The collapse of the AMOC would be catastrophic. How will we know when it’s coming?

On a sunny evening in July, a research ship called the Sir David Attenborough was working near the Sorgenfri glacier in south-eastern Greenland when a kilometre-long wall of craggy, bulging ice collapsed into the fjord. The scientists on deck watched in stunned silence as an estimated 18 million tonnes of ice slowly flipped over, sending up vast plumes of spray. Iceberg calving on this scale was once relatively rare – but it has become commonplace in recent years as Earth warms. And that acceleration could have catastrophic impacts that extend far beyond Greenland. It is weakening a system of ocean currents in the North Atlantic that is vital to societies in Europe and around the world. If the Atlantic Meridional Overturning Circulation (AMOC) were to shut down, the continent would plunge into “ice age” winters with year-round long droughts, and agriculture would become close to impossible. Yet whether the AMOC really will fail, when that might happen and how quickly – these points all remain highly uncertain. And because of that, the threat remains a distant one in policy-makers’ minds. That is why an £81 million plan is now afoot to build an early warning system that could alert humanity if AMOC is nearing a dangerous tipping point. Advertisement Data now being collected by the Attenborough research ship is core to that plan. “We might be committed to centuries of change without knowing it,” said ocean scientist Paul Holland at the British Antarctic Survey (BAS) as he prepared to set sail from the port of Harwich, UK, some weeks earlier. “We might be passing these tipping points without realising.” So how worried should we be about the AMOC collapsing, what will it take to build an early warning system, and will the alarm bells persuade governments to pull back from the point of no return – or at least to prepare for the consequences? The AMOC is a vast current system that circulates in the ocean between North America and Europe, driven by temperature and salinity. Around the Gulf of Mexico, the harsh sun heats the ocean and drives evaporation. The resulting warm, salty water flows towards the seas between Greenland and the UK in a part of the AMOC called the Gulf Stream. As the circulation pushes north, the atmosphere cools the salty water, so that it becomes denser and cascades towards the ocean floor like a subsurface waterfall. From there, water flows southward along the seafloor back towards the Americas. To replace the water sinking in the North Atlantic, more warm water must then flow from the Gulf of Mexico, powering the circulation. The AMOC is a crucial section of an undulating but well-established conveyor belt of ocean currents that flow around the globe. Traversing the entire belt takes a typical “parcel” (about 1 cubic metre) of water about 1000 years. Like the rest of this conveyor belt, the AMOC, which circulates between North America and Europe, is driven by temperature and salinity. Around the Gulf of Mexico, the harsh sun heats the ocean and drives evaporation. The resulting warm, salty water flows towards the seas between Greenland and the UK in a part of the AMOC called the Gulf Stream. As the circulation pushes north, the atmosphere cools the salty water, so that it becomes denser and cascades towards the ocean floor like a subsurface waterfall. From there, water flows southward along the seafloor back towards the Americas. To replace the water sinking in the North Atlantic, more warm water must then flow from the Gulf of Mexico, powering the circulation. Without the AMOC, models predict that sea ice would cover the North Sea down to Norfolk in the UK and Friesland in the Netherlands. London would experience an average winter temperature of 2°C (36°F), with cold snaps of -20°C (-4°F). Similar cold would descend on Amsterdam, Brussels, Dublin and Paris. Oslo would see deep freezes of -48°C (-54°F) or below at least every 10 years. “The winter is like being in the ice age, and in Scotland, it’s in the ice age for half the year,” says Earth system scientist Tim Lenton at the University of Exeter, UK. Yet this vital circulation is also the section of the global conveyor belt most likely to slow down, or shut down completely, in the coming decades or centuries due to climate change. This possibility was first suggested in a 1987 paper by geochemist Wallace Broecker at Columbia University in New York. Fluctuations in Earth’s climate aren’t always gradual, he wrote, and human-made “global warming”– a term he popularised – could disrupt the feedbacks fuelling the AMOC. At the time, evidence was mounting that AMOC had shut down in the past because of natural changes in the planet’s temperature. About 12,900 years ago, as the last glacial period was ending, the northern hemisphere suddenly slipped back into near-glacial conditions for 1300 years – a period called the Younger Dryas, after an Arctic flower that spread through Europe then. Shortly after Broecker’s paper