Why you need to know what is happening to the AMOC

If the Atlantic Meridional Overturning Circulation (AMOC) stops or slows, my climate will run amok, and so may yours.

Dec 01, 2024

Chart of the Gulf Stream by Benjamin Franklin and Timothy Folge, 1769

A failure of the AMOC would adversely affect the climate of the UK and Europe and parts of North America (with a knock-on effect elsewhere), disrupting agriculture and causing mass migration and food shortages, affecting national security. Sea level rise will also be involved. We don't know when it will happen, but it has been slowing down for years.

Background

The global thermohaline circulation is a 'conveyor belt' of water continuously circulating around the oceans of the world, mixing the waters. The portion of the current in the Atlantic Ocean is called the Atlantic Meridional Overturning Circulation (AMOC). The portion of the AMOC which flows from the Gulf of Mexico, through the Straits of Florida and up the U.S. East Coast is called the Gulf Stream. It then veers east near North Carolina and moves toward Northwest Europe, where it is called the North Atlantic Current, though in Britain I think we tend to still call it the Gulf Stream as it passes us, keeping us relatively warm, even though London is north of Calgary, Canada.

Water from the warmer south heads northward near the surface, bringing warmth with it. It is also saltier than average, as it comes from an area where there is more evaporation. Being saltier, it is denser, and when it cools in the north it sinks and begins the return journey south. However, as a result of global warming the ocean remains warmer, plus melting ice sheets are adding more and more less-dense fresh water to the mix. Studies show that the AMOC has been weaker than before the Industrial Revolution for years. However, it is not known how much of that is due to natural variability - it has varied in the past, for example during the during the Late Pleistocene, as can be seen in Greenland which has been greener in the past. It is predicted to carry on weakening, however.

Climate models have been playing catch-up when it comes to factoring in the slowing down of the AMOC, which interacts with other causes and symptoms of global warming.

A weakening AMOC would affect average air temperatures over Scandinavia, Great Britain and Ireland, which it currently warms.

It would also affect North America by accelerating sea level rise.

If it collapsed completely Europe would become considerably cooler on average with more rain and snow. This would be a major tipping point which it would be hard to recover from.

How we know

While seafarers no doubt knew about the Gulf stream earlier, Juan Ponce de León is the first European known to have mentioned it, in 1513. In 1751 Henry Ellis discovered that there was cold water at depth in the Atlantic, but did not realise the significance. In 1769 Benjamin Franklin had a chart of the Gulf stream printed based on the observations of his whaling captain cousin, Timothy Folger (see picture above).

Henry Stommel wrote a paper about how there can be "free convection between two interconnected reservoirs, due to density differences maintained by heat and salt transfer to the reservoirs" back in 1961 and thought it might be at play in oceanic circulation.

Wallace S. Broecker published the idea of a global conveyor belt back in 1997 (he popularized the term global warming back in 1975). He first conceived of the idea in 1984. He said in 1997 that "Were the ongoing increase in atmospheric CO2 levels to trigger another such reorganization, it would be bad news for a world striving to feed 11 to 16 billion people."

I first heard about the AMOC years ago when I read that it was slowing down and that that would send Britain (where I live), North East America and parts of Europe into a big freeze, so naturally I have been paying attention. William H. Calvin warned of a "drastic cooling—a catastrophe that could threaten the survival of civilization" in 1998. The possibility that intense regional cooling could be triggered by global warming was included in the 2001 IPCC report. In 2003 Peter Schwartz and Doug Randall wrote a report for the Pentagon, "Imagining the Unthinkable", which discussed the implications for United States National Security. They 'created a climate change scenario that although not the most likely, is plausible, and would challenge United States national security in ways that should be considered immediately'. The 2004 American science fiction disaster film The Day After Tomorrow implicated the AMOC.

This is one way how, counterintuitively, global warming can cause colder weather in places.

In the decades since then, predictions as to how fast it was slowing and which parts of the world - if any - would freeze have varied, as more aspects were studied, and the warnings are getting stronger.

How much of that variability was due to developments in the science, and how much to journalistic license, I don't know. Some 'journalists' tend to cherry pick and exaggerate those parts of studies which will get them the most clickbait (though it may only be those who write the headlines, and the actual articles are OK). And scientists tend to vary in their degree of optimism or pessimism like everyone else - glass half full or glass half empty. But more data keeps coming in and the warnings are not going away.

In 2004, the UK's Rapid Climate Change (RAPID) project began providing hourly temperature and salinity data from instruments moored in the Atlantic ocean rather than 5-yearly ones from survey ships. 20 years on they have built up a much better picture of the variability of the current.

