SPOTLIGHT TOPIC

Is the AMOC on course for shutdown?

A commentary on recent studies

A recent study by van Westen et al. 2024 [1] has suggested that the Atlantic meridional overturning circulation (AMOC) may be on course for an abrupt shutdown. Due to the potential climatic consequences on the North Atlantic region and much of the Northern Hemisphere, an AMOC shutdown has been one of the figureheads of climate change related impacts since the late 1980s [2][3][4][5]. This new study has re-ignited the conversation about AMOC stability and the climate impacts of an AMOC shutdown and received considerable media attention.

In this article, EPOC experts reflect on the scientific evidence to date.

Is the AMOC slowing and will it reach a tipping point?

Studies based on paleo-observations (i.e. proxies of past ocean conditions that are stored in sea sediments) have suggested that the AMOC is at its weakest for the last millennium [6][7]. In contrast, contemporary AMOC estimates based on direct observations of ocean (temperature, salinity, velocities) and atmospheric (wind) conditions acquired by the RAPID-MOCHA observing system in the North Atlantic do not show any significant weakening of the AMOC during the last 20 years [8]. That period may be too short to detect a longer term weakening trend though [9]. Whether the AMOC strength is currently reducing is therefore still an open question being debated in the ocean- and climate-community [6][10].

What about a possible AMOC tipping point?

Again, paleo observations show that the AMOC is likely to have undergone rapid changes during the last ice age [11][12][13]. Evidence of abrupt changes in the past has been one of the key motivations for many later and current studies and indeed for the current AMOC observing systems. How likely is an AMOC shutdown in response to global warming and what are the warning signs we should be looking out for? Whereas the paleo proxies show us that the AMOC has undergone rapid changes, numerical models are ideal tools to test likely future AMOC scenarios. It is accepted that if subjected to large enough perturbations, the AMOC shuts down in the latest generation of high resolution models [14]. High resolution models also suggest that the AMOC may be bistable – i.e., after a shutdown the AMOC can remains in the “off” state (i.e., severe reduction in AMOC strength) long after the freshwater perturbation has stopped [14]. Observations and high resolution ocean models suggest that presently there is a net freshwater transport out of the Atlantic ocean at 34°S [1][14][15]. Van Westen, and to some extent an earlier study by Hawkins et al. [16], suggest that the AMOC-driven freshwater transport at the southern boundary of the Atlantic reaches a minimum shortly before an abrupt decline in AMOC strength and van Westen also identify a coincident change in temporal variability of the freshwater flux as a precursor of the AMOC change. In classical AMOC stability theory, a net freshwater transport out of the Atlantic at its southern boundary is considered an indicator of AMOC bistability. This view has been questioned by some authors though, and the debate about the role of the AMOC-driven freshwater transport is not yet settled [14][17].
Melting glaciers contribute a large freshwater input around the Arctic

How likely is an AMOC shutdown in response to global warming?

Mechanisms and feedback in the Earth climate system may cause the AMOC to shut down or to switch to a different stable state. However, it is unclear if the warming would be strong enough for the climate system to reach this tipping point. Since the 1990s, the question many studies, including van Westen, have been trying to answer is whether the AMOC will shut down as the climate warms due to human-induced greenhouse gas emissions and the ocean receives increased freshwater input from ice sheets such as Greenland (starting with Manabe & Stouffer [18] and many others since). Over the years, the pendulum has swung from a marked AMOC decline / shutdown being considered likely to this being an unlikely scenario. Currently, the prevalent view is somewhere between the two and in the latest IPCC report, an AMOC collapse in the 21st Century is considered unlikely – a statement made with a medium level of confidence. If and when we could reach the point where the AMOC would switch off is therefore still an open question. There is no consensus yet in the AMOC science community and the recent study by van Westen has to be viewed in this context. The study is a valuable addition to the ongoing discussion about the AMOC and its stability but in the following we summarise why it does not reduce the present uncertainty about a future AMOC shutdown.

An aerial view of the Greenland Icesheet

What are the limitations of the van Westen study?

