Closed world seas under pressure from climate change

Enclosed mariginal seas (EMS), such as the Baltic or the Mediterranean, are bodies of water largely surrounded by land and isolated from the waters of the open ocean. On the one hand, they play a key role in the economy, creating conditions for fishing, transportation and tourism. On the other, they are among the most stressed marine areas in the world. Overlapping here are the effects of human activity – pollution, eutrophication or overfishing – and climate change, particularly warming and acidification of the waters.

The results of the study, published April 4, 2026 in the journal Communications Earth & Environment, present a global analysis of temperature changes and the occurrence of marine heat waves in the world’s 19 largest enclosed seas, showing the scale of the risks and possible future scenarios.

Warming faster than global average

Analysis of data collected since 1950 shows a clear increase in EMS surface temperature, which has further accelerated after 2010. In most of the studied basins, the rate of warming exceeds the average value for the entire world ocean. Particularly pronounced differences apply to seas with temperate, cool and polar waters, where the anomaly was up to twice as high as the global value.

Climate models show that assuming a scenario of no reduction in greenhouse gas emissions, the temperature in the enclosed seas could rise by about 2°C by the end of the 21st century, and as much as 7°C in places. The strongest changes are forecast in the polar regions, where warming could exceed 6°C on average, while in temperate and cool zones it is likely to be 4-5.5°C. Even in tropical seas, which are warming somewhat more slowly, projected increases exceed 3°C in some cases.

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Unprecedented pace of change

According to the study’s authors, of particular concern is the rate of warming. Around the year 2000, a clear peak of it was recorded – a precedent in the history of measurements. In the future, especially with high greenhouse gas emissions, there is a high risk of such extreme trends occurring again, or even exceeding them. In some seas, the warming rate in 30-year terms could be three or even four times higher than before.

Scenarios that meet the goals of the Paris Agreement significantly reduce this risk. In low-carbon options, the probability of unprecedented trends drops to less than 10 percent, which the authors describe as a relatively safe value for marine ecosystems.

Sea heat waves are becoming the norm

With warming, the incidence and intensity of marine heat waves are increasing. So far, the risk has increased about threefold compared to the beginning of the 21st century. In the case of enclosed seas, the problem is particularly serious, as limited water exchange promotes heat accumulation.

The study’s authors introduce the concept of a quasipermanent state, in which heat waves cover almost the entire sea for most of the year. Such a state is considered when more than 90 percent of a body of water experiences extreme temperatures for more than 330 days a year. In a high emissions scenario, this becomes very likely for the vast majority of the seas studied. With effective emission reductions, the risk decreases to just a few cases.

Even ambitious climate policy won’t stop change

Limiting global warming to levels consistent with the Paris Agreement significantly reduces the magnitude of risks, but does not eliminate them. Even in an optimistic low-emissions scenario, temperatures in enclosed seas will rise by an average of about 1.5-1.8°C by the middle of the 21st century relative to those of the pre-industrial era. The warming will be accompanied by a sustained increase in the number and extent of heat waves, which will cover more than 60 percent of the surface of these bodies of water.

By the end of the century, most of the studied seas will reach a state where at least half of their surface area will be covered by heat waves on an average annual basis. This means a fundamental change in environmental conditions, even with effective climate policies.

Closed seas: ecosystems in deep transition

Already there are serious effects of warming seas, such as shifting species ranges, population declines and changes in ecosystem structure. This is accompanied by a loss of biodiversity and a weakening of the functions the seas perform for humans, including fisheries productivity and carbon storage capacity.

These changes are set to get even worse in the future. The high rate of warming could force species migrations on an unprecedented scale, and paleoclimatic data suggest that up to 70 percent of areas with the highest marine biodiversity could be threatened.

Conclusions: double action required

The study’s authors stress that the enclosed seas are already under severe thermal stress. Low-emission scenarios clearly reduce the risk of the most extreme changes, but they cannot stop them completely. This means that the future of these ecosystems depends not only on emissions reductions, but also on their ability to adapt.

Developing early warning systems for heat waves and carrying out measures to reduce local pressures, such as improving water quality, therefore becomes a priority. Without simultaneous implementation of a mitigation and adaptation strategy, changes in the enclosed seas could prove irreversible, both for nature and for the economies that depend on these vulnerable ecosystems.

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Bibliography:

Gröger, M., Börgel, F., Dutheil, C. et al. The world’s enclosed seas highlight the need for urgent emission reductions and societal adaptation. Commun Earth Environ 7, 312 (2026). https://doi.org/10.1038/s43247-026-03412-3