The effect of CO2 ramping rate on the transient weakening of the Atlantic Meridional Overturning Circulation
Edited by Eric Rignot, University of California, Irvine, CA; received June 9, 2024; accepted October 26, 2024
Significance
The Atlantic Meridional Overturning Circulation (AMOC) is an important ocean circulation that is expected to slow down under future anthropogenic climate change, with likely impacts on the climate of the North Atlantic region and beyond. In this study, I find that in a modern global climate model, the AMOC slows down more when the rate of atmospheric CO2 change is faster, even when the level of CO2 change is the same, and that this can be explained by a positive feedback cycle. This work highlights how the same amount of carbon emissions released over different amounts of time can lead to qualitatively different climates, making the timing of future emissions an important consideration for policy decisions.
Abstract
The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate that is projected to weaken under future anthropogenic climate change. While many studies have investigated the AMOC’s response to different levels and types of forcing in climate models, relatively little attention has been paid to the AMOC’s sensitivity to the rate of forcing change, despite it also being highly uncertain in future emissions scenarios. In this study, I isolate the AMOC’s response to different rates of CO2 increase in a state-of-the-art global climate model and find that the AMOC undergoes more severe weakening under faster rates of CO2 change, even when the magnitude of CO2 change is the same. I then propose an AMOC-ocean heat transport-sea ice feedback that enhances the decline of the circulation and explains the dependence on the rate of forcing change. The AMOC’s rate-sensitive behavior leads to qualitatively different climates (including differing Arctic sea ice evolution) at the same CO2 concentration, highlighting how the rate of forcing change is itself a key driver of global climatic change.
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Data, Materials, and Software Availability
Model data and python code used to generate the results and figures in this work can be found at Zenodo (66).
Acknowledgments
I thank Carl Wunsch and Eli Tziperman for their helpful feedback, and Jon Proctor and Kaitlyn Loftus for their aesthetic input. This work was supported by NSF Grant 2303486 of the Paleo Perspective on Present and Projected Climate program, National Center for Atmospheric Research Small Exploratory Computing Grant UHAR0020, and the Cooperative Institute for Climate, Ocean, and Ecosystem Studies Postdoctoral Fellowship Program.
Author contributions
C.H. designed research; performed research; analyzed data; and wrote the paper.
Competing interests
The author declares no competing interest.
Supporting Information
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References
1
H. Stommel, Thermohaline convection with two stable regimes of flow. Tellus 13, 224–230 (1961).
2
R. X. Huang, J. R. Luyten, H. M. Stommel, Multiple equilibrium states in combined thermal and saline circulation. J. Phys. Oceanogr. 22, 231–246 (1992).
3
S. Manabe, R. J. Stouffer, Two stable equilibria of a coupled ocean-atmosphere model. J. Clim. 1, 841–866 (1988).
4
S. Rahmstorf, Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle. Nature 378, 145–149 (1995).
5
A. Schiller, U. Mikolajewicz, R. Voss, The stability of the north Atlantic thermohaline circulation in a coupled ocean-atmosphere general circulation model. Clim. Dyn. 13, 325–347 (1997).
6
E. Hawkins et al., Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport. Geophys. Res. Lett. 38, L10605 (2011).
7
P. Wu, J. Ridley, A. Pardaens, R. Levine, J. Lowe, The reversibility of co2 induced climate change. Clim. Dyn. 45, 745–754 (2015).
8
L. Jackson, R. Wood, Hysteresis and resilience of the AMOC in an eddy-permitting GCM. Geophys. Res. Lett. 45, 8547–8556 (2018).
9
D. Rind et al., Multicentury instability of the Atlantic meridional circulation in rapid warming simulations with GISS ModelE2. J. Geophys. Res.: Atmos. 123, 6331–6355 (2018).
10
R. M. van Westen, M. Kliphuis, H. A. Dijkstra, Physics-based early warning signal shows that AMOC is on tipping course. Sci. Adv. 10, eadk1189 (2024).
