The ‘tipping point’ that could drop Britain’s future temperatures by 10C

Sea ice on the coast of Scotland with bitterly cold, stormy winters and temperatures in Europe plunging by 10 degrees. It doesn’t sound like the average climate prediction.

But it’s a realistic scenario, and could happen over the space of a couple of decades, experts are warning - with the culprit is human-made CO2 emissions.

While the world has warmed over the past few decades, there are signs that the North Atlantic has failed to keep up - and that could be a warning sign of what’s to come, explains Professor Jon Robson of the University of Reading. The potential culprit is the AMOC (Atlantic Meridional Overturning Circulation), a system of ocean currents in the North Atlantic that moves heat northward through to higher latitudes.

Professor Robson says, ‘As that water moves northward, it gets colder. It gets denser, and it sinks and then it returns. It's part of the reason why the North Atlantic is relatively warmer, and so the European climate is relatively warmer than, say, North American climate.’

But scientists are increasingly worried that the AMOC might rapidly weaken due to climate change warming water in the Atlantic - sending temperatures in Europe plunging.

Why are scientists worried?

Some measurements point to the AMOC is already weakening, with some research suggesting it might hit a ‘tipping point’ which would potential see it rapidly change.

One of the reasons scientists are concerned is that there is evidence of something similar happening in Europe 12,000 years ago, Professor Robson explains.

"One thing that worries scientists is that there’s ‘paleo’ evidence - evidence of the climate from the past - in stuff like ice cores, that there have been very rapid and abrupt changes in the North Atlantic region in the past. Large cooling events over the space of a few decades," Prof Robson explains.

"These have been linked to changes in the AMOC. The last time this happened was about 12,000 years ago. As we were coming out of the last ice age, there’s a rapid cooling of the North Atlantic region in particular."

Robson says that scientists fear that a "similar thing" might occur due to climate change, with a rapid weakening event triggered by warming water meaning that the AMOC could reduce quickly over the space of a couple of decades.

Why could the AMOC weaken?

The AMOC is driven by the cooling and densification of the water in the North Atlantic - but climate change will make the water in the North Atlantic warmer and less dense.

Much of the warming caused by man-made climate change is absorbed by the oceans, and over time, this could cause changes to currents such as the AMOC.

Professor Robson says: "This slows down the formation of this dense water that goes south, completing the circuit. That's the main reason why we think it's going to slow down.

"It will transport less heat into the North Atlantic. And so could potentially change the way the North Atlantic temperatures change over the next 100 years or so."

Could there be a 'tipping point'?

But some scientists fear that this change could occur rapidly, Professor Robson warns - not overnight, but over the space of a few decades.

"A tipping point can most simply be thought of as some sort of threshold where changes in the climate system - the AMOC in this case - change very rapidly. Evidence suggests that such an abrupt weakening of AMOC has happened before at times of large climate change, such as the last deglaciation, and climate models suggest that the AMOC can weaken abruptly under specific conditions.

"Tipping points are also associated with changes that are hard to reverse. So, there is a worry that as the climate continues to warm we could cross a critical threshold that leads to a rapid and abrupt decrease in the AMOC even if we hit net zero emissions. If the AMOC does collapse it could also remain "off" for hundreds of years.

"Tipping points are thought to exist in the AMOC because it is thought to sustains itself. This is because it moves water that is salty, as well as warm, into the North Atlantic. The salt helps to drive the higher density of water needed to form the headwaters of the AMOC. So, if the AMOC weakens, it may come to a point where it can no longer sustain its own strength and weakens.

"However, how close to such a critical threshold we really are, or whether such a threshold really exists at all in the real world, is not well understood."

What would this mean for Britain?

If the AMOC weakened rapidly, temperatures in Britain would change over the space of a couple of decades - dropping by up to 10 degrees.

Professor Robson says: "Western European temperatures might decrease by 10 degrees. You would have sea ice down the coast of Scotland, along with a large increase in sea level across Western Europe - with windier, stormy winters.’

Research in 2020 suggested that crop production in Britain could plummet if the AMOC weakened rapidly.

Land suitable for arable farming could plunge by a quarter, reducing crop value by £346 billion a year.

Are there signs this is happening?

There are some signs that the AMOC could be declining, but further research is needed, Robson says.

"The North Atlantic is warming less fast than the rest of the globe, especially when you look over the last 100 years. Some people are pointing to this and saying that this is consistent with what we’d expect if the AMOC is declining.

But Professor Robson says that these signs are not conclusive.

"We’ve only been observing the AMOC for the last 20 years or so, and during that time there’s been a bit of a weakening but not much, and the variability is huge. There may already be a long-term weakening of the AMOC but it’s hard to spot it in the observations we have so far."

Further research with better climate models can help us to understand whether the AMOC will hit a ‘tipping point’ in the near future, Professor Robson says.

"A lot of this work about abrupt climate changes relies on climate models, because we have no other laboratory. But they are not perfect - and we are trying to test these hypotheses using very-state-of-the-art models compared to the ones which are cheaper to run."

What can be done to stop it?

There’s no alternative way to stop the weakening of the AMOC bar limiting human climate emissions, Professor Robson says.

The more carbon we emit into the atmosphere, the more likely the AMOC will decrease very abruptly. To stay protected, we need to bring down emissions as quickly as possible - there’s no other reasonable approach.