Context: Tipping Points - Gulf Stream / Atlantic Meridional Overturning Circulation (AMOC) Collapse

1. Overview – Thresholds

(from McKay et al, Science (2022))

Type – Feedback mechanisms weakening system leading to potential major system tipping-point tipping to an ‘off’ state
Temperature Threshold (estimated) at +4 degrees C (min. 1.4 C, max – 8 C).
Abrupt Change Possible – Yes (medium confidence)
Reversibility – Reversible but only within centuries timeframe (high confidence)
Time-frame to threshold (estimated): 100 years (min – 15 years, max– 300 years), assuming no change in current emissions pathways
Impact Scale – Global and Regional

2. Mechanisms

Thermohaline Circulation system

The Atlantic Meridional Overturning Current or AMOC operates as a large scale current system that operates as an enormous heat conveyor, delivering warm waters from the tropics to the northern upper latitudes of Northern Europe and Greenland where it discharges the heat to the atmosphere. AMOC is often referred to, as the Gulf Stream but which is only the north easterly warm current part of the system delivering warmer temperatures to the British Isles and neighbouring coastal Continent.

According to Rahmstorf (2024), the AMOC delivers approximately one petawatt of heat (10 to the power of 15 or one quadrillion), equivalent to 50 times the total global energy consumption, to North West Europe, keeping the region significantly more temperate than its Canadian counterparts on the same latitude. It is also, as Rahmstorf indicates, the reason that the whole Northern Hemisphere is on average 1.4C warmer than the Southern Hemisphere and the reason that the thermal equator is 10 degrees north of the geographic equator.

The following diagram from Rahmstorf illustrates the current system in operation – The first thing to note is that the Northern Atlantic AMOC is itself part of a much larger current circulation system again called the thermohaline system, ‘thermo’ because the ocean currents are driven by relative differences in water heat and densities, as opposed to winds and tidal currents and ‘haline’ because it is dense salt waters that drives the current as will be seen below.

Starting the Northern leg off the coast of Central America, evaporation makes these warm waters more salty and therefore more dense. These warm, dense waters travels across up and across the North Atlantic where they discharge the heat to the atmosphere and cool. These cooler, more dense waters then sink in what is called ‘deep water formation’ and travel down the Americas at great depths to renew the conveyor system. This deep return current is what draws further warm salty surface waters up from the tropics to the northerly latitudes in what Carbon Brief describes the AMOC as a ‘self reinforcing system’.

Salt transport feedback – However as Ramstorf (2024) describes, the system can be weakened when the waters are diluted in the North Atlantic by the discharge of melting freshwater ice from the Greenland Ice Sheet and increased rainfall and become less dense and therefore lighter and will sink less. This reduced delivery of dense waters weakens or slows the AMOC and the related heat flows in what he describes as the ‘salt transport feedback’. As the circulation system slows, less warm salty water is transported to northern latitudes, less sinking occurs and the system slows further. From recent research, Carbon Brief reports that the AMOC has already weakened by around 15% in the past 75 years.

Tipping Point?

This system slow down can reach a classic tipping point where the AMOC stops functioning. Ramstorf (2024) states ‘beyond a critical threshold, it becomes a self-amplifying vicious circle and the AMOC grinds to a halt.’ The question then is what is that tipping point, when would we reach it and do we get any early warning signs?

One of the most difficult and controversial debates in the study of feedbacks and tipping-points is at what point the AMOC may stop or slow dramatically which would have substantially the same impacts – from the literature, it appears the consensus is that a real shutdown would not happen below 3C or 4C warming. However it is also the case that the continuous data necessary to test and calibrate the computer modelling of an AMOC shutdown is only about 50 years long and due to the natural variances in the current, the data set is not considered to be sufficient to provide tighter parameters or more accurately predict a stopping point. The result is that the estimated distance to that tipping point in terms of temperature increase and time varies a lot in the modelling and with a wide uncertainty range.

Model Biases

It also caused some recent concern to Ramsdorf and others that the computer models may be incorrectly configured with an unjustified bias towards stability of the system ie. a false bias based on the AMOCs current state. Indeed the IPCC in the most recent 6^th Assessment Report picks up on this concern in the following passage quoted in TippingPoints.Info:

“Tuning towards stability and model biases… provides CMIP models [the complex climate models used by the IPCC] a tendency toward unrealistic stability (medium confidence). By correcting for existing salinity biases, Liu et al., (2017) demonstrated that AMOC behaviour may change dramatically on centennial to millennial timescales and that the probability of a collapsed state increases.”

