Key Atlantic current could collapse soon

 The Atlantic Ocean's currents are on the verge of collapse : Impacting the entire world in future    

Leading climate scientists ring alarm bell on key Atlantic Ocean current collapse in an open letter. Scientists are concerned that the Atlantic Ocean’s system of currents may be about to reach a tipping point. If it does, it’ll have severe consequences for all of us. Leading climate scientists signed an open letter warning about the collapse of the Atlantic Meridional Overturning Circulation (AMOC), which includes the Gulf Stream. Forty-four of the world's leading climate scientists have called on Nordic policymakers to address the potentially imminent and "devastating" collapse of key Atlantic Ocean currents. Icy winds howl across a frozen Thames, ice floes block shipping in the Mersey docks and crops fail across the UK. Meanwhile, the US east coast has been inundated by rising seas and there’s ecological chaos in the Amazon as the wet and dry season have switched around… The world has been upended. While these scenes sound like something from a Hollywood disaster movie, a new scientific study investigating a key element of Earth’s climate system, the Atlantic Meridional Overturning Circulation (AMOC), says this could occur for real as soon as 2050. So what is it and why is it important? Are such catastrophic events, like those mentioned here, likely to happen if it disrupted, and what can we do about it? 

In an open letter, University of Pennsylvania climatologist Michael Mann and other eminent scientists say the risks of weakening ocean circulation in the Atlantic have been greatly underestimated and warrant urgent action. The currents in question are those forming the Atlantic Meridional Overturning Circulation (AMOC), a giant ocean conveyor belt which includes the Gulf Stream and transports vital heat to the Northern Hemisphere. Research shows the AMOC is slowing down and could soon reach a tipping point due to global warming, throwing Earth's climate into chaos. "Such an ocean circulation change would have devastating and irreversible impacts especially for Nordic countries, but also for other parts of the world," the scientists wrote in the letter. The Nordic countries include Denmark, Iceland, Norway, Finland and Sweden.

Importance of AMOC

The AMOC, often called ‘the great ocean conveyor’, is a large system of ocean currents which includes the Gulf Stream. It circulates warm, salty water from the tropics northward into the North Atlantic, where it cools and becomes denser. As this water cools, it sinks and flows back southward at deeper levels, before eventually rising back to the surface throughout the rest of the ocean, creating a continuous loop of circulation. It transports vast amounts of heat around the Atlantic, equivalent to boiling about a thousand billion kettles. It’s responsible for 25 per cent of the total heat flowing into the northern hemisphere by the ocean and atmosphere. This heat warms the winds that blow in off the Atlantic over Northwest Europe, contributes to the relatively mild climate these regions experience and helps to stop Arctic sea ice from spreading down beyond Norway. But its effects are felt more globally, too. If the AMOC gets weaker and causes the northern hemisphere to cool, all the climate belts of the world, including the equatorial rainfall belt, will be shifted further south.

Changes in the location and strength of winds blowing across the ocean can also alter the AMOC. During the Ice Age, stronger winds led to a stronger Gulf Stream, helping strengthen parts of it. In the opposite case, a future warmer world, models predict that winds may contribute to a weaker AMOC. An AMOC collapse would lead to major cooling and extreme weather in Nordic countries. This would enlarge and deepen a strange "cold blob" that has already developed over the eastern North Atlantic due to the slowdown of heat-carrying currents. Collapsing ocean currents are also likely to precipitate climate impacts across the Northern Hemisphere, threatening agriculture in Northwestern Europe. It is therefore a key component of the stability of the global climate system. As global warming continues, the surface of the high-latitude North Atlantic is getting much warmer. It’s also getting fresher because of the melting of the Greenland ice sheet and Arctic sea ice, and due to increased precipitation. This makes the surface ocean less dense, which prevents it from sinking, in turn weakening this important climate system. Other regions would feel the effects, too, the scientists said. Should the AMOC grind to a halt, it would trigger a southward shift in tropical monsoon systems, with catastrophic consequences for agriculture and ecosystems. Halted ocean currents could also further heighten sea levels along the American Atlantic coast and send marine ecosystems and fisheries into a state of "upheaval."

What is changing?