11
B. I. Moat et al., Atlantic meridional overturning circulation observed by the RAPID-MOCHA-WBTS (RAPID-Meridional Overturning Circulation and Heatflux Array-Western Boundary Time Series) array at 26N from 2004 to 2022 (v2022.1) (NERC EDS British Oceanographic Data Centre NOC, 2023). https://doi.org/10.5285/223b34a3-2dc5-c945-e063-7086abc0f274. Accessed 12 March 2024.
12
E. Frajka-Williams et al., Atlantic meridional overturning circulation: Observed transport and variability. Front. Mar. Sci. 6, 260 (2019).
13
E. L. Worthington et al., A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline. Ocean Sci. 17, 285–299 (2021).
14
Y. Fu et al., Seasonality of the meridional overturning circulation in the subpolar North Atlantic. Commun. Earth Environ. 4, 181 (2023).
15
L. Jackson et al., Global and European climate impacts of a slowdown of the AMOC in a high resolution GCM. Clim. Dyn. 45, 3299–3316 (2015).
16
K. Bellomo et al., Impacts of a weakened AMOC on precipitation over the Euro-Atlantic region in the EC-Earth3 climate model. Clim. Dyn. 61, 3397–3416 (2023).
17
W. Liu, A. V. Fedorov, S. P. Xie, S. Hu, Climate impacts of a weakened Atlantic meridional overturning circulation in a warming climate. Sci. Adv. 6, eaaz4876 (2020).
18
A. Timmermann et al., The influence of a weakening of the Atlantic meridional overturning circulation on ENSO. J. Clim. 20, 4899–4919 (2007).
19
B. Orihuela-Pinto, A. Santoso, M. H. England, A. S. Taschetto, Reduced ENSO variability due to a collapsed Atlantic meridional overturning circulation. J. Clim. 35, 5307–5320 (2022).
20
R. Zhang, T. L. Delworth, Impact of the Atlantic multidecadal oscillation on north pacific climate variability. Geophys. Res. Lett. 34, L23708 (2007).
21
R. Zhang et al., A review of the role of the Atlantic meridional overturning circulation in Atlantic multidecadal variability and associated climate impacts. Rev. Geophys. 57, 316–375 (2019).
22
M. E. Hamouda, C. Pasquero, E. Tziperman, Decoupling of the Arctic Oscillation and North Atlantic Oscillation in a warmer climate. Nat. Clim. Change 11, 137–142 (2021).
23
C. Deser, On the teleconnectivity of the “Arctic Oscillation’’. Geophys. Res. Lett. 27, 779–782 (2000).
24
D. B. Bonan, A. F. Thompson, E. R. Newsom, S. Sun, M. Rugenstein, Transient and equilibrium responses of the Atlantic overturning circulation to warming in coupled climate models: The role of temperature and salinity. J. Clim. 35, 5173–5193 (2022).
25
P. Nobre et al., AMOC decline and recovery in a warmer climate. Sci. Rep. 13, 15928 (2023).
26
C. Bitz, J. Chiang, W. Cheng, J. Barsugli, Rates of thermohaline recovery from freshwater pulses in modern, last glacial maximum, and greenhouse warming climates. Geophys. Res. Lett. 34, L07708 (2007).
27
L. Jackson, Shutdown and recovery of the AMOC in a coupled global climate model: The role of the advective feedback. Geophys. Res. Lett. 40, 1182–1188 (2013).
28
M. D. Thomas, A. V. Fedorov, Mechanisms and impacts of a partial AMOC recovery under enhanced freshwater forcing. Geophys. Res. Lett. 46, 3308–3316 (2019).
29
R. K. Haskins, K. I. Oliver, L. C. Jackson, S. S. Drijfhout, R. A. Wood, Explaining asymmetry between weakening and recovery of the AMOC in a coupled climate model. Clim. Dyn. 53, 67–79 (2019).
30
L. C. Jackson et al., Understanding AMOC stability: The North Atlantic hosing model intercomparison project. Geosci. Model Dev. Discuss. 2022, 1–32 (2022).
31
P. Wu, L. Jackson, A. Pardaens, N. Schaller, Extended warming of the northern high latitudes due to an overshoot of the Atlantic meridional overturning circulation. Geophys. Res. Lett. 38, L24704 (2011).