More worryingly, the passage continues with reference to the risks of Greenland ice sheet melting:

“None of the CMIP6 models features an abrupt AMOC collapse in the 21st century, but they neglect meltwater release from the Greenland ice sheet and a recent process study reveals that a collapse of the AMOC can be induced even by small-amplitude changes in freshwater forcing (Lohmann and Ditlevsen, 2021).” IPCC AR6 WG1 Chapter 9 (9.2.3.1) [emphasis & explanatory notes added]

This uncertainty is reflected in the broad temperature range at which a shut down could occur as indicated in a recent article from Ditlvesen & Ditlevesen (2023) which gained widespread coverage – what is significant is not the full range of estimation but the lower end from indirect, early warning signs that a shutdown may occur which the indicate as between 2025 up to 2095 and likeliest on this model in 2050. Quoting from the article:

Here we calculate when the EWS [early warning signals] are significantly above the natural variations. Furthermore, we have provided a method to not only determine whether a critical transition will happen but also an estimate of when it will happen. We predict with high confidence the tipping to happen as soon as mid-century (2025–2095 is a 95% confidence range).

As the commentary in TippingPoints.Info explains, early warning signals provide statistical markers for water salinity or other features of the AMOC current which might indicate that a tipping is approaching but they are not themselves direct proof of a tipping point. So, in the model used in the Ditlvesen & Ditlevesen 2023 paper, the values at which a tipping points occurs are predetermined rather than demonstrated and the model itself is much simplified. Nevertheless as TippingPoints.Info notes the closeness of the early warning signals provided by the model to real life observations supports its analysis of a much sooner tipping point than previously considered likely.

Other AMOC Tipping Points

While the focus in this Context article (and most of the academic articles) has been on the northern branch of the AMOC, TippingPoints.Info points out how the IPCC have in their most recent 6^th Assessment Report also now included the southern branch, the Southern Meridional Overturning Circulation (SMOC) as having the same dynamic with a potential tipping point and irreversible/abrupt event. This is predicted to operate as a reinforcing feedback by the melting of the massive Antarctic Ice sheets as in the the Greenland Ice-sheet melting in the Northern hemisphere.

It is also reported in TippingPoints.Info how a portion of the AMOC might turn off much sooner than the rest of the AMOC called the ‘sub polar gyre’ in the region south of Greenland and east of Newfoundland where some models have found that the deep water mixing in the Labrador-Irmniger Seas could collapse separately to the wider AMOC. Those models predicted collapse of the sub polar gyre was triggered at a lower +1.8C degrees and only taking ten years to collapse.

Further Evidence

It is appropriate to also point out two other features related to the AMOC of significance. Firstly is the famous ‘blue blob‘ which appears in ocean heat maps illustrating a warming ocean generally but with the distinct exception of a cooling location to the south east of Greenland, exactly where the deep water formation is primarily located: it is postulated that this cooling trend is an indicator of a weakening AMOC, though acknowledge that other explanations are possible. More particularly, certain models suggest this cold blob is the consequence of the sub system, the Sub Polar Gyre, discussed above.

The second element is that we know that in the geological past that the AMOC did stop completely during periods what are know as ‘Heinrich events’ where mass outbursts and freshwater release from the North American Laurentide Ice Sheet melting at the time resulted in a compete breakdown of the AMOC.

Irreversibility

Ramsdorf explains how the AMOC is actually stable in two regimes: an ‘On’ and an ‘Off’ mode. In the On or operating mode, the current continues to function despite disturbances, primarily freshwater inflows, and can revert to full operation once the forcing ends (‘monostable’ regime’) . In the other Off mode, a tipping point is reached, as discussed above, where the system has been significantly weakened by the freshwater forcing that the warm water sinking stops resulting in the large overturning current circulation stopping and the systems switches to an ‘Off’ mode which is also a stable system state.

This means that following a further, temporary forcing, the AMOC flow will not restart again but will continue in the ‘Off’ mode and the conditions to resume the flow and turn the system back to the ‘On’ mode must be much greater than the final step to turn the system off so can remain in the off position indefinitely for centuries and millenia (‘bistable regime’). This is in fact a classic example of what a climate ‘tipping point’ is and how it functions – sudden and irreversible (in century scales).

Ramsdorf (2024) explains the two regimes in the following terms, with reference to the diagram below:

‘Climate change can drive the AMOC away from the equilibrium line, following something like the blue path in Figure 3a, because modern global warming proceeds too fast for the ocean to fully adjust. After crossing the dashed line, the AMOC will be attracted toward the “off” state even without further pushing. Note that the AMOC is all the more vulnerable to more rapid forcing (Stocker and Schmittner, 1997). That means that the very slow equilibrium-tracing experiments understate how close the AMOC tipping point is in a situation of rapid climate change, as we are in today.’