Simulating an AMOC shutdown scenario using a CMIP6 class model is an impressive achievement by van Westen, as running such a model for thousands of years is computationally very expensive. It is both time and energy consuming. However, it should be noted that similar results were obtained by Hawkins et al. [16] who were the first to simulate an AMOC shutdown (and hysteresis) in a 3D-coupled ocean atmosphere model – albeit one not as complex as in van Westen. Common to both Hawkins et al. [16] and van Westen is that the freshwater perturbations applied over thousands of years amount to water discharge volumes which exceed the total volume of freshwater contained in the Greenland Ice Sheet. The freshwater flux perturbations of 0.1 to 0.6 Sverdrups (Sv) used in van Westen are small when compared to, for example, the 10 Sv pulse applied over 10 years in Mecking et al. [14], but by the time the AMOC shuts down after about 1700 years about 15 million km³ of fresh water has been discharged. This is about 6 times the fresh water volume currently stored in the Greenland Ice Sheet.

The total freshwater volume discharged in Mecking et al. [14] corresponds to the freshwater stored in the Greenland Ice Sheet. Neither the sharp and extremely strong freshwater perturbation used in Mecking et al. [14] nor the weaker but much more prolonged perturbation used in van Westen are possible in the real world. It is also worth mentioning that in van Westen, the freshwater hosing experiment is performed under pre-industrial conditions – i.e. without enhanced greenhouse gas forcing – leaving the background climate cooler than today.

Does the background climate matter?

It is not clear what the response would be if the model experiment were performed in a warming climate scenario. This would have been a more realistic choice given that rising temperatures are consistent with an increased meltwater discharge from Greenland. Under pre-industrial conditions there is no reason for the Greenland Ice Sheet to melt. Admittedly, one could argue that in a greenhouse gas warming scenario the AMOC may be more sensitive to freshwater hosing and that the AMOC could already shut down in response to a smaller freshwater input [19].

What can model resolution tell us?

Finally, even though complex, the Earth System Model used by van Westen has a resolution of about 100 km in the ocean. At this resolution, a model cannot simulate fast-flowing currents such as the Gulf Stream and the associated sharp temperature and salinity fronts are not adequately resolved. In reality, a freshwater discharge would not spread evenly across the entire basin between 20°N and 50°N but would be entrained in the fast boundary currents around Greenland [20][21] and subsequently the Subpolar Gyre. How much a higher model resolution, and a more realistic discharge rate of fresh water along the coast of Greenland, would affect the sensitivity of the AMOC to freshwater perturbations is not yet known.

Neither do we know if the freshwater discharges we can realistically expect from the Greenland Ice Sheet in the coming decades to centuries are large enough to push the AMOC past a tipping point. The past can only serve as a partial analogue for the future here: the rapid changes found in the paleo-record during ice ages happened in a climatic background very different to today’s, with lower temperatures and about three times as much fresh water locked in land ice than today (global sea level was about 130 m lower than today during the Last Glacial Maximum). On the other hand, there is emerging evidence that the AMOC may have undergone prolonged phases (up to 1000+ years) of reduction during previous interglacial periods (the warm periods between ice ages) [22] suggesting that large AMOC changes can occur in warm climates.

The AMOC’s likely future fate remains an important question, though one that we cannot yet answer based on our current level of understanding.

Spotlight article by Joel Hirschi1, Hege-Beate Fredriksen2, Laura de Steur2, Adam Blaker1, David Thornally3, Yevgeny Aksenov1, Eleanor Frajka-Williams4 & Jon Robson5
1 National Oceanography Centre, UK; 2 Norwegian Polar Institute, Norway; 3 University College London, UK; 4 Universitaet Hamburg, Germany; 5 University of Reading, UK 

You may also be interested in…

The Atlantic Ocean’s currents are on the verge of collapse. This is what it means for the planet – article by Prof. David Thornalley in BBC Science Focus Magazine (October 2024)