In November 2007 a program called Argo which had been proposed in 1999 achieved its planned array of 3,000 profiling floats used to observe temperature, salinity and currents (map).

More recent news

A 2015 paper described how the AMOC has been weakening since 1975 (though it is variable), saying "Further melting of Greenland in the coming decades could contribute to further weakening of the AMOC". The melting of the Greenland ice sheets affects the AMOC and vice versa. A 2019 paper said that this melting was not being incorporated into the climate model used by policy makers.

In 2020 news, "University of Exeter scientists show that, while warming Britain is expected to boost food production, if the AMOC collapses it would not just wipe out these gains but cause the “widespread cessation of arable (crop-growing) farming” across Britain." It could have profound consequences for global food security. The science shows that "economic and land-use impacts of such a tipping point are likely to include widespread cessation of arable farming with losses of agricultural output that are an order of magnitude larger than the impacts of climate change without an AMOC collapse". It was considered unlikely to collapse this century, but scientists have underestimated the speed of climate change before, so I am not betting on it. And the collapse of the AMOC is only one of the tipping points that we risk. They can cascade.

A series of studies around 2018 reinforced the conclusion that the AMOC is weakening, and Prof. Stefan Rahmstorf explained how the North Atlantic can be cooler yet Europe still have warm summers - "the heat transport in the Atlantic has not yet decreased strongly enough to cause cooling also over the adjacent land areas – but the cold of the sea surface is sufficient to influence the air pressure distribution. It does that in such a way that an influx of warm air from the south into Europe is encouraged".

In 2021 a paper was published entitled "Current Atlantic Meridional Overturning Circulation weakest in last millenium". Co-author Prof. Stefan Rahmstorf gives an overview with diagrams here.

In 2022 a new paper described how the effects of a collapsed AMOC would not be confined to the Atlantic, as Prof. Matt England explained. Another study gave details of the effects of previous collapses, for example "a torrential increase in rainfall in Northeast Brazil and a sharp drop in rainfall in Venezuela and the far north of Amazonia".

In mid 2023 a study warned the AMOC could collapse by the middle of the century, possibly by 2025, but other researchers had their doubts. Prof Eleanor Frajka-Williams explains what is known from direct observation - the AMOC is very variable and can even briefly reverse due to wind.

In February this year a study looked at spotting early warning signals and showed that when the tipping point is reached - and it may be close - it could happen in as little as 100 years. In March global climate models still did not include a realistic amount of Greenland ice sheet meltwater. In June a study estimated that the AMOC would collapse between 2037 and 2067, with a probability of it happening before the year 2050 of 59 +/- 17%, and still scientists wrote in October that the UK is ‘flying blind’.

Also in October, a group of climate scientists wrote an open letter to the Nordic Council of Ministers which was meeting, warning that the "risk has so far been greatly underestimated".

Only in November 2024 was a paper published which fully accounted for meltwater input, which improved the match between modelling and observation. Modelling has been underestimating the slowdown up to now.

Why the news can seem confusing

Some studies quoted by climate deniers cite papers with narrow focuses, for example studying a relatively short period when the AMOC was not demonstrably slowing (see postscript here).
The AMOC is a very large current near the surface of the ocean on its way north and in the depths of the ocean on its way south, and is weakening. The surface currents or eddies, however, have been growing stronger and could be worth including in future models.
As mentioned earlier, it is counterintuitive that global warming can cause more freezing in places.
The IPCC reports which policy-makers use tend to be small 'c' conservative, being agreed by committees, with less strong warnings.
One of the pieces of evidence of global warming which affects the AMOC is the appearance of a cold blob in the Atlantic.
The Gulf Stream is not the AMOC, only part of it.
Many articles have similar headlines.

So here we are

So now you are up to date. You should be planning for short term warming, or longer term freezing; more rain or less. But which? Definitely expect the food supply to be affected. Keep an eye on it - as more data comes in and models are improved, we should have a better idea of what to expect.

Some reference articles

Article: 'Thermohaline circulation' by Wikipedia.
Article: 'Atlantic meridional overturning circulation' by Wikipedia.
Article: 'What is the Atlantic Meridional Overturning Circulation?' by Met Office.
Article: 'Tipping points in the climate system' by Wikipedia.
Article: 'What is the Atlantic Meridional Overturning Circulation (AMOC)?' by NOAA.
Article: 'Methodology' by RAPID.
Article: 'Argo (oceanography)' by Wikipedia.
Article: 'Gulf Stream' by Wikipedia.
Article: 'Cold blob' by Wikipedia.