was published, geochemists studying the composition of shells trapped in marine sediments found that the AMOC was sending less surface water to the seafloor during the Younger Dryas. Broecker and others argued that the melting of an ice sheet over North America had injected fresh water into the North Atlantic, diluting the salty water arriving via the Gulf Stream and slowing its cascade towards the seafloor. “We play Russian roulette with the climate,” he wrote at the time. Today, a similar process may be happening again, only with fresh water from Greenland. “There are 200 fjords around Greenland… pumping out icebergs and gushing out meltwater five times faster than they were 30 years ago,” says marine geophysicist Kelly Hogan, who is part of the BAS mission. Since 2004, sensors measuring water flow have been anchored across the Atlantic as part of the Rapid Climate Change programme. Although it has found early indications that AMOC is weakening, it will take several further decades of data collection to confirm this. Alongside this monitoring, an analysis of how historical Atlantic surface temperatures have shifted concluded that AMOC has slowed 15 per cent since 1950. And the appearance of a “cold blob” in the North Atlantic – the only part of Earth’s surface that has been cooling over the past 150 years – suggests the AMOC is delivering less warm water to the area than before. These aren’t good omens. Yet climate models still vary hugely in terms of how soon they predict the AMOC could reach a tipping point, and how soon after that it would shut down. In June, for instance, a study found a 10 to 20 per cent chance the AMOC has already crossed the point of no return, in which case it could collapse as soon as 2060. But other research this year found the AMOC would weaken only gradually or, if humanity emits carbon at a lesser rate, not at all. Building an early warning system The Advanced Research and Invention Agency (ARIA), a UK government-backed science agency, is building an early warning system to improve these forecasts and update them in almost real time – rather than waiting years, or decades, for studies to come to fruition. To do this, researchers first need to figure out the amount of meltwater from Greenland that is entering the AMOC, what form it is in and how that is altering its flow. On the Attenborough research ship before the expedition, Holland pulled up a map of results from the UK government’s flagship climate model on a screen. Red pixels south of Greenland indicated where dense, salty water is sinking in the AMOC today. Then Holland switched to a forecast for intense global warming in 2090. The red sinking pixels had almost completely disappeared. “This is a really terrifying prospect. So the question is, is this realistic?” he said. Roughly half of the fresh water from Greenland comes from the surface of the ice sheet, which is being melted by warming air temperatures. The other half comes from the fjords, where warming ocean waters melt the glaciers from below, and where icebergs calve off, like the enormous ice face that toppled in front of the ship. The second half is difficult to model. For instance, many fjord glaciers rest on relatively shallow sills of bedrock. If they recede into deeper regions of the fjord, the calving can speed up. “It’s an unstable, self-sustaining process,” says oceanographer Pierre Dutrieux at BAS. “It’s going to retreat like crazy until it finds another sill.” As a result, the UK’s top climate model may be either under- or overestimating the fresh water coming from Greenland. And because icebergs can float thousands of kilometres before they melt, it may be incorrectly predicting where and when that fresh water hits the AMOC. “We need to measure the entire chain of processes,” said Holland. To do that, researchers on the ARIA-funded Greenland Ice to Atlantic (or GIANT) mission have a suite of gadgets at their disposal. “GIANT is the James Bond of science projects,” joked Athena Dinar, head of communications at BAS. Boaty McBoatface, an autonomous underwater vehicle shaped like a fat yellow torpedo, will scan the position of icebergs and bergy bits in the fjords; DriX, a red surface robot with a sonar pod underneath it, will map the geometry of ice sheet faces; and small bundles of sensors will, once delivered by a robot, automatically drill themselves into glacier fronts, all to measure how much of the retreat is due to calving versus ice melt. This information will then be fed into ice-sheet models, which will be included in climate models to improve AMOC tipping-point forecasts. The next phase of the ARIA project will focus on finding a way to give routine updates about these risks. One idea is to build an AI model that can reproduce these forecasts without weeks or months of supercomputing time. The AI would automatically analyse satellite imagery of ice fronts to spot accelerations in glacier loss that could increase the freshwater input to the AMOC. As a side point, regular warnings like these would be