Weakening of the Atlantic Meridional Overturning Circulation driven by subarctic freshening since the mid-twentieth century by Gabriel M. Pontes & Laurie Menviel, Nov 18, 2024, Nature Geoscience (2024)
Abstract
The Atlantic Meridional Overturning Circulation is the main driver of northward heat transport in the Atlantic Ocean today, setting global climate patterns. Whether global warming has affected the strength of this overturning circulation over the past century is still debated: observational studies suggest that there has been persistent weakening since the mid-twentieth century, whereas climate models systematically simulate a stable circulation. Here, using Earth system and eddy-permitting coupled ocean–sea-ice models, we show that a freshening of the subarctic Atlantic Ocean and weakening of the overturning circulation increase the temperature and salinity of the South Atlantic on a decadal timescale through the propagation of Kelvin and Rossby waves. We also show that accounting for upper-end meltwater input in historical simulations significantly improves the data–model agreement on past changes in the Atlantic Meridional Overturning Circulation, yielding a slowdown of 0.46 sverdrups per decade since 1950. Including estimates of subarctic meltwater input for the coming century suggests that this circulation could be 33% weaker than its anthropogenically unperturbed state under 2 °C of global warming, which could be reached over the coming decade. Such a weakening of the overturning circulation would substantially affect the climate and ecosystems.
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Valuable 30 Min. AMOC overview by Prof Stefan Rahmstorf July 7, 2024:
Prof. Stefan Rahmstorf @rahmstorf:
One of the most ominous risks for Europe is that of a major change in Atlantic ocean currents.
Recent science suggests it has been greatly underestimated in the past -including by me, having worked on it for over 30 years. …
Dr. Rahmstorf’s talk above is covered in his published article with excellent easy to grasp visuals:
Is the Atlantic Overturning Circulation Approaching a Tipping Point? by Stefan Rahmstorf, Published Online: April 10, 2024, Oceanography, The Official Magazine of The Oceanography Society, View Issue TOC, Volume 37, No. 3, Pages 16 – 29
- https://doi.org/10.5670/oceanog.2024.501
- Full Article: PDF
…
What does this mean for our future? Let’s first look at the impacts of an AMOC slowdown or collapse, and then discuss the implications.
How Bad Would It Be?
The current cold blob is already affecting our weather, though not in the way that might be expected: a cold subpolar North Atlantic correlates with summer heat in Europe (Duchez et al., 2016). The cooling of the sea surface is enough to influence the air pressure distribution in a way that encourages an influx of warm air from the south into Europe. For example, in summer 2015, the subpolar Atlantic was the coldest since records began in the nineteenth century—while Europe suffered a strong heatwave. Subsequent study has shown that heatwaves are increasing three to four times faster in Europe than in other regions of the Northern Hemisphere, related to changes in the jet stream that may well be influenced by the cold blob (Rousi et al., 2022).
Several studies show that if the AMOC weakens, sea levels on the American northeast coastline will rise more sharply (e.g., Levermann et al., 2005; Yin et al., 2010). The Coriolis force pushes moving water, in this case, in the Gulf Stream, to the right, away from the American coast. When the Gulf Stream weakens, less water is moved northward, causing water levels to rise inshore of the Gulf Stream, with models projecting a 15–20 cm rise by 2100 from this effect alone, in addition to other causes of rising seas. Coastal erosion, the frequency of nuisance flooding, and extent of storm surge damage will substantially increase.
A collapse of convection in the subpolar gyre would significantly magnify these problems. Figure 14 shows the expected temperature change in this case. It is not so much the absolute change, but the changes in temperature contrast between neighboring regions—here, the cold ocean relative to the adjacent warm land masses—that will greatly change the dynamics of the weather, as temperature gradients drive weather activity in ways we can’t foresee in detail. Even this limited oceanic change will shift tropical rainfall belts, though not by as much as a full AMOC shutdown.
| FIGURE 14. Temperature changes in the model-mean before and after a collapse of convection in the subpolar gyre region are plotted here. From Swingedouw et al. (2021). > High res figure |
A full shutdown of the AMOC would have truly devastating consequences for humanity and many marine and land ecosystems.
