Abstract
Enclosed marginal seas are hotspots of endemic biodiversity that sustain societies through fisheries, tourism, and vital ecosystem services. Their small size makes them highly sensitive to global warming but systematic assessments of future climate change are lacking. Using climate model projections and reanalysis data, we assess two key thermal stressors across 19 seas: the rate of warming that limits the adaptive capacity of ecosystems, and the emergence of near-permanent marine heatwaves. We find these seas have already entered an unprecedented warming phase following the reversal of aerosol cooling effect in the late 20th century. Under unmitigated future scenarios, many seas would experience climate warming rates three to four times higher than previously observed, with 15 seas at risk of entering near-permanent heatwave states. Limiting global warming to below 2 °C prevents such extremes, but seas still warm substantially: 13 seas exceed >1 °C warming above preindustrial levels, and over 60% of the seas area transforms to a near-permanent heatwave state at mid-century. These findings highlight that even under the most optimistic scenarios, safeguarding the ecological integrity and socioeconomic value of marginal seas demands transformative adaptation, proactive conservation, and large-scale restoration efforts.
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Data availability
This study uses output from the MPI Grand Ensemble (MPI-GE) generated with the Max Planck Institute for Meteorology Earth System Model. The data are distributed through the Earth System Grid Federation (ESGF) and can be accessed via the Deutsches Klimarechenzentrum ESGF node (project: mpi-ge, https://esgf-data.dkrz.de). Users must register with ESGF and agree to the CC BY-SA 4.0 license prior to download. Data source files to reproduce the figures are available under https://doi.org/10.5281/zenodo.18723187.
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Acknowledgements
The research presented in this study is part of the Baltic Earth program (Earth System Science for the Baltic Sea region; see www.baltic.Earth) and a contribution to the BMBF funded project CoastalFutures (03F0911E). We thank the Max Planck Institute for Meteorology for providing the MPI-GE.
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M.G. designed the study, performed the analysis, and wrote the text. CD, MM, SK, GV, KS, FB contributed with comments about scientific content/analysis, read and approved the manuscript.
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Communications Earth & Environment thanks Phillip Williamson and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Olusegun Dada and Alice Drinkwater. A peer review file is available.
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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 (2026). https://doi.org/10.1038/s43247-026-03412-3
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DOI: https://doi.org/10.1038/s43247-026-03412-3
