In waters off Geomun Island, blooming coral is a wondrous sight — and a warning

GEOMUN ISLAND — Fifteen meters (49 feet) below the surface of the waters off Geomun Island, everything goes quiet, save for the hiss of a regulator and the occasional flick of a fin. The seafloor here doesn't look like a coral reef most people picture — sparse and pale, clinging to rock.

Instead, great colonies of soft coral rise up out of the dark like something out of an overgrown garden. A cluster of carnation coral — usually delicate, branching out — has grown so large that a diver hovering beside it looks almost small by comparison, its arms stretching up nearly to the height of a person.

It's a striking sight. It is also, marine researchers say, a warning sign.

Located off the southern coast, part of Yeosu, South Jeolla, Geomun Island isn't easy to reach. It takes about three hours by boat from Nokdong Port in Goheung before the island comes into view. On a recent Tuesday, the island was quiet — quieter, even, than the empty ferry route suggested. At the pier, there seemed to be more cats than people.

"We've had an unusual number of high-wave advisories since last year, so the boat route keeps getting cut off, and fewer visitors have been coming to the island," said Jeong Min-gyo, who runs a diving business on the island. "It's because the sea has gotten rougher."

Jeong wasn't the only one at the dock. Members of a national parks conservation organization stepped off one boat and hurried to gather their diving gear before climbing aboard Jeong's boat. They'd come to the island to survey how its underwater ecosystem is changing.

"Geomun Island sits along the path of the Tsushima Current, a branch of the Kuroshio Current, so a wide range of species are concentrated here," said Jeong In-cheol, the group's secretary general. The Kuroshio is a warm ocean current that flows north from the tropics past Japan, and the Tsushima Current is the branch that splits off into the waters between Korea and Japan, carrying that warmth — and the marine life that comes with it — into the region.

"It's one of the clearest places to see the effects of ocean climate change."

Beneath the surface, that change was easy to spot.

"As water temperatures have risen sharply, the soft coral here has been growing at an extremely fast rate for the past three to four years, and more subtropical fish species have taken up residence," Jeong In-cheol said. "We've also found coral that appears to have dissolved and died from heat stress."

In waters as hot as 30 degrees Celsius (86 degrees Fahrenheit), toxic fish species native to subtropical waters are being observed more frequently as well. For instance, divers recently documented a large school of rabbitfish, which carry venomous spines capable of delivering a painful sting to humans, stripping a kelp forest bare. The species travels in groups and feed on seaweed, and their feeding activity is known to surge once water temperatures surpass 26 degrees Celsius.

"If they strip a marine forest bare the way a swarm of locusts sweeps through a cornfield, it could set off a chain reaction across the marine ecosystem — including the loss of spawning grounds for fish," Jeong In-cheol said.

Korean seas are getting hotter, fast 

Geomun Island isn't an outlier. The seas around the Korean Peninsula are warming faster than almost anywhere else in the world. According to the National Institute of Fisheries Science, water temperatures in Korean waters rose 1.6 degrees Celsius (2.9 degrees Fahrenheit) over the past 58 years, from 1968 to 2025 — more than double the global average increase of 0.76 degrees Celsius.

The pace has only accelerated. From 2016 to 2025, temperatures rose by 0.09 degrees Celsius a year on average — three times faster than the warming rate from 1991 to 2025. This year, water temperatures in the first half already set a new record, and the high-temperature conditions have persisted through summer.

The warming has reshaped the marine food web from the bottom up. Phytoplankton, the base of the ocean food chain, has declined 58 percent compared with the late 1990s, while sightings of jellyfish and other toxic species that tend to appear in summer have risen 60 percent.

"As marine heat waves become more frequent and more intense, they're affecting plankton at the bottom of the food chain, which is reducing the ocean's basic productivity," said Han In-seong, head of the institute's climate change research division. "As those effects move up the food web, they can eventually hit the fishing industry as well."

Locals suffer 

Island residents on the front lines of the climate crisis have felt the impact directly. As fish farms suffer growing losses from high water temperatures, some operators have shut down entirely, while others have given up fishing as catches have plummeted.

Geomun Island local Jeong Min-gyo is one of them.

"I used to run a tuna cage farm, but a red tide came through and wiped out most of the tuna I was raising, so I stopped," the now-diving school owner said, referring to a harmful algae bloom.

As making a living on the island has grown harder, more residents have left altogether. The population of Samsan-myeon, the township that includes Geomun Island, in Yeosu, has fallen to 1,948 — down about 17 percent from 2,344 a decade ago. The area once had four elementary schools; now there's just one, and even that school is at risk of closing for lack of students.

The island's shrinking population has made its environmental problems worse. There is no longer a large enough work force to keep up with the marine debris that keeps washing ashore.

Experts warn that population decline and an aging populace have left islands like Geomun Island without the resilience to withstand such rapid climate change.

"The climate is changing so fast and so unpredictably that adapting to it has become genuinely difficult," said Hong Sun-kee, a professor at Mokpo National University and director of its Institution for Marine and Island Cultures. "Preventing these islands from disappearing will require a comprehensive response — one that accounts for climate change alongside the ecological and population crises together."

BY CHON KWON-PIL [[email protected]]