South Korea's seas are heating up — and by 2100, marine heat waves could last nearly all year

Warming in the seas surrounding South Korea is accelerating, and marine heat waves are becoming a regular occurrence. The number of marine heat wave days over the past decade has risen to 3.7 times the long-term average, and projections suggest that more than 80 percent of the year could qualify as a marine heat wave period by the end of this century.

The Korea Meteorological Administration on Wednesday released an analysis of past changes and future projections for sea surface temperatures and marine heat waves in waters surrounding South Korea.

A marine heat wave is defined as a period of five or more consecutive days during which the daily average sea surface temperature exceeds the top 10 percent of the historical distribution — the oceanic equivalent of a land heat wave. The analysis found that the average sea surface temperature in surrounding waters over the past decade (2016–2025) was 18.1 degrees Celsius, 0.7 degrees above the long-term average for 1991–2020.

The rate of warming has also picked up sharply. Since 1991, the overall warming trend stood at 0.3 degrees Celsius per decade, but over the most recent 10 years alone it reached 1.2 degrees per decade. By contrast, the rate of increase was close to zero per decade between 1991 and 2015.

The annual average number of marine heat wave days over the past decade also surged to 83.3 days, roughly 3.7 times the long-term average of 22.6 days. What was once about 3.2 weeks of ocean heat per year has grown to an average of roughly 2.8 months. The intensity of marine heat waves also rose to 2.1 degrees Celsius above baseline, up from the long-term average of 1.9 degrees.

The warming around South Korea stands out even by global standards. Over the past decade, the global average sea surface temperature rose 0.3 degrees Celsius above the long-term average, and the northwestern Pacific rose 0.5 degrees, while waters surrounding South Korea climbed 0.7 degrees.

Warming has been particularly pronounced in the East Sea. The average sea surface temperature there over the past decade was 18.4 degrees Celsius, 0.8 degrees above the long-term average. Marine heat wave days reached 96.1, far above the long-term average of 23.5 days, and intensity climbed to 2.2 degrees above baseline, compared with the long-term average of 1.9 degrees.

The East Sea has also seen heat accumulate not just at the surface but deep into the water column. Ocean heat content measures the total heat stored in a given depth range of seawater. The KMA said heat stored within the East Sea has increased more than in other surrounding waters, and that this accumulated subsurface heat appears to be a key background condition driving the more pronounced rise in sea surface temperatures and marine heat wave activity there compared with other regions.

The concern is that these trends are likely to continue. Applying climate change scenarios based on the Intergovernmental Panel on Climate Change's Sixth Assessment Report, the KMA projected that even under a low-carbon scenario with significant greenhouse gas reductions (SSP1-2.6), sea surface temperatures in surrounding waters would rise 1.5 degrees Celsius above the past decade's average by the end of this century. Under an intermediate scenario (SSP2-4.5), the increase widens to 2.2 degrees.

Marine heat waves are expected to increase even more dramatically. By the end of this century, the annual average number of marine heat wave days is projected at about 304 days under the low-carbon scenario and about 320 days under the intermediate scenario — increases of 181 days and 231 days, respectively, compared with the past decade's average of 83.3 days. That would mean roughly 83 percent and 88 percent of the year, respectively, would qualify as marine heat wave days by current standards.

The KMA said that because both scenarios project increases in sea surface temperatures and in the frequency and intensity of marine heat waves, ocean warming driven by the cumulative effect of past greenhouse gas emissions is likely to persist for a long time. However, the agency said the marked difference in projected change between the two scenarios shows that cutting greenhouse gas emissions remains a critical factor in limiting future ocean warming risks.

"The ocean is one of the core components of the Earth system, interacting closely with the atmosphere, and its changes can have wide-ranging effects on people's daily lives through their connection to extreme weather events," KMA Commissioner Lee Mi-seon said. "We will do our utmost to strengthen proactive responses to the climate crisis."