Direct instrumental measurements of the strength of the AMOC only began in 2004, when the RAPID array was installed across the Atlantic. After nearly 20 years of continuous measurements, the data show a 10-per-cent decrease in its strength, but because the observations show such a lot of change from year to year, we can’t be sure that this is a meaningful, long-term decline. To look further back in time beyond 2004, we have to rely on indirect measurements of AMOC strength. Sea surface temperature data from the south of Greenland reveal a cooling trend over the past several decades, which is unusual given the general global warming pattern. At the same time, salty water, which is normally transported into the North Atlantic as part of the AMOC, has been building up in the South Atlantic. These two patterns are thought to be a result of the reduced heat and salt transport by the weakening system. Palaeoclimatologists have also used ocean sediment cores containing mud and the shells of once-living organisms that settled in layers on the seafloor over millennia, to study changes further back in time. Results suggest the current weakening is unprecedented in the last 1,600 years. This evidence indicate that the system may have already weakened by about 15 per cent. The indirect nature of this evidence means we’re not certain if there has been a substantial decrease in the AMOC, however. Without urgent climate action, the AMOC could collapse in the next few decades, although there is huge uncertainty in predicting the timescales. The latest report by the Intergovernmental Panel on Climate Change (IPCC) states that "there is medium confidence that the Atlantic Meridional Overturning Circulation will not collapse abruptly before 2100," but the scientists say this is an underestimate. The IPCC estimate is not only imprecise but also worrisome, according to the letter.

Future estimates

Climate models suggest that the AMOC could weaken by 30–50 per cent by the end of the 21st century if greenhouse gas emissions continue at current rates. This will lead to altered weather patterns and more extreme weather in Europe; altered tropical rainfall patterns, which could potentially lead to droughts or floods; as well as regional changes in sea levels, including a faster increase along the US eastern seaboard by several tens of centimetres. But a more catastrophic future may lie ahead: even with only a small amount of climate warming, the AMOC may switch from strong to very weak, or shut down, in decades. This is called a tipping point and models suggest it could lead to the severe climate impacts outlined earlier. "The purpose of this letter is to draw attention to the fact that only 'medium confidence' in the AMOC not collapsing is not reassuring, and clearly leaves open the possibility of an AMOC collapse during this century," the scientists wrote. "Even with a medium likelihood of occurrence, given that the outcome would be catastrophic and impacting the entire world for centuries to come, we believe more needs to be done to minimize this risk."

Will AMOC Collapse?

The AMOC may cross a tipping point due to the ‘salt feedback’ mechanism. Observations suggest the AMOC imports salty surface water into the Atlantic and exports less salty water in the deep ocean. If the AMOC weakens, the surface of the Atlantic starts to get fresher, which causes further weakening in a runaway process that can lead to sudden collapse. It might only take a small initial forcing (the ongoing melting of the Greenland ice sheet, for example) to kick-start this process. Simple mathematical models revealed this tipping point behaviour in the early 1960s, but until recently most climate models, such as those used for Intergovernmental Panel on Climate Change (IPCC) reports, didn’t show this behaviour. This means the 2021 IPCC consensus view is that an AMOC collapse before 2100 is unlikely. There are concerns, though, that the AMOC in climate models is biased towards being too stable. Also, most future model simulations don’t include the impacts of the melting of the Greenland ice sheet. Statistical analysis of AMOC indicators has recently been used to suggest an impending mid-century AMOC tipping point. These studies have been questioned, however, as they’re derived from the longer, yet indirect, time series of AMOC, not the RAPID array data. New advances are being made using a complex climate model to explore tipping point behaviour and to determine the best location and metric for detecting early warning signs of a ‘salt-feedback’ induced tipping point. By applying this model-based knowledge to observational datasets, one yet-to-be-reviewed study estimates the probability of an AMOC collapse before the year 2050 at somewhere between 42–76 per cent. 

Conclusion

An AMOC collapse could disrupt food supplies and water resources, increase energy demands for heating and strain infrastructure. This requires us to address the underlying cause, global warming, by reducing atmospheric greenhouse gas emissions as quickly as possible. We can also reduce the associated risks by strengthening climate resilience. The scientists addressed the letter to the Nordic Council of Ministers, an intergovernmental forum tasked with promoting cooperation among the Nordic countries. They urged policymakers to seriously consider the risks posed by an AMOC collapse and to increase pressure on international partners to stay close to the targets of the 2015 Paris Agreement, which aims to keep average global temperature rise to 2.7 degrees Fahrenheit (1.5 degrees Celsius) above preindustrial levels.