Scientists say we could have underestimated the prospect of the main Atlantic current collapsing.
They said there could be a ‘significantly higher risk’ than we realised of the Atlantic meridional overturning circulation, the AMOC, shutting down due to climate change.
We have this current to thank for people in the UK not being able to turn their gardens into ice rinks in winter, like they do in Canada – so if it collapsed, that would be a problem (even if you love ice hockey).
That’s a massively simplified look at what the current does, but essentially it helps with our relatively temperate climate. Without it, we could face drier summers and much colder, stormier winters.
New research from the Royal Netherlands Meteorological Institute (KNMI) and others extended climate models further than 2100, where IPCC simulations have typically stopped.
In the longer running models, going to 2300 and 2500, many showed that it would ‘decline significantly this century, only to shut down completely shortly thereafter’.
They said this process could take 50 to 100 years to complete, so to really understand the picture, we need to look further ahead.
It all might seem far into the future, but the ‘tipping points’ which might set in motion could be much closer.
Dr Jon Robson, of the University of Reading, told Metro that it is ‘very challenging’ to quantify the risk of the AMOC collapsing.
If it happened, things certainly wouldn’t look good. The UK and western Europe could face conditions like the ‘Little Ice Age’, while rainfall could shift south from the Tropics to fall over the ocean instead, causing drought and food shortages. It could even worsen sea level rise around the US East Cost and in Western Europe, as less water would be pulled away from the regions.
Dr Robson said: ‘The paper reveals that we may have been underestimating the risk that the AMOC collapses at some point in the future.’
He cautioned: ‘We still don’t know the overall likelihood – the models may be too sensitive, and they are using extremely high CO2 emissions.
‘We need urgently need to work out whether it is plausible, but the overall result is definitely concerning.’
How could climate change make the AMOC collapse?
The current relies on cold, salty water sinking in the northern Atlantic.
A warming climate makes ocean surface temperature warmer, while melting fresh water from ice caps, as well as increased rainfall, makes it less salty.
This means the circulation is weakened, and less warm water is pulled up from the tropical southern ocean.
Earlier this year, the Met Office published a paper saying that collapse of the AMOC this century was unlikely, and aspects may be ‘more robust’ than we previously thought, even though it is ‘very likely’ to weaken.
But the paper published last week warned that even if the current keeps circulating past 2100, a tipping point which would eventually stop it could be reached far before.
It found that if carbon emissions keep rising, 70% of the models led to AMOC collapse, while an intermediate scenario would lead to collapse in 37% of models. Even in a low emissions future, a collapse eventually happened in a quarter of models.
The study warms that deep convection could stop ‘in the next decade or two’, which could push the current ‘into a terminal decline’.
Deep convection is the process by which cold water in the northern ocean sinks down as it is loaded with salt and becomes denser, pulling warmer water north from the tropics.
To complicate things further, it is also hard to quantify the impact of melting fresh water from the Greenland ice sheet on this convection of saltier cold water.
Lead study author Sybren Drijfhout from the Royal Netherlands Meteorological Institute said: ‘The deep overturning in the northern Atlantic slows drastically by 2100 and completely shuts off thereafter in all high-emission scenarios, and even in some intermediate and low-emission scenarios. That shows the shutdown risk is more serious than many people realise.’
Dr Robson said tipping points were ‘complex’ and we might not even know if we had passed one ‘until it was much too late’.
‘Ultimately, the the only real way to be safe about tipping points is to reduce emissions as soon as possible,’ he said.
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