References

  1. van Westen, R.M. et al. (2024) Physics-based early warning signal shows that AMOC is on tipping course. Science Advances. https://doi.org/10.1126/sciadv.adk1189
  2. Manabe, S. & Stouffer, R.J. (1988) Two stable equilibria of a coupled ocean-atmosphere model. Journal of Climate. https://doi.org/10.1175/1520-0442(1988)001%3C0841:TSEOAC%3E2.0.CO;2
  3. Vellinga, M. & Wood, R.A. (2002) Global climatic impacts of a collapse of the Atlantic thermohaline circulation. Climatic Change. https://doi.org/10.1023/A:1016168827653
  4. Vellinga, M. & Wood, R.A. (2008) Impacts of thermohaline circulation shutdown in the twenty-first century. Climatic Change. https://doi.org/10.1007/s10584-006-9146-y
  5. Jackson, L.C. et al. (2015) Global and European climate impacts of a slowdown of the AMOC in a high resolution GCM. Climate Dynamics. https://doi.org/10.1007/s00382-015-2540-2
  6. Caesar, L. et al. (2022) Reply to: Atlantic circulation change still uncertain. Nature Geoscience. https://doi.org/10.1038/s41561-022-00897-3
  7. Thornalley, D.J. et al. (2018) Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years. Nature. https://doi.org/10.1038/s41586-018-0007-4
  8. Worthington, E.L. et al. (2021) A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline. Ocean Science. https://doi.org/10.5194/os-17-285-2021
  9. Jackson, L.C. et al. (2022) The evolution of the North Atlantic meridional overturning circulation since 1980. Nature Reviews Earth & Environment. https://doi.org/10.1038/s43017-022-00263-2
  10. Kilbourne, K.H. et al. (2022) Atlantic circulation change still uncertain. Nature Geoscience. https://doi.org/10.1038/s41561-022-00896-4
  11. Dansgaard, W. et al. (1993) Evidence for general instability of past climate from a 250-kyr ice-core record. Nature. https://doi.org/10.1038/364218a0
  12. Broecker, W.S. et al. (1985) Does the ocean–atmosphere system have more than one stable mode of operation? Nature. https://doi.org/10.1038/315021a0
  13. Loriani, S.Y. et al. (2023) Global Tipping Points 2023 Report, Chapter 1.4: ‘Tipping points in ocean and atmosphere circulations’. In Lenton, T.M. et al. (eds) The Global Tipping Points Report 2023, University of Exeter, Exeter, UK. https://global-tipping-points.org
  14. Mecking, J.V. et al. (2016) Stable AMOC off state in an eddy-permitting coupled climate model. Climate Dynamics. https://doi.org/10.1007/s00382-016-2975-0
  15. Deshayes, J. et al. (2013) Oceanic hindcast simulations at high resolution suggest that the Atlantic MOC is bistable. Geophysical Research Letters. https://doi.org/10.1002/grl.50534
  16. Hawkins, E. et al. (2011) Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport. Geophysical Research Letters. https://doi.org/10.1029/2011GL048997
  17. Weijer, W. et al. (2019) Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis. Journal of Geophysical Research: Oceans. https://doi.org/10.1029/2019JC015083
  18. Manabe, S. & Stouffer, R.J. (1993) Century-scale effects of increased atmospheric CO₂ on the ocean–atmosphere system. Nature. https://doi.org/10.1038/364215a0
  19. Lin, Y.J. et al. (2023) Mean state AMOC affects AMOC weakening through subsurface warming in the Labrador Sea. Journal of Climate. https://doi.org/10.1175/JCLI-D-22-0464.1
  20. Marsh, R. et al. (2010) Short-term impacts of enhanced Greenland freshwater fluxes in an eddy-permitting ocean model. Ocean Science. https://doi.org/10.5194/os-6-749-2010
  21. Frajka-Williams, E. et al. (2016) Greenland melt and the Atlantic meridional overturning circulation. Oceanography 29. https://doi.org/10.5670/oceanog.2016.96
  22. Galaasen, E.V. et al. (2020) Interglacial instability of North Atlantic deep water ventilation. Science. https://doi.org/10.1126/science.aay6381

 

Further reading

Boyle, E.A. & Keigwin, L. (1987) North Atlantic thermohaline circulation during the past 20000 years linked to high-latitude surface temperature. Nature. https://doi.org/10.1038/330035a0

Caesar, L. et al. (2021) Current Atlantic meridional overturning circulation weakest in last millennium. Nature Geoscience. https://doi.org/10.1038/s41561-021-00699-z

Rühlemann, C. et al. (1999) Warming of the tropical Atlantic Ocean and slowdown of thermohaline circulation during the last deglaciation. Nature. https://doi.org/10.1038/990069

Stocker, T.F. & Schmittner, A. (1997) Influence of CO₂ emission rates on the stability of the thermohaline circulation. Nature. https://doi.org/10.1038/42224

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