valuable to local Inuit who fish and hunt seals and whales in Greenland’s coastal fjords. They would know that an outpouring of meltwater or icebergs was coming that might affect these crucial hunting grounds. An early warning system would also need to analyse changes in AMOC ocean circulation. Climate scientist Jon Robson at the University of Reading, UK, is on one of several ARIA-funded teams that are focusing on a crucial part of the AMOC called the subpolar gyre, a vast swirl of ocean currents south of Greenland where warm, salty water is cooled by the atmosphere and sinks. The ice sheet’s melt is expected to slow down the rotation and sinking of that water. Then, a sudden shock could push the weakened AMOC past its tipping point. Because we have never directly observed AMOC collapse – or any climate tipping point – we need to figure out what to look for, says Robson. He and his colleagues are trying to find potential shocks, such as a sudden influx of fresh water from disintegrating Arctic sea ice. “It may be that there is a very definite threshold in the system,” he says. “But it could be that it’s much more blurry.” Preparing for AMOC collapse Although it is challenging to pin down exactly when the AMOC could shut down, its impacts are more certain. Besides lowering winter temperatures across Europe, AMOC collapse would amplify the North Atlantic storm track, bringing more frequent winter storms and causing wild swings between these cold snaps and warm spells. European summers would be almost as hot as today, thanks largely to continued global warming. But the weather would be drier year-round, especially in southern Europe. Droughts like the one that has helped fuel record wildfires in France and Spain this summer would become more frequent and intense. The cold and especially the dryness would wipe out agriculture in some places. Only 7 per cent of the UK would be arable, down from 32 per cent today. AMOC collapse would also diminish monsoon rainfall by almost 30 per cent in West Africa and 20 per cent in India. This could undermine food supplies not only in these areas, where most agriculture relies on the annual rains, but also around the world, since India is the biggest producer of rice and the Ivory Coast of cocoa. “You’re getting a whole-system shock,” says René van Westen at Utrecht University in the Netherlands. “Our global society is built around a strong AMOC.” The ARIA team is aiming to have a prototype early warning system ready by 2030. They hope that by giving governments and key industries an evolving forecast of how imminent a tipping point is, it could spur them to invest in adapting to the potential consequences, as well as in trying to avoid collapse altogether. Discussions about how this would work in practice are only just beginning. Each year, the Met Office, the UK’s weather service, issues a forecast of global temperature and weather trends over the next decade, such as the per cent chance that global warming will exceed 1.5°C. That could be expanded to include the latest probability of AMOC collapse, says Robson. In addition, an early warning system could issue monthly reports of the risks. These could range from a level one, representing no elevated threat, to level three, indicating imminent subpolar gyre collapse, to level five, showing the AMOC tipping point has been crossed, says Lenton. In July, ARIA met with representatives from the UK and other European governments, as well as from industries like water, insurance and finance, to ask what information they would find useful. One idea is that the system could give a running menu of the options available for adapting to AMOC collapse, such as building reservoirs or desalination plants, and insulating houses. “[We could put] it in terms of, well, in 10 years’ time, those opportunities might be more limited or more expensive,” says Gemma Bale, programme director at ARIA. Researchers’ biggest fear isn’t that the ARIA programme will fail to build an early warning system, but that governments and industry will fail to heed the warning. They have been alerting politicians to the dire consequences of climate change, including AMOC collapse, for almost 40 years. Other attempts to warn of risk thresholds, such as the goal to limit global warming to 1.5°C above pre-industrial levels, have failed. Even today, in the UK’s relatively mild and stable climate, 13 per cent of households have trouble keeping their homes warm, while the government doesn’t keep grain reserves and no new water reservoirs have been built since 1992. Van Westen sees little point in building an early warning system. “The risk of reaching the tipping point… is too high to ignore,” he says. “That should be already enough motivation.” But Hogan argues that the ARIA programme is our best hope of avoiding these catastrophic outcomes before it’s too late. She has a 16-year-old daughter who could live to see the societal upheaval caused by a slowing AMOC. “I look at her future and think, what’s it going to be like for them?”