Further reading (ascending date order)

Article: 'Juan Ponce de León' by Wikipedia.
Article: '1785: Observations on the Gulf Stream' by Benjamin Franklin.
Article: 'Thermohaline Convection with Two Stable Regimes of Flow' by Henry Stommel.
Article: 'Climatic Change: Are We on the Brink of a Pronounced Global Warming?' by Wallace S. Broecker.
Article: 'Will Our Ride into the Greenhouse Future be a Smooth One?' by Wallace S. Broecker.
Article: 'Thermohaline Circulation, the Achilles Heel of Our Climate System: Will Man-Made CO2 Upset the Current Balance?' by Wallace S. Broecker.
Article: 'What If the Conveyor Were to Shut Down? Reflections on a Possible Outcome of the Great Global Experiment' by W. S. Broecker.
Article: 'The Great Climate Flip-Flop' by William H. Calvin.
Article: 'Climate Change 2001: The Scientific Basis' by IPCC.
Movie: "The Day After Tomorrow".
Article: 'A sea change' by Quirin Schiermeier.
Book: The Great Ocean Conveyor: Discovering the Trigger for Abrupt Climate Change by Wallace Broecker.
Article: 'Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation' by Stefan Rahmstorf, Jason E. Box, Georg Feulner, Michael E. Mann, Alexander Robinson, Scott Rutherford & Erik J. Schaffernicht.
Article: 'Observing the Atlantic Meridional Overturning Circulation yields a decade of inevitable surprises' by M. A. Srokosz and H. L. Bryden.

2018

Article: 'Atlantic ‘conveyor belt’ has slowed by 15% since mid-20th century by Robert McSweeney.
Article: 'The oceans’ circulation hasn’t been this sluggish in 1,000 years. That’s bad news. by Chris Mooney.
Article: 'Ocean circulation is changing, and we need to know why by Nature.
Article: 'Climate change is slowing Atlantic currents that help keep Europe warm by Peter T. Spooner .
Article: 'The fast-melting Arctic is already messing with the ocean’s circulation, scientists say (about the Oltmanns paper) by Chris Mooney.
Article: 'Greenland ice mass loss during the Younger Dryas driven by Atlantic Meridional Overturning Circulation feedbacks' by Eleanor Rainsley, Laurie Menviel, Christopher J. Fogwill, Chris S. M. Turney, Anna L. C. Hughes & Dylan H. Rood.

2019

Article: 'Global environmental consequences of twenty-first-century ice-sheet melt' by Nicholas R. Golledge, Elizabeth D. Keller, Natalya Gomez, Kaitlin A. Naughten, Jorge Bernales, Luke D. Trusel & Tamsin L. Edwards .
Article: 'Climate tipping points — too risky to bet against' by Timothy M. Lenton, Johan Rockström, Owen Gaffney, Stefan Rahmstorf, Katherine Richardson, Will Steffen & Hans Joachim Schellnhuber.
Article: 'Deep oceans can help us understand our climate' by .

2020

Article: 'Atlantic circulation collapse could cut British crop farming' by Tim Lenton.
Article: 'Map of the week – Argo floats' by European Marine Observation and Data Network (EMODnet).
Article: 'Running AMOC in the farming economy' by Tim G. Benton.
Article: 'Shifts in national land use and food production in Great Britain after a climate tipping point' by Paul D. L. Ritchie, Greg S. Smith, Katrina J. Davis, Carlo Fezzi, Solmaria Halleck-Vega, Anna B. Harper, Chris A. Boulton, Amy R. Binner, Brett H. Day, Angela V. Gallego-Sala, Jennifer V. Mecking, Stephen A. Sitch, Timothy M. Lenton & Ian J. Bateman.
Article: 'Could the Atlantic Overturning Circulation ‘shut down’?' by Richard Wood, Laura Jackson.
Article: 'Likely weakening of the Florida Current during the past century revealed by sea-level observations' by Christopher G. Piecuch.

Scary stuff. I admit I could only quickly scan. Would that people wake up before things worsen. Would that they/we could wake up the powers that be.

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Richard ball

Dec 1, 2024

Our job is to make the voices of all these smart researchers loud and widely heard. There are many more large planet wide cycles that are weather connected tying to balance out the imbalance. Of solar heating going on in constant change since the ice ages. My small opinion is that the buffering in the whole system can not be fixed . The political portion 0f our culture is far from any meaningful action .we need to prepare for a collapse far beyond the known effects forecast..

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