Figure 15 shows the model of Liu et al. (2017) after a doubling of CO2, with an AMOC collapse caused by this CO2 increase. The cold air temperatures then expand to cover Iceland, Britain, and Scandinavia. The temperature contrast between northern and southern Europe increases by a massive 4°C, likely with major impact on weather, such as unprecedented storms.
| FIGURE 15. Annual-mean near-surface air temperature change resulting from a CO2 doubling and AMOC breakdown. While Earth is much warmer, the northern Atlantic region has become colder. In winter, the cooling there is much larger still. From Liu et al. (2017). > High res figure |
Figure 16 shows the precipitation changes in this model. As we have seen in the paleoclimate data for Heinrich events, major precipitation shifts in the tropics would likely cause drought problems in the northern tropics of America as well as Asia. Seasonal changes will be even larger than these annual mean changes. Other simulations predict a significant increase in winter storms in Europe and a “strong reduction of crop yield and pasture” there (Jackson et al., 2015).
| FIGURE 16. Annual-mean precipitation change resulting from a CO2 doubling and AMOC breakdown. Most concerning is the southward shift in tropical rainfall belts and a generally drier Europe. From Liu et al. (2017). > High res figure |
The IPCC summarized the impacts: “If an AMOC collapse were to occur, it would very likely cause abrupt shifts in the regional weather patterns and water cycle, such as a southward shift in the tropical rain belt, and could result in weakening of the African and Asian monsoons, strengthening of Southern Hemisphere monsoons, and drying in Europe” (IPCC, 2021, TS p. 73). Some further consequences include major additional sea level rise especially along the American Atlantic coast, reduced ocean carbon dioxide uptake, greatly reduced oxygen supply to the deep ocean, and likely ecosystem collapse in the northern Atlantic.
Implications: Uncertainty Is Not Our Friend
The risk of a critical AMOC transition is real and very serious, even if we cannot confidently predict when and whether this will happen. We have already left behind the stable Holocene climate in which humanity has thrived (Osman et al., 2021), and the latest IPCC report warns us that beyond 1.5°C of global warming, we move into the realm of “high risk” with respect to climate tipping points (IPCC, 2023).
Also at risk is the Southern Hemisphere equivalent of the northern Atlantic deep-water formation: the Antarctic bottom-water formation. A recent study by Australian researchers concluded that the increasing meltwater inflow around Antarctica is set to dramatically slow down the Antarctic overturning circulation, with a potential collapse this century (Q. Li et al., 2023). That will slow the rate at which the ocean takes up CO2 (hence, more will accumulate in the atmosphere), and it will reduce the oxygen supply for the deep sea.
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.
The Global Tipping Points Report 2023 was published in December 2023, a 500-page effort by 200 researchers from 90 organizations in 26 countries (Lenton et al., 2023). Its summary conclusion reads: “Harmful tipping points in the natural world pose some of the gravest threats faced by humanity. Their triggering will severely damage our planet’s life-support systems and threaten the stability of our societies.”
For the AMOC and other climate tipping points, the only action we can take to minimize the risk is to phase out fossil fuel use and stop deforestation as fast as possible. If we can reach zero emissions, further global warming will stop within years, and the sooner this happens the smaller the risk of passing devastating tipping points. It would also minimize many other losses, damages, and human suffering from “regular” global warming impacts (e.g., heatwaves, floods, droughts, harvest failures, wildfires, sea level rise), which are already happening all around us even without the passing of major climate tipping points.
As another Climate Tipping Points report published in December 2022 by the Organisation for Economic Co-operation and Development (OECD) concludes: “Yet, the current scientific evidence unequivocally supports unprecedented, urgent and ambitious climate action to tackle the risks of climate system tipping points” (OECD, 2022).
It would be irresponsible, even foolhardy, if policymakers, business leaders, and indeed the voting public continue to ignore those risks.
Refer also to Republic of Ireland taking prompt serious action:
2023: Wise! Republic of Ireland Planning Board says no to unclean unnatural frac’d LNG
2017: Ireland is set to become the first country to completely stop funding fossil fuels
2017: Republic of Ireland: President Michael Higgins signs bill making fracking illegal!