[A collapse would bring catastrophic climate impacts but
scientists disagree over the new analysis]
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GULF STREAM COULD COLLAPSE AS EARLY AS 2025, STUDY SUGGESTS  
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Damian Carrington
July 25, 2023
Guardian
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_ A collapse would bring catastrophic climate impacts but scientists
disagree over the new analysis _

Perpetual Ocean - Gulf Stream, by NASA Goddard Photo and Video (CC BY
2.0)

 

The Gulf Stream system could collapse as soon as 2025, a new study
suggests. The shutting down of the vital ocean currents, called the
Atlantic Meridional Overturning Circulation (Amoc) by scientists,
would bring catastrophic climate impacts.

Amoc was already known to be at its weakest in 1,600 years
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to global heating and researchers spotted warning signs of a tipping
point
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2021.

The new analysis estimates a timescale for the collapse of between
2025 and 2095, with a central estimate of 2050, if global carbon
emissions are not reduced. Evidence from past collapses
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changes of temperature of 10C in a few decades, although these
occurred during ice ages.

Other scientists said the assumptions about how a tipping point would
play out and uncertainties in the underlying data are too large for a
reliable estimate of the timing of the tipping point. But all said the
prospect of an Amoc collapse was extremely concerning and should spur
rapid cuts in carbon emissions.

Amoc carries warm ocean water northwards towards the pole where it
cools and sinks, driving the Atlantic’s currents. But an influx of
fresh water from the accelerating melting of Greenland’s ice cap
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other sources is increasingly smothering the currents.

A collapse of Amoc would have disastrous consequences around the
world, severely disrupting the rains that billions of people depend on
for food in India, South America and west Africa. It would increase
storms and drop temperatures in Europe, and lead to a rising sea level
on the eastern coast of North America. It would also further endanger
the Amazon rainforest and Antarctic ice sheets.

“I think we should be very worried,” said Prof Peter Ditlevsen, at
the University of Copenhagen in Denmark, and who led the new study.
“This would be a very, very large change. The Amoc has not been shut
off for 12,000 years.”

The Amoc collapsed and restarted repeatedly in the cycle of ice ages
that occurred from 115,000 to 12,000 years ago. It is one of the
climate tipping points scientists are most concerned about as global
temperatures continue to rise.

Research in 2022 showed five dangerous tipping points
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already have been passed due to the 1.1C of global heating to date,
including the shutdown of Amoc, the collapse of Greenland’s ice cap
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an abrupt melting of carbon-rich permafrost
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The new study, published in the journal Nature Communications
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temperature data stretching back to 1870 as a proxy for the change in
strength of Amoc currents over time.

The researchers then mapped this data on to the path seen in systems
that are approaching a particular type of tipping point called a
“saddle-node bifurcation”. The data fitted “surprisingly
well”, Ditlevsen said. The researchers were then able to extrapolate
the data to estimate when the tipping point was likely to occur.
Further statistical analysis provided a measure of the uncertainty in
the estimate.

The analysis is based on greenhouse gas emissions rising as they have
done to date. If emissions do start to fall, as intended by current
climate policies, then the world would have more time to try to keep
global temperature below the Amoc tipping point.

The most recent assessment by the Intergovernmental Panel on Climate
Change concluded that Amoc would not collapse this century. But
Divlitsen said the models used have coarse resolution and are not
adept at analysing the non-linear processes involved, which may make
them overly conservative.

The potential collapse of Amoc is intensely debated by scientists, who
have previously said it must be avoided “at all costs”
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Prof Niklas Boers, from the Potsdam Institute for Climate Impact
Research in Germany, revealed the early warning signs of Amoc
collapse
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2021. “The results of the new study sound alarming but if the
uncertainties in the heavily oversimplified model [of the tipping
point] and in the underlying [sea temperature] data are included, then
it becomes clear that these uncertainties are too large to make any
reliable estimate of the time of tipping.”

Prof David Thornalley, at University College London, UK, agreed the
study had large caveats and unknowns and said further research was
essential: “But _if_ the statistics are robust and a relevant way
to describe how the actual Amoc behaves, then this is a very
concerning result.”

Dr Levke Caesar, at the University of Bremen, Germany, said using sea
surface temperatures as proxy data for the strength of the Amoc
currents was a key source of uncertainty: “We only have direct
observational data of the Amoc since 2004.”

The extrapolation in the new analysis was reasonable, according to
Prof Tim Lenton, at the University of Exeter, UK. He said the tipping
point could lead to a partial Amoc collapse, for example only in the
Labrador Sea, but that this would still cause major impacts. Divlitsen
said he hoped the debate would drive new research: “It’s always
fruitful when you do not exactly agree.”

Prof Stefan Rahmstorf, at the University of Potsdam, Germany, said:
“There is still large uncertainty where the Amoc tipping point is,
but the new study adds to the evidence that it is much closer than we
thought. A single study provides limited evidence, but when multiple
approaches have led to similar conclusions this must be taken very
seriously, especially when we’re talking about a risk that we really
want to rule out with 99.9% certainty. Now we can’t even rule out
crossing the tipping point in the next decade or two.”

* Climate Change
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* Global warming
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* gulf stream
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