Korea's seas warming at twice the global rate as ocean temperatures hit record highs

'The heat we feel now is not the real heat'

South Korea has yet to shake off the summer swelter even after Cheo-seo — the traditional solar term marking the start of cooler weather — has passed. But the heat Koreans are enduring represents only a fraction of what climate change is actually producing.

In numbers, that fraction is roughly 10 percent. Where does the remaining 90 percent go? Into the ocean — the planet's vast heat reservoir.

This summer, global sea surface temperatures reached their highest level since satellite observations began in 1979. What makes the record all the more striking is that the peak arrived well before the season typically associated with the warmest ocean readings.

The situation in Korean waters is particularly alarming. Sea surface temperatures around the peninsula are rising at roughly twice the global average rate.

The damage has been severe. With sea surface temperatures continuing to climb, millions of farmed fish have died en masse along the southern coast, underscoring the mounting toll of an overheating ocean.

According to the EU's Copernicus Climate Change Service (C3S), the daily average sea surface temperature across the world's oceans between 60 degrees south and 60 degrees north latitude reached 21.10 degrees Celsius on Saturday — surpassing the previous record of 21.09 degrees set in March 2024, the hottest year on record.

That is the highest reading in roughly 47 years since satellite observation data used for comparison began in 1979. Analysts say the timing of the record is itself extraordinary.

Global sea surface temperatures typically peak around March or April, after the Southern Hemisphere's summer — which covers most of the world's ocean area — draws to a close. This year, however, the annual high was recorded in August, a significant departure from the norm.

C3S said monthly sea surface temperatures have repeatedly hit or approached record highs this year, sustaining an abnormally warm state across the world's oceans.

The dominant driver is a powerful El Niño — a phenomenon in which sea surface temperatures in the central and eastern equatorial Pacific remain above normal for several months, disrupting temperature and precipitation patterns around the globe.

In its seasonal outlook released July 31, the World Meteorological Organization said the current El Niño is developing rapidly into a strong event and is expected to intensify further between August and October. The phenomenon is projected to peak in November and could push average temperatures even higher next year.

C3S said El Niño is adding heat on top of an ocean baseline already raised by decades of human-induced climate change. Abnormally high sea temperatures are now being recorded simultaneously across multiple ocean regions, not just in the equatorial Pacific where El Niño directly operates.

The situation around Korea is more severe. According to the National Institute of Fisheries Science's "2026 Climate Change Impact Briefing Book for the Ocean and Fisheries Sector," surface water temperatures in Korean waters rose by 1.60 degrees Celsius over the 58-year period from 1968 to 2025. The global average surface temperature increase over the same period was 0.76 degrees — meaning Korean waters have warmed more than twice as fast as the world average.

The pace of warming is also accelerating. The long-term rate of sea surface temperature increase in Korean waters from 1968 to 2025 averaged 0.0276 degrees Celsius per year. But looking only at the decade since 2016, the annual rate has climbed to about 0.09 degrees — roughly three times faster than the long-term trend.

This summer's record-breaking heat wave has been accompanied by a pronounced high-water-temperature event. Data released by the Korea Meteorological Administration on Monday showed that sea surface temperatures in Korean waters averaged 26.9 degrees Celsius during the week of Aug. 9–15, some 1.1 degrees above the seasonal norm. The agency forecast that water temperatures in the region will continue to rise from September through November, with a strong El Niño likely to develop.

The human cost is mounting. Fishermen are suffering livelihood-threatening losses, with mass die-offs of farmed fish from high water temperatures continuing along the southern coast even after Cheo-seo.

According to South Gyeongsang Province, about 1.106 million farmed fish — including Korean rockfish, mullet, filefish, black rockfish and olive flounder — died at 44 fish farms across three cities and counties, including Tongyeong and Hadong-gun, through Monday.

By species, Korean rockfish accounted for the largest share at 901,000. Mullet followed at 140,000, with filefish at 32,000, black rockfish at 20,000 and olive flounder at 13,000. This year also marked the first time sea squirts — 95 strings' worth — were killed by high water temperatures.

Farmed fish exposed to high water temperatures for extended periods generally eat less and suffer weakened immune systems. Warmer water also holds less dissolved oxygen. In the confined conditions of a fish farm, where large numbers of fish are packed into a small space, the combination of stress and oxygen depletion sharply raises the risk of mass die-offs.

The government has moved to contain the damage. The Ministry of Oceans and Fisheries has raised and maintained its high-water-temperature disaster alert at the "Serious I" level. Concerned that a prolonged high-water-temperature event could disrupt the production and distribution of key farmed species such as flounder and Korean rockfish, the ministry has also begun monitoring supply and price trends.

In effect, even this damage is only the surface of the problem. The ocean acts as the planet's enormous heat sink, absorbing the energy accumulating in the Earth's system. The trouble is that water warms more slowly than air — but once heated, it cools far more slowly too. Heat stored in the ocean exerts a lasting influence on climate and ecosystems alike.

Marine heat waves — prolonged periods of abnormally warm ocean temperatures — are also on the rise. According to C3S, the number of days per year affected by marine heat waves worldwide has more than tripled compared with the early 1990s.

As sea temperatures rise, more water evaporates from the ocean surface, pumping greater amounts of water vapor into the atmosphere. When that moisture encounters weather systems such as fronts or low-pressure areas, it can fuel intense rainfall. Warm seas also supply the energy that drives the development of typhoons and other powerful storms.

Sea levels are rising as well. Water expands in volume as it warms — a process known as thermal expansion. As the ocean heats up, the water itself swells, contributing to sea level rise and increasing the long-term risk of coastal flooding.

Ultimately, the significance of this record lies not simply in the fact that ocean temperatures hit 21.10 degrees Celsius on a single day. The deeper concern is that decades of climate change have raised the ocean's baseline temperature — and that underlying shift is the more serious problem.

Samantha Burgess, head of climate strategy at the European Centre for Medium-Range Weather Forecasts (ECMWF), called the record "another clear signal that the ocean is under increasing stress," adding that while El Niño is contributing additional heat, "behind it lies decades of human-induced warming." She warned that as ocean temperatures continue to rise, the risks to marine ecosystems and coastal communities will only grow.