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. What’s going on?
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.
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So what is it and why is it important? Are such catastrophic events, like those mentioned above, likely to happen if it disrupted, and what – if anything – can we do about it?
Why the AMOC matters and what's changing
The AMOC – often called ‘the great ocean conveyor’ – is a large system of ocean currents that 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.
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.
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.
Is it definitely 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.
These lines of 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.
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What about the future?
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.
Why might the AMOC shut down?
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. A worryingly high number.
What can we do about the AMOC risk?
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.
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