32
L. Ackermann, C. Danek, P. Gierz, G. Lohmann, AMOC recovery in a multicentennial scenario using a coupled atmosphere-ocean-ice sheet model. Geophys. Res. Lett. 47, e2019GL086810 (2020).
33
H. Alkhayuon, P. Ashwin, L. C. Jackson, C. Quinn, R. A. Wood, Basin bifurcations, oscillatory instability and rate-induced thresholds for Atlantic meridional overturning circulation in a global oceanic box model. Proc. R. Soc. A 475, 20190051 (2019).
34
T. F. Stocker, A. Schmittner, Influence of CO2 emission rates on the stability of the thermohaline circulation. Nature 388, 862–865 (1997).
35
R. J. Stouffer, S. Manabe, Response of a coupled ocean-atmosphere model to increasing atmospheric carbon dioxide: Sensitivity to the rate of increase. J. Clim. 12, 2224–2237 (1999).
36
M. F. Jansen, L. P. Nadeau, T. M. Merlis, Transient versus equilibrium response of the ocean’s overturning circulation to warming. J. Clim. 31, 5147–5163 (2018).
37
M. Nikurashin, G. Vallis, A theory of the interhemispheric meridional overturning circulation and associated stratification. J. Phys. Oceanogr. 42, 1652–1667 (2012).
38
E. Butler, K. Oliver, J. J. M. Hirschi, J. Mecking, Reconstructing global overturning from meridional density gradients. Clim. Dyn. 46, 2593–2610 (2016).
39
R. K. Haskins, K. I. Oliver, L. C. Jackson, R. A. Wood, S. S. Drijfhout, Temperature domination of AMOC weakening due to freshwater hosing in two GCMs. Clim. Dyn. 54, 273–286 (2020).
40
R. J. Stouffer et al., Investigating the causes of the response of the thermohaline circulation to past and future climate changes. J. Clim. 19, 1365–1387 (2006).
41
J. Gregory et al., A model intercomparison of changes in the Atlantic thermohaline circulation in response to increasing atmospheric CO2 concentration. Geophys. Res. Lett. 32, L12703 (2005).
42
P. Bakker et al., Fate of the Atlantic Meridional Overturning Circulation: Strong decline under continued warming and Greenland melting. Geophys. Res. Lett. 43, 12–252 (2016).
43
Y. C. Lee, W. Liu, The weakened Atlantic meridional overturning circulation diminishes recent arctic sea ice loss. Geophys. Res. Lett. 50, e2023GL105929 (2023).
44
C. W. Arnscheidt, D. H. Rothman, Routes to global glaciation. Proc. R. Soc. A 476, 20200303 (2020).
45
U. Feudel, Rate-induced tipping in ecosystems and climate: The role of unstable states, basin boundaries and transient dynamics. Nonlinear Process. Geophys. 30, 481–502 (2023).
46
P. D. L. Ritchie, H. Alkhayuon, P. M. Cox, S. Wieczorek, Rate-induced tipping in natural and human systems. Earth Syst. Dyn. 14, 669–683 (2023).
47
F. Sévellec, A. V. Fedorov, W. Liu, Arctic sea-ice decline weakens the Atlantic Meridional Overturning Circulation. Nat. Clim. Change 7, 604–610 (2017).
48
H. Li, A. Fedorov, W. Liu, AMOC stability and diverging response to Arctic sea ice decline in two climate models. J. Clim. 34, 5443–5460 (2021).
49
B. S. Ferster, A. Simon, A. Fedorov, J. Mignot, E. Guilyardi, Slowdown and recovery of the Atlantic Meridional Overturning Circulation and a persistent north Atlantic warming hole induced by Arctic Sea ice decline. Geophys. Res. Lett. 49, e2022GL097967 (2022).
50
W. Liu, S. P. Xie, Z. Liu, J. Zhu, Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate. Sci. Adv. 3, e1601666 (2017).
51
J. Mecking, S. Drijfhout, L. Jackson, M. Andrews, The effect of model bias on Atlantic freshwater transport and implications for AMOC bi-stability. Tellus A 69, 1299910 (2017).
52
R. M. van Westen, H. A. Dijkstra, Asymmetry of AMOC hysteresis in a state-of-the-art global climate model. Geophys. Res. Lett. 50, e2023GL106088 (2023).
