation aids for indigenous mariners, who followed their flight paths to land.

Similar reef-based ecosystems exist in all the world's tropical and subtropical oceans. In total, as many as 1 billion people depend on fisheries associated with reefs as their primary source of protein, Eakin said.

"When those reefs go, entire communities lose a major source of food," he said. "Communities with healthy reefs are also more protected from waves due to storms and tsunamis than those with degraded reefs. In places like South Florida, the value of reefs as coastal protection is huge."

After the 2004 Indian Ocean tsunami that killed more than 200,000 people, Eakin noted, research showed that communities in Sri Lanka with degraded reefs suffered more damage than those with healthy reefs.

Recognizing these benefits, reinsurance companies have launched programs that pay hotels and other large, seaside developments in places like Mexico to protect their reefs, he said.

"This model is being discussed for U.S. reefs as well, and may be used to fund reef restoration," he added. "There are also discussions with FEMA and the (U.S. Army) Corps of Engineers about including reefs in green shoreline protection projects."

Accelerating Bleaching, Desperate Measures

Up until a few years ago, the worst bleaching generally coincided with ocean-warming cycles, but that wasn't so with the most recent case in Australia.

"The most frightening thing about this bleaching event is that it doesn't appear to be linked to El Niño," Eakin said. "Instead, it appears to be driven primarily by human-caused climate change. Because of the repeated bleaching of the Great Barrier Reef, there's been almost no recovery. Most of the hardest-hit reefs are still severely damaged."

There's some evidence that a few coral species are trying to migrate to more suitable areas, he said. Coral larvae have traveled poleward, spreading some species north into new parts of Japan, Florida and the Gulf of Mexico, and down into southwestern Australia.

"However, this doesn't mean reefs are moving, he said. "These are weedy species and have evolved to recruit far and wide. A few weedy species don't make a reef.".

Although exact global reef numbers may be uncertain, the trends are clear, said Verena Schoepf, a University of Amsterdam coral scientist who has recently studied reefs that persist in extreme conditions.

Studies show that there won't be any reefs left untouched by bleaching in 20 years, so Schoepf said scientists charged with preserving them need to keep all options open, including selective breeding and transplantation of heat-tolerant corals that may be able to survive in Earth's future climate conditions.

"Even if we cap global warming by 2050 we can only speculate what will happen, but we know we will have lost a lot of reefs globally by then," she said. "There will only be a few left and they will look very different from today's reefs. The reefs that do survive will have fewer species and fewer of the big branching corals that help protect the rest of the reef from storm damage."

If possible, restoration efforts should be partly focused on some of those key reef species that could provide a foundation for long-term recovery, she said. Paying attention to lesser-known reefs that persist in suboptimal conditions could also pay off in the long run. In some of the places she's worked, there are "new corals growing on degraded reefs that are doing pretty well," she said. "We should explore how they are doing that."

Sale, the University of Windsor reef ecologist, said that technical solutions won't solve the root problem—climate change driven by a societal disregard for the biosphere—but he has some hope for corals.

"The one thing that does keep me optimistic about corals is, they've been around for 400 million years, since the Ordovician period," he said.

Even during extreme climate periods lasting millions of years when there were no reefs, corals persisted as individual organisms floating in the oceans.

"The reefs we know, however, won't be around for my grandchildren," he said. He added that there was "every reason to believe reefs will appear again some time in the future. But first we have to get an ocean that's suitable for reef development."