As mentioned earlier and highlighted in the above extract, the major concern reflected here and in the IPCC considerations is that computer models are likely not correctly configured to accurately model the abrupt regime changing tipping point and may therefore be lulling into a false sense of security.

3. Impacts

With all the discussion on AMOC tipping points and estimations about when that may occur and the uncertainties, it is perhaps easy to become distracted from the significance and impact of such an event if it should occur. On this, all the writers are unanimous; that the impact would be devastating, to an existential degree on the North West Atlantic region including the British Isles and North West Europe. Carbon Brief quotes Dr Richard Wood of the Met Office Hadley Centre who states a shutdown would cause:

“widespread cooling around the whole of the northern hemisphere, but particularly around western Europe and the east coast of North America”, says Wood. This could be in the order of “several degrees, possibly 5C”.

TippingPoints.Info indicates how this would also have the effect of shifting the Jet Stream, that fast moving air mass northwards and also affect the Atlantic storm tracks and consequently change weather patterns throughout Europe. Separately, a collapse of the Sub Polar Gyre would also have significant but lesser effects though as we have seen could happen in the more immediate future.

Rahmstorf in the Carbon Brief explainer notes how:

‘the whole North Atlantic ecosystem is adapted to the existence of this overturning circulation which really sets the conditions – the seasonal cycle, the temperature, the nutrient conditions – in the North Atlantic, and so the intricate web of the Atlantic ecosystem will be substantially disrupted if allow such a massive change in the ocean circulation to happen.’

In particular, as the Carbon Brief article highlights, the impact of an AMOC collapse would directly and severely impact farming in North West Europe with one report in Nature Food indicating that arable farming would no longer be economically feasible in Britian (or by extension Ireland and North West Europe) ‘with losses of agricultural output that are an order of magnitude larger than the impacts of climate change without an AMOC collapse’. Whilst that is easily stated, the economic, political and social consequences would be quiet simply enormous.

Other impacts indicated in the reports include:

Rainfall – would be affected and reduced as explained by Dr Woods to Carbon Brief as there would be less evaporation from the North Atlantic.

Sea Level rises – Rahmstorf notes several studies which show that as the AMOC weakens, sea levels on the American northeast coast will rise as the AMOC and aligned Gulf Stream weaken, particularly where the sub polar gyre weakens.

West Antarctic Ice – Rahmstorf notes with the slowing of SMOC, this would result in reduced water transported heat away from the low Southern hemisphere consequently adding to the melting of the Antarctic and generate a further feedback loop of freshening waters and further reducing the SMOC, this southern leg of the thermohaline circulation.

Monsoons–illustrating the interconnectedness of the worlds major climatic systems, the West African Monsoon as well as the Asian monsoons are indicated in the IPCC 6^th Assessment Report to weaken whilst the Southern Hemisphere monsoons would strengthen. This would put the Amazon forest under further drought stress as well as impact populations in Central America, West Africa and parts of Asia as TippingPoints.Info indicates. This impact was detected in the recent paleoclimatic past, during the Younger Dryas event (approx. 12k years ago), when the AMOC slowed significantly and monsoon rainfall was directly impacted as illustrated in the following map from TippingPoints.Info:

Reduced Oxygen delivery to deep ocean – This impact would follow directly from the reduced deep water formation in which vast amounts of water sink which also carry enormous volumes of oxygen from the upper levels of water to deeper levels which are vital for the deep-water ecosystem, further impacting the delicate balance of nature.

Conclusion

Returning to the estimations of the weakening of the AMOC, the timeframes and potentials for a switch to an ‘Off’ state, Rahmstorf questions how any of these outcomes are worth the risk, however that risk is framed, in view of the major consequences they bring. He states:

A full AMOC collapse would be a massive, planetary-scale disaster. We really want to prevent this from happening.

In other words: we are talking about risk analysis and disaster prevention. This is not about being 100% or even just 50% sure that the AMOC will pass its tipping point this century; the issue is that we’d like to be 100% sure that it won’t. That the IPCC only has “medium confidence” that it will not happen this century is anything but reassuring, and the studies discussed here, which came after the 2021 IPCC report, point to a much larger risk than previously thought.

That is both a fitting conclusion to the analysis to date and yet another specific reminder to us all of the urgent need to stop heating the planet for the planetary risks it creates.