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Ocean stratification in a warming climate

Nature Reviews Earth & Environment volume 6, pages 637–655 (2025)Cite this article

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Abstract

The ocean is highly stratified. Warm, fresh water sits on top of cold, salty water, influencing vertical oceanic exchange of heat, carbon, oxygen and nutrients. In this Review, we examine observed and projected stratification shifts and their impacts. Changes in ocean temperature and salinity have altered the ocean density field, leading to a 0.8 ± 0.1% dec−1 (90% confidence interval) increase in stratification in the global upper 2,000 m since the 1960s. These increases are most pronounced in the tropics and are primarily temperature driven. Model simulations project ongoing stratification increases in the future, with global 0–2,000 m stratification increasing 0.7 [0.3,1.1; 13–87% confidence interval], 1.4 [0.9,1.8] and 2.9 [2.1,3.8]% dec−1 by 2090–2100 relative to 2010–2020 under Shared Socioeconomic Pathways SSP1-2.6, SSP2-4.5 and SSP5-8.5, respectively; regional patterns of projected stratification changes generally follow observed trends. These observed and projected ocean stratification changes have important climate and ecological consequences, including alterations in ocean heat uptake, ocean currents, vertical mixing, tropical cyclone intensity, marine ecosystems and elevation of marine extremes. Further research should better quantify stratification change at critical layers and understand their drivers and impacts.

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Fig. 1: Explaining ocean stratification.
Fig. 2: Global and regional ocean stratification.
Fig. 3: Observed and projected global ocean stratification changes.
Fig. 4: Observed regional stratification changes.
Fig. 5: Future stratification change sensitivity to emission scenario.
Fig. 6: Ocean stratification impacts on physical and biogeochemical ocean systems.
Fig. 7: Quantifying the impact of stratification on the climate system.

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Acknowledgements

The authors acknowledge support from the National Natural Science Foundation of China (grant numbers 42206208, 42261134536, 42076208), the International Partnership Program of the Chinese Academy of Sciences (grant no. 060GJHZ2024064MI), Asian Cooperation Fund, the new Cornerstone Science Foundation through the XPLORER PRIZE, Youth Innovation Promotion Association, Chinese Academy of Sciences, National Key Scientific and Technological Infrastructure project ‘Earth System Science Numerical Simulator Facility’ (EarthLab), Young Talent Support Project of Guangzhou Association for Science and Technology, and Ocean Negative Carbon Emissions (ONCE). They acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and thank the climate modelling groups for producing and making available their model output through the Earth System Grid Federation. Argo data were collected and made freely available by the International Argo Program and the national programmes contributing to it (https://argo.ucsd.edu, https://www.ocean-ops.org). The Argo Program is part of the Global Ocean Observing System. The observation and model data used in this Review are available at http://www.ocean.iap.ac.cn/. NSF-NCAR is sponsored by the US National Science Foundation. The authors acknowledge discussions with F. Liu, Q. Liu, Y. Gong, S. Li and F. Song. The authors also thank W. Cai, G. Wang, C. Wang, Y. Gong, W. Mei and S. Li for providing data in Fig. 7 of this Review, and W. Zhang for processing global-mean surface temperature data from CMIP6 simulations.

Author information

Authors and Affiliations

  1. State Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

    Lijing Cheng  (成里京)

  2. Eco-Environmental Monitoring and Research Center, Pearl River Valley and South China Sea Ecology and Environment Administration, Ministry of Ecology and Environment, Guangzhou, China

    Guancheng Li  (李冠城)

  3. Key Laboratory of Marine Hazards Forecasting, Ministry of Natural Resources, Hohai University, Nanjing, China

    Shang-Min Long  (龙上敏) & Xuhua Cheng  (程旭华)

  4. College of Oceanography, Hohai University, Nanjing, China

    Shang-Min Long  (龙上敏) & Xuhua Cheng  (程旭华)

  5. Key Laboratory of Ocean Observation and Forecasting & Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China

    Yuanlong Li  (李元龙) & Zhenhua Xu  (徐振华)

  6. Mercator Ocean International, Toulouse, France

    Karina von Schuckmann

  7. National Center for Atmospheric Research, Boulder, CO, USA

    Kevin E. Trenberth

  8. Department of Physics, University of Auckland, Auckland, New Zealand

    Kevin E. Trenberth

  9. Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA, USA

    Michael E. Mann

  10. School of Engineering, University of St. Thomas, St. Paul, MN, USA

    John Abraham

  11. Guangdong Key Laboratory of Ocean Remote Sensing, State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China

    Yan Du  (杜岩)

  12. Department of Atmospheric Sciences, Yunnan University, Kunming, China

    Hailong Liu  (刘海龙)

  13. Laboratory for Ocean Dynamics and Climate, Qingdao Marine Science and Technology Center, Qingdao, China

    Zhenhua Xu  (徐振华)

  14. Department of Atmospheric and Oceanic Sciences, Peking University, Beijing, China

    Maofeng Liu  (刘茂峰)

  15. Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, USA

    Qihua Peng  (彭启华)

  16. State Key Laboratory of Physical Oceanography, Institute of Oceanographic Instrumentation, Shandong Academy of Sciences, Qingdao, China

    Xun Gong  (宫勋)

  17. Shandong Computer Science Center, Qilu University of Technology (Shandong Academy of Sciences), Jinan, China

    Xun Gong  (宫勋)

  18. College of Meteorology and Oceanography, National University of Defense Technology, Changsha, China

    Zhanhong Ma  (马占宏)

  19. Computer Network Information Center, Chinese Academy of Sciences, Beijing, China

    Huifeng Yuan  (原惠峰)

Authors
  1. Lijing Cheng  (成里京)
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  2. Guancheng Li  (李冠城)
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  3. Shang-Min Long  (龙上敏)
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  4. Yuanlong Li  (李元龙)
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  5. Karina von Schuckmann
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  6. Kevin E. Trenberth
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  7. Michael E. Mann
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  8. John Abraham
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  9. Yan Du  (杜岩)
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  10. Xuhua Cheng  (程旭华)
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  11. Hailong Liu  (刘海龙)
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  12. Zhenhua Xu  (徐振华)
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  13. Maofeng Liu  (刘茂峰)
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  14. Qihua Peng  (彭启华)
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  15. Xun Gong  (宫勋)
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  16. Zhanhong Ma  (马占宏)
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  17. Huifeng Yuan  (原惠峰)
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Contributions

All authors contributed to writing and editing the article. L.C. led the overall conceptual design and the activity and led and coordinated the writing in collaboration with K.T. and the editor. L.C., G.L., K.v.S., K.T., M.M. and J.A. jointly designed the structure of this Review. G.L. led the data analyses; Y.L. led the section on regional stratification and its seasonal variation; L.C. led the introduction and summary sections. X.C., H.L., Z.X., M.L., Q.P., G.X., Z.M. and H.Y. led different topics in the consequences section. All authors contributed to reviewing the stratification changes and the impacts, editing the manuscript and analysing the results.

Corresponding author

Correspondence to Lijing Cheng  (成里京).

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Nature Reviews Earth & Environment thanks Ryohei Yamaguchi, Chellappan Gnanaseelan and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Cheng, L., Li, G., Long, SM. et al. Ocean stratification in a warming climate. Nat Rev Earth Environ 6, 637–655 (2025). https://doi.org/10.1038/s43017-025-00715-5

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