Warming has left Gulf Stream at 'weakest point in a millennium'
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The Gulf Stream is at its weakest point in more than a thousand years, say scientists who reconstructed the flow history of the powerful ocean current that brings mild temperatures to the eastern United States and western Europe.
A study by researchers in Ireland, Britain and Germany found “compelling and consistent” evidence that reinforces views the Atlantic Meridional Overturning Circulation, as the Gulf Stream system is known, has slowed to never-before-seen levels in recent decades.
Paleoclimate data taken from deep sea sediments and ice cores dating back 1,600 years allowed the team, whose study was published Thursday in Nature Geoscience, to calculate how the strength of the current has evolved over the centuries.
Evidence of its decline during the 20th century is now unmistakable, says Stefan Rahmstorf, of the Potsdam Institute for Climate Impact Research in Germany, who is one of the study’s authors.
“This is something that climate models have long predicted as a consequence of global warming,” he told RFI.
“We have compiled and statistically analysed 11 independent estimates … that all consistently saw an unprecedented weakening in the 20th century, with relatively stable circulation in the centuries before then.”
Just out: our new paper affirming the unprecedented slowdown of the Gulf Stream System (aka Atlantic meridional overturning circulation, 𝗔𝗠𝗢𝗖) in Nature Geoscience! @NatureGeosci A thread. 1/11 pic.twitter.com/2GovKz5evk— Stefan Rahmstorf 😷 (@rahmstorf) February 25, 2021
'Giant conveyor belt'
Bringing warm surface water from the tropics to the North Atlantic Ocean, the Gulf Stream works “like a giant conveyor belt”. While warm salty water is sent north, colder water that is low in salt sinks to the bottom and is then pumped southwards.
It’s a mighty process that transports some 20 million cubic meters of water per second, with a profound effect on the climate of the North Atlantic – which has already developed a stubborn cold spot.
Climate change caused by humans not only warms the water, but it also adds freshwater to the North Atlantic in the form of rain and meltwater from the Greenland ice sheet, Rahmstorf explains.
“This dilutes the surface waters there, making it harder for those waters to sink.”
Risk of total shutdown
The study is the “strongest, clearest” evidence the Gulf Steam has weakened by at least 15 percent since the mid-20th century, says Rahmstorf – adding that it’s unlikely those changes are a natural part of climate variability.
“It's an observed fact that the North Atlantic is getting less salty," he says. "We know the meltwater comes from global warming and there's no natural explanation for this phenomenon.”
In its fifth assessment, the UN’s Intergovernmental Panel on Climate Change projected the Gulf Stream would continue to weaken even further this century.
Scientists are also talking of a “tipping point” at which the system would shutdown altogether, the consequences of which would be disastrous for countries whose climates depend on the Gulf Stream.
While this is not expected in the coming decades, a complete shutdown could happen under continued global warming, warns Rahmstorf.
The cool spot that has already begun in the North Atlantic as a result of the weakened Gulf Stream would be extended, bringing freezing temperatures to countries on both sides of the Atlantic.
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