53
R. M. Van Westen, H. A. Dijkstra, Persistent climate model biases in the Atlantic ocean’s freshwater transport. Ocean Sci. 20, 549–567 (2024).
54
S. Rahmstorf, On the freshwater forcing and transport of the Atlantic thermohaline circulation. Clim. Dyn. 12, 799–811 (1996).
55
P. de Vries, S. L. Weber, The Atlantic freshwater budget as a diagnostic for the existence of a stable shut down of the meridional overturning circulation. Geophys. Res. Lett. 32 (2005).
56
H. A. Dijkstra, Characterization of the multiple equilibria regime in a global ocean model. Tellus A 59, 695–705 (2007).
57
S. E. Huisman, M. Den Toom, H. A. Dijkstra, S. Drijfhout, An indicator of the multiple equilibria regime of the Atlantic Meridional Overturning Circulation. J. Phys. Oceanogr. 40, 551–567 (2010).
58
W. Weijer et al., Stability of the Atlantic Meridional Overturning Circulation: A review and synthesis. J. Geophys. Res.: Oceans 124, 5336–5375 (2019).
59
A. J. Weaver, M. Eby, M. Kienast, O. A. Saenko, Response of the Atlantic Meridional Overturning Circulation to increasing atmospheric CO2: Sensitivity to mean climate state. Geophys. Res. Lett. 34, L05708 (2007).
60
Y. J. Lin, B. E. Rose, Y. T. Hwang, Mean state AMOC affects AMOC weakening through subsurface warming in the Labrador Sea. J. Clim. 36, 3895–3915 (2023).
61
L. C. Jackson et al., Impact of ocean resolution and mean state on the rate of AMOC weakening. Clim. Dyn. 55, 1711–1732 (2020).
62
A. Levermann, J. Mignot, S. Nawrath, S. Rahmstorf, The role of northern sea ice cover for the weakening of the thermohaline circulation under global warming. J. Clim. 20, 4160–4171 (2007).
63
M. J. Roberts et al., Sensitivity of the Atlantic Meridional Overturning Circulation to model resolution in CMIP6 HighResMIP simulations and implications for future changes. J. Adv. Model. Earth Syst. 12, e2019MS002014 (2020).
64
P. Chang et al., An unprecedented set of high-resolution earth system simulations for understanding multiscale interactions in climate variability and change. J. Adv. Model. Earth Syst. 12, e2020MS002298 (2020).
65
T. J. McDougall, P. M. Barker, Getting started with TEOS-10 and the Gibbs seawater (GSW) oceanographic toolbox. SCOR/IAPSO WG 127, 1–28 (2011).
66
C. Hankel, Data and code for “The effect of CO2 ramping rate on the transient weakening of the Atlantic Meridional Overturning Circulation”. Zenodo. https://doi.org/10.5281/zenodo.14219963. Deposited 26 November 2024.
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Copyright © 2024 the Author(s). Published by PNAS. This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
Data, Materials, and Software Availability
Model data and python code used to generate the results and figures in this work can be found at Zenodo (66).
Submission history
Received: June 9, 2024
Accepted: October 26, 2024
Published online: December 23, 2024
Published in issue: January 7, 2025
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Acknowledgments
I thank Carl Wunsch and Eli Tziperman for their helpful feedback, and Jon Proctor and Kaitlyn Loftus for their aesthetic input. This work was supported by NSF Grant 2303486 of the Paleo Perspective on Present and Projected Climate program, National Center for Atmospheric Research Small Exploratory Computing Grant UHAR0020, and the Cooperative Institute for Climate, Ocean, and Ecosystem Studies Postdoctoral Fellowship Program.
Author contributions
C.H. designed research; performed research; analyzed data; and wrote the paper.
Competing interests
The author declares no competing interest.
Notes
This article is a PNAS Direct Submission.
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The effect of CO2 ramping rate on the transient weakening of the Atlantic Meridional Overturning Circulation, Proc. Natl. Acad. Sci. U.S.A.
122 (1) e2411357121,
https://doi.org/10.1073/pnas.2411357121
(2025).
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