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For over 1,000 years, the Gulf Stream system has not been as weak as it has been in previous decades.
A team of Irish, British and German scientists came to the conclusion in the journal Nature Geosciences.
The finding is alarming because the gigantic ocean circulation is one of the most important heat transport systems on earth: its weakening could have noticeable consequences for the climate.
The Atlantic Meridional Circulating Current (AMOC), as the Gulf Stream system is actually called, works “like a giant conveyor belt that transports warm surface water from the equator to the north and sends cold, low-salt deep water back to the south,” explains Stefan Rahmstorf, researcher at Potsdam Institute for Climate Impact Research (PIK).
As a result, huge people are transported heat.
more on the subject
Gulf Stream stable in the North Atlantic: Our large circulation pump is still working
Alarming study on the consequences of climate change: The Gulf Stream system is limping up A guest contribution by Stefan Rahmstorf
Rahmstorf writes guest posts for the SPIEGEL website at regular intervals.
In one of these he pointed out the weakening of the Gulf Stream in September 2020 (here).
Rahmstorf had already shown in the distant past that the important ocean current has slowed by around 15 percent since the middle of the 20th century, which has been associated with human-made global warming.
However, there were no reliable statements about the long-term AMOC development.
Important parameters have only been measured since 2004.
Natural witnesses to the past
Now the scientists put together so-called proxy data, which can be described as natural witnesses of the past.
"We used a combination of three different types of data to obtain information about ocean currents: the temperature changes in the Atlantic, the distribution of water masses and the grain sizes of the deep-sea sediments, with the individual archives going back 100 to around 1600 years," says Levke Caesar from Maynooth University in Ireland, who is a visiting scholar at PIK.
While individual proxy data are imperfect in depicting the AMOC development, the combination of all three produced a robust picture of the overturning circulation: After a long and relatively stable period, there was an initial weakening since around 1850 with a more drastic decline since the middle of the 20th century .
Already in the special report on the ocean of the Intergovernmental Panel on Climate Change (IPCC) it was read, "that the Atlantic meridional overturning current has become weaker compared to 1850-1900".
Result: more extreme weather events
The system is driven by differences in the density of the water in the oceans: warm, salty surface water flows from south to north, where it cools down and becomes denser.
As a result, it sinks into deeper ocean layers and flows back to the south.
It is precisely this mechanism that is thrown out of balance by global warming.
Increased precipitation and the melting of the Greenland ice sheet are bringing fresh water to the northern Atlantic, reducing the density of the water, preventing it from sinking and ultimately weakening the flow of the AMOC circulation.
That weakening could also have caused a "cold bubble" that has developed in the northern Atlantic over the past few decades.
more on the subject
Consequences of climate change: Greenland's ice sheet is lost
Special report of the Intergovernmental Panel on Climate Change: Land under Von Susanne Götze
The weaker Gulf Stream affects both sides of the Atlantic, explains Caesar: "The northward flowing surface current of the AMOC leads to a deflection of water masses to the right, away from the US east coast." This is due to the earth's rotation, which moves objects such as currents on the Divert the northern hemisphere to the right and in the southern hemisphere to the left.
"When the current slows down, this effect is weakened and more water can accumulate on the US east coast." This can lead to an increased rise in sea levels.
In Europe, on the other hand, a slowdown in the AMOC could cause more extreme weather events and intensify winter storms over the Atlantic.
Other studies cite extreme heat waves or greater drought in summer as possible consequences.
According to the experts, the exact consequences are still the subject of current research
"If we continue to push global warming in the future, the Gulf Stream system will weaken further - by 34 to 45 percent by 2100, according to the latest generation of climate models," says Rahmstorf.
“That could bring us dangerously close to the tipping point at which the current becomes unstable.” In November 2020, experts attested stability to the Gulf Stream in the North Atlantic - even more (read more here).
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