International
A coral reef at the Palmyra Atoll National Wildlife Refuge in the Pacific Ocean. JIM MARAGOS/USFWS

As Oceans Warm, Tropical Corals Seek Refuge in Cooler Waters Yale Environment 360

Due to soaring temperatures, tropical coral reefs are facing a bleak future. But recent research shows that some of these corals are migrating to cooler subtropical seas, offering a measure of hope that these ecosystems can survive the existential threat of climate change.

From the shores of Florida to the islands of Japan, from the Midway atoll to southern Australia, an unheralded ecological regeneration may be underway. Reef-building coral, which has become ever more threatened in the superheated waters of the tropics, is making a bid for survival by migrating to subtropical climes that meet its temperature requirements.

The coral reefs of the tropics have looked doomed. Bleached by marine heatwaves, suffering mass die-offs, and stuck to the sea bed, they have no obvious escape as the oceans warm. Some experts say they will be gone by mid-century, the first great ecosystem casualty of the climate emergency.

But the news is not entirely grim. It turns out that young corals can be surprisingly mobile, able to move in ocean currents, if their homes become inhospitable, and relocate to more equable waters. “I do believe there is a glimmer of hope for them,” says Nichole Price of the Bigelow Laboratory for Ocean Sciences in Maine, the lead author of the first global study of their sporadic recovery.

Marine ecologists are reporting migration of tropical coral into subtropical regions, part of a wider “tropicalization” of ocean ecosystems as species seek cooler waters away from the Equator. While struggling in their former habitat, they are proliferating between 20 and 35 degrees north and south of the Equator, with young refugee corals creating new reefs hundreds of miles from home.

With coral on the move, the map of the world’s reef-building tropical coral is being redrawn.

Price cautions that the migration does not come close to compensating for tropical decline. The percentage increase in the establishment of new corals in the subtropics, while encouraging, is from a low level. “Many more corals are being lost near the equator than are migrating to the subtropics,” she says. So overall, there has been a global decline in recruitment of 82 percent over the past four decades. But, even so, wipe-out may not loom.

In southern Japan, for example, at 33 degrees north, more than 70 coral species now occupy most of Tatsukushi Bay. In the United States, staghorn and elkhorn corals are extending around the Florida peninsula into the Gulf of Mexico. And in Australia, coral appear to be migrating south from the Great Barrier Reef to the coast of New South Wales around Sydney, at 30 degrees north.

This has been going on for a while, but until now nobody had assembled the complete picture. Price’s six-nation study, published last month, finds that with coral on the move, the map of the world’s reef-building tropical coral is being redrawn.

Her study has two headline findings. First the bad news: The recruitment of new young coral on tropical reefs has declined by a staggering 85 percent since the 1970s. Even reefs that appear healthy are simply not reproducing. But the good news is that there has been a 78 percent increase in recruitment on sites studied outside the tropics. Coral recruitment has been “shifting poleward,” Price concludes.

Corals grow alongside seaweed in temperate waters off southern Japan.
Corals grow alongside seaweed in temperate waters off southern Japan. SOYOKA MUKO/NAGASAKI UNIVERSITY

Tropical coral reefs are among the most complex ecosystems in nature. Submerged within a few feet of the sea’s surface, they are drenched in sunlight, which drives their ecology. Most reefs host a dazzling array of creatures: worms and snails, limpets and conches, crabs and eels, sea cucumbers, and sharks, all consuming and being consumed.

Reefs grow either as platforms extending from shorelines, or upwards from submerged mountains on the seabed. The Great Barrier Reef off Australia is more than 1,400 miles long, and took an estimated 500,000 years to reach that size. The Enewetak atoll in the Marshall Islands extends down more than half a mile and is the product of some 60 million years of growth.

The corals that make these reefs are soft-fleshed creatures, related to sea anemones. But they produce hard exoskeletons that accumulate in the billions and fuse to rocky seabeds, creating reefs. Their famous kaleidoscopic color is provided by microscopic algae that live inside the translucent corals in a symbiotic relationship. In return for shelter, the algae provide the corals with their main source of nourishment.

Coral reefs occupy only about 1 percent of the world’s oceans — from the Caribbean to the “coral triangle” centered on Indonesia in Southeast Asia — but they are home to about a quarter of all marine species. They have survived destructive fishing, pollution, mining for construction materials, the anchors of cruise ships, and even nuclear bomb tests. But climate change has proven to be their nemesis.

If water temperatures rise as little as 2 degrees Fahrenheit above normal for periods of more than a few weeks, the stressed coral expel their algae, and the multi-colored reefs turn a ghostly white. This “bleaching” has reached epidemic proportions and is among the most unambiguous signs of ecological decline from global climate change.

Bleaching need not be lethal. But if the waters do not cool enough to allow the symbiotic algae to return within a few weeks, the corals starve to death. And death may come more quickly if the temperatures are sufficiently high, as happened on the Great Barrier Reef in 2016, says Tracy Ainsworth of the University of New South Wales.

Tropical corals could be gone as early as mid-century. But research published in recent weeks suggests that all may not be lost. Christian Voolstra, a geneticist at the University of Konstanz, in Germany, showed that some corals can adapt to changing environments by expelling one lot of symbiotic algae and adopting a new set better attuned to new conditions. Price’s findings of mass migration by refugee coral from overheated tropical reefs suggest a whole new route to survival.

Read full Yale Environment article . . .

Fred Pearce is a freelance author and journalist based in the U.K. He is a contributing writer for Yale Environment 360 and is the author of numerous books, including The Land Grabbers, Earth Then and Now: Amazing Images of Our Changing World, and The Climate Files: The Battle for the Truth About Global Warming. MOREABOUT FRED PEARCE →