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Peer-reviewedCC BYPhilosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences

Global stability of the Atlantic overturning circulation: edge state, long transients and boundary crisis under CO2 forcing

  • Reyk Börner1,2
  • Oliver Mehling3,2
  • Jost von Hardenberg3,4
  • Valerio Lucarini5
5 affiliations
  1. 1Department of Mathematics and Statistics, University of Reading 1 , Reading, UK
  2. 2Institute for Marine and Atmospheric Research Utrecht, Utrecht University 2 , Utrecht, The Netherlands
  3. 3Department of Environment, Land and Infrastructure Engineering, Politecnico di Torino 3 , Torino, Italy
  4. 4Institute of Atmospheric Sciences and Climate, Consiglio Nazionale delle Ricerche 4 , Torino, Italy
  5. 5School of Computing and Mathematical Sciences, University of Leicester 5 , Leicester, UK
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Abstract

The Atlantic Meridional Overturning Circulation (AMOC), a major ocean current system, could transition to a weak state. Despite severe associated climate impacts, assessing the AMOC's response under global warming and its proximity to possible critical thresholds remains difficult. To understand future Earth system stability, a global dynamical view is needed beyond the local stability analysis associated with classical early-warning methods. Using an intermediate-complexity climate model, we explore the stability landscape of the AMOC for different atmospheric CO2 concentrations. We explicitly compute the edge state (or Melancholia state), a chaotic saddle on the basin boundary separating the strong and weak AMOC attractors found in the model. Despite being unstable, the edge state can govern the transient climate for centuries, supporting centennial AMOC oscillations driven by atmosphere-ice-ocean interactions in the North Atlantic. At increased CO2 levels projected for the near future, we reveal a boundary crisis where the current AMOC attractor disappears by colliding with the edge state. Under crisis overshoot, long chaotic transients owing to a 'ghost state' lead to ensemble splitting under time-varying forcing. Rooted in dynamical systems theory, our results offer an explanation of large ensemble variance and apparent 'stochastic bifurcations' observed in earth system models under intermediate forcing scenarios. This article is part of the theme issue 'Critical transitions and intelligent control in complex systems'.

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Cite

BibTeXRIS
BibTeX
@article{borner2026global,
  title = {{Global stability of the Atlantic overturning circulation: edge state, long transients and boundary crisis under CO2 forcing}},
  author = {Reyk Börner and Oliver Mehling and Jost von Hardenberg and Valerio Lucarini},
  journal = {Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences},
  year = {2026},
  month = jun,
  doi = {10.1098/rsta.2025.0087},
  url = {https://doi.org/10.1098/rsta.2025.0087},
  note = {CC BY},
}
RIS
TY  - JOUR
TI  - Global stability of the Atlantic overturning circulation: edge state, long transients and boundary crisis under CO2 forcing
AU  - Reyk Börner
AU  - Oliver Mehling
AU  - Jost von Hardenberg
AU  - Valerio Lucarini
PY  - 2026
DA  - 2026/06/18
JO  - Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences
DO  - 10.1098/rsta.2025.0087
UR  - https://doi.org/10.1098/rsta.2025.0087
AB  - The Atlantic Meridional Overturning Circulation (AMOC), a major ocean current system, could transition to a weak state. Despite severe associated climate impacts, assessing the AMOC's response under global warming and its proximity to possible critical thresholds remains difficult. To understand future Earth system stability, a global dynamical view is needed beyond the local stability analysis associated with classical early-warning methods. Using an intermediate-complexity climate model, we explore the stability landscape of the AMOC for different atmospheric CO2 concentrations. We explicitly compute the edge state (or Melancholia state), a chaotic saddle on the basin boundary separating the strong and weak AMOC attractors found in the model. Despite being unstable, the edge state can govern the transient climate for centuries, supporting centennial AMOC oscillations driven by atmosphere-ice-ocean interactions in the North Atlantic. At increased CO2 levels projected for the near future, we reveal a boundary crisis where the current AMOC attractor disappears by colliding with the edge state. Under crisis overshoot, long chaotic transients owing to a 'ghost state' lead to ensemble splitting under time-varying forcing. Rooted in dynamical systems theory, our results offer an explanation of large ensemble variance and apparent 'stochastic bifurcations' observed in earth system models under intermediate forcing scenarios. This article is part of the theme issue 'Critical transitions and intelligent control in complex systems'.
SN  - 1364-503X
SN  - 1471-2962
ER  -