Scientists have declared a mass global bleaching event. But some reefs are still hanging on — and even thriving.
Coral reefs around the world are turning white and dying.
Today scientists from the National Oceanic and Atmospheric Administration declared the world is currently experiencing its fourth global bleaching event on record.
Bleaching is bad. During long spells of extreme heat, the relationship between coral and the algae that live inside its tissues breaks down. Those algae give the corals most of their food and their brilliant color in exchange for nutrients and a place to absorb sunlight. White, or “bleached,” corals aren’t dead; they are starving to death.
Since early last year, NOAA scientists have confirmed mass bleaching in the Atlantic, Pacific, and Indian oceans, including along the coastlines of Florida, the Caribbean, and the Great Barrier Reef. The first bleaching event on a global scale was in 1998. “As the world’s oceans continue to warm, coral bleaching is becoming more frequent and severe,” Derek Manzello, a coral reef ecologist at NOAA, said Monday in a statement.
This is bleak for pretty much everyone on the planet. Coral reefs operate like seawalls, helping minimize flooding during hurricanes. They provide homes to roughly a quarter of all marine species including the fish people eat at one point or another. And they are an engine of the tourism economy in many places, such as the Florida Keys, Mexico, and Australia.
But amid all this destruction — which will almost certainly get worse in the decades to come — there are still some signs of hope. Not all coral reefs are dying. Indeed, some are teeming with life.
In March, a team of marine biologists was diving off the coast of Cambodia when they witnessed something that filled them with awe.
An explosion of life
Once a year, after dark, a bit of magic happens in the ocean. Within tropical waters worldwide, large chunks of coral — those colorful rocklike structures in shallow, coastal seas, each a colony of living animals — start puffing out hundreds of little pearl-sized balls. Some are pink. Others are red, orange, or yellow. For a few minutes, the ocean is a snow globe, and then the balls float away.
This phenomenon, known as spawning, is how many corals reproduce. Each ball is a bundle of eggs and sperm from an individual coral colony. Different colonies of the same species somehow know how to spawn on the same day and same time, so their eggs and sperm can meet and form baby corals.
Spawning is incredibly hard to observe. Again, it happens only once a year, and often only for a few minutes at night. Plus, bleached corals are less likely to spawn successfully.
Yet, in March, a team of marine biologists got lucky: They witnessed a massive spawning event off the coast of Cambodia, in the Gulf of Thailand. Not long after sunset, several different kinds of coral filled the water with pearls.
The team, led by Fauna & Flora International, an environmental group, was able to capture the event on video, shown in a series of clips below.
“It was like it was snowing,” Tharamony Ngoun, a marine species and ecosystems officer at Fauna & Flora, who observed the spawn, told Vox. “It was so amazing.”
Spawning on Cambodia’s reefs is not only thrilling to witness (I’ve been lucky enough to see coral spawning before, though not in Cambodia). It also offers hope for these important ecosystems as many of them are under siege.
Globally, coral reefs have declined by half since the 1950s, largely due to climate change. Indeed, the leading scientific authority on climate change suggests that if the world warms by 1.5 degrees Celsius, relative to pre-industrial times, coral reefs could decline by 70 percent to 90 percent. And we’re basically already there.
The reefs in Cambodia and in the broader East Asian region, however, appear to be bucking this trend. Surveys indicate that they haven’t declined in recent decades, perhaps because they’re more resilient to warming. Their secret to survival may ultimately help safeguard ailing reefs elsewhere.
Southeast Asia’s reefs are hanging on
The dire outlook for coral makes this spawning event even more special. While many reefs are disappearing, others are relatively healthy and capable of producing a new generation of corals.
“The coral is thriving,” said Matt Glue, a marine technical specialist at Fauna & Flora, which sent a team in March to try to observe spawning. “Everywhere we would go we would see more colonies that were spawning. It’s very hopeful.”
The reefs in Cambodia are not free of problems. Overfishing has diminished larger predators, like groupers, which help maintain the health of the reef. And although the amount of coral has remained relatively stable, contrasting global declines, it’s likely that more sensitive species have become less abundant and others more abundant, tweaking the makeup of the ecosystem.
Nonetheless, this reef does seem more resilient, according to Glue. While this region has experienced plenty of marine heat and some amount of bleaching, the heat typically doesn’t cause a mass die-off like what you see elsewhere.
The secret to the reef’s survival may be in the diversity of its corals. East Asia has a huge number of coral species and a lot of genetic diversity within individual species. The more varieties of coral a reef has, the more likely it is that some of them may have slightly more or less tolerance to various stresses, such as high temperatures. During a bout of severe warming, some coral colonies may die off, but others can take their place, Glue said.
Within East Asia, “high coral cover and diversity on the coral reefs within this critically important region may have conferred a degree of natural resistance to elevated [sea surface temperatures],” coral scientists wrote in a 2020 report.
Matt Glue/Fauna & Flora
What’s more is that these corals may help reefs elsewhere withstand the worsening wrath of climate change. Research has found that tolerance to heat is baked into the DNA of some coral colonies. And importantly, two heat-tolerant parents tend to produce heat-tolerant babies.
“If these corals are indeed unusually tolerant in whatever manner, the fact that they are actively producing larvae provides the direct possibility for those larvae to disperse to adjacent reef areas,” said Margaret Miller, one of the top coral experts in the US and research director at the conservation group Secore International. (She was not part of the team that observed the spawning.) In other words, all of that new spawn may help seed the ocean with more resilient corals.
“It feels really great to be part of this,” Glue said, of observing spawning with his team. “And hopeful — hopeful for the future of reefs in the Gulf of Thailand.”
Update, April 15, 11 am ET: This story was originally published on March 14 and has been updated with NOAA’s declaration of a fourth global bleaching event.