Sea surface temperatures in waters around South Korea and East Asia hit some of their highest levels on record last year, accelerating climate risks to marine ecosystems and the country’s fisheries, government data showed.
South Korea’s National Institute of Fisheries Science (NIFS) said the average sea surface temperature in East Asian waters reached 20.84°C in 2024, the second-highest level since records began in 2000. The country’s surrounding seas averaged 17.66°C, above the long-term norm.
Monthly data showed temperatures broadly in line with the 2001–2020 average from January to May, but from June to October the average jumped to 26.44°C, setting a new seasonal record, according to NIFS.
Observations from scientific survey vessels operating in South Korea’s exclusive economic zone since the 1960s showed that from August to December 2025, sea surface temperatures averaged 21.27 °C – the second-highest on record after 2024’s annual average of 22.28°C.
NIFS attributed the unusually high temperatures to the early expansion of the North Pacific high-pressure system, prolonged heat linked to the Tibetan high from late July, and increased inflows of warm, low-latitude waters from the Tsushima Current extending from summer into autumn.
“Climate observation data show that the rate of temperature rise in seas around Korea is extremely steep,” NIFS head Choi Yong-seok said in a statement on Monday, adding the agency would step up research into climate forecasting and adaptation technologies for the fisheries sector.
The findings come as the Ministry of Oceans and Fisheries published a “2025 Climate Change Impact Briefing Book” in May, assessing the effects of warming on marine ecosystems and the fishing industry.
Over the past 57 years, from 1968 to 2024, global average sea surface temperatures rose by 0.74°C, the report said. In South Korean waters, temperatures climbed by 1.58°C – more than twice the global pace.
The sharpest increase was recorded in the Sea of Japan, also known as East Sea in South Korea, where surface temperatures rose by 2.04°C.
Indicators of marine productivity are also deteriorating. Chlorophyll-a concentrations, a measure of primary production, have generally declined since 2003, with particularly steep drops in the Yellow Sea and the central East Sea. In 2024, productivity fell 21.6 per cent from the previous year, the report said.
Economic losses are mounting. Prolonged high-temperature conditions through late September last year caused KRW143 billion (US$106 million) in damage to aquaculture, the largest figure since records began in 2012. Coastal and offshore fishery output has also fallen sharply, from about 1.51 million tonnes in the 1980s to around 910,000 tonnes in the 2020s, sliding further to 841,000 tonnes in 2024.
“Climate change is progressing faster and more severely than expected across our seas and fisheries,” Choi said in May, adding the briefing aimed to strengthen scientific understanding and improve response capacity.
The findings align with broader global trends. A peer-reviewed assessment shows that global ocean heat content – a measure of the heat stored in the upper 2,000 metres of the world’s oceans – rose to its highest level on record in 2025, with heat accumulation increasing sharply over the past decade as oceans absorb more than 90 per cent of excess heat from greenhouse gas emissions.
Scientists have increasingly described the current state of the oceans as approaching a “global boiling” regime, with marine heatwaves spanning vast areas, prolonged warming, and record sea surface temperatures contributing to more frequent extreme weather, shifts in marine ecosystems, and impacts on global fisheries and coastal communities.
