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Observational constraints imply limited future Atlantic meridional overturning circulation weakening

Nature Geoscience volume 18, pages 479–487 (2025)Cite this article

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Abstract

The degree to which the Atlantic meridional overturning circulation (AMOC) weakens over the twenty-first century varies widely across climate models, with some predicting substantial weakening. Here we show that this uncertainty can be greatly reduced by using a thermal-wind expression that relates the AMOC strength to the meridional density difference and the overturning depth in the Atlantic. This expression captures the intermodel spread in AMOC weakening, with most of the spread arising from overturning depth changes. The overturning depth also establishes a crucial link between the present-day and future AMOC strength. Climate models with a stronger and deeper present-day overturning tend to predict larger weakening and shoaling under warming because the present-day North Atlantic is less stratified, allowing for a deeper penetration of surface buoyancy flux changes, larger density changes at depth and, consequently, larger AMOC weakening. By incorporating observational constraints, we conclude that the AMOC will experience limited weakening of about 3–6 Sv (about 18–43%) by the end of this century, regardless of emissions scenario. These results indicate that the uncertainty in twenty-first-century AMOC weakening and the propensity to predict substantial AMOC weakening can be attributed primarily to climate model biases in accurately simulating the present-day ocean stratification.

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Fig. 1: Relationship between the present-day and future AMOC strength.
Fig. 2: Controls on AMOC weakening at the end of the twenty-first century.
Fig. 3: Relationship between present-day and future North Atlantic stratification.
Fig. 4: Schematic depicting the processes linking the present-day and future AMOC strength.
Fig. 5: Constraints on AMOC weakening at the end of the twenty-first century.

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Data availability

We thank the climate modelling groups for producing and making available their model output, which is accessible on the Earth System Grid Federation (ESGF) Portal (https://esgf-node.llnl.gov/search/cmip6/).

Code availability

The code needed to calculate the overturning depth scale is available on Zenodo at https://doi.org/10.5281/zenodo.15103083 (ref. 53).

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Acknowledgements

This work was supported by the National Science Foundation (NSF) Graduate Research Fellowship Program under NSF Award DGE1745301 (D.B.B.), the Cooperative Institute for Climate, Ocean, and Ecosystem Studies (CICOES) under NOAA Cooperative Agreement NA20OAR4320271, Contribution No. 2025-1450 (D.B.B.), the David and Lucile Packard Foundation and NSF Award OCE-1756956 (A.F.T.), Schmidt Sciences, LLC (T.S. and L.Z.), and NSF Awards OCE-1850900 and AGS-1752796 (K.C.A.).

Author information

Author notes

  1. David B. Bonan

    Present address: Department of Atmospheric and Climate Science, University of Washington, Seattle, WA, USA

  2. David B. Bonan

    Present address: Cooperative Institute for Climate, Ocean, and Ecosystem Studies, University of Washington, Seattle, WA, USA

Authors and Affiliations

  1. Environmental Science and Engineering, California Institute of Technology, Pasadena, CA, USA

    David B. Bonan, Andrew F. Thompson & Tapio Schneider

  2. Courant Institute of Mathematical Sciences, New York University, New York City, NY, USA

    Laure Zanna

  3. Department of Atmospheric and Climate Science, University of Washington, Seattle, WA, USA

    Kyle C. Armour

  4. School of Oceanography, University of Washington, Seattle, WA, USA

    Kyle C. Armour

  5. Laoshan Laboratory, Qingdao, China

    Shantong Sun

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Contributions

D.B.B. conceived of the project, conducted the analysis, generated the figures and wrote the paper. A.F.T. and T.S. contributed to paper revision and supervised the project. L.Z. and K.C.A. contributed to paper revision and helped interpret results. S.S. contributed to project design and paper revision and helped interpret results.

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Correspondence to David B. Bonan.

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Nature Geoscience thanks Wei Cheng and Yen-Ting Hwang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: James Super, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Comparison of the AMOC weakening calculated in GCMs and predicted by Eq. (3).

Bar plots showing the residual error between the AMOC weakening calculated in GCMs and predicted by the thermal- wind expression (Eq. 3) for each emission scenario at years 2071–2100.

Extended Data Fig. 2 Contributions of each variable to the intermodel spread in Term B from Eq. (3).

The implied AMOC strength change from Term B at years 2071–2100 of the SSP5-8.5 emission scenario. Each panel shows the magnitude of Term B for each GCM when considering the full intermodel spread of all terms (〈Δyρ〉, H, δH) and from H, δH, and (〈Δyρ〉 separately).

Extended Data Fig. 3 Observed present-day AMOC strength implied by ECCO and the RAPID array.

The present-day annual-mean AMOC strength calculated from ECCO and RAPID array at 26.5∘N. The ECCO period is 1992–2017. The RAPID array period is 2005–2021.

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Bonan, D.B., Thompson, A.F., Schneider, T. et al. Observational constraints imply limited future Atlantic meridional overturning circulation weakening. Nat. Geosci. 18, 479–487 (2025). https://doi.org/10.1038/s41561-025-01709-0

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