Sooner than they thought?
"Critical Atlantic Ocean current system is showing
early signs of collapse, prompting warning from
scientists
A crucial system of ocean currents may already be on
course to collapse, according to a new report, with
alarming implications for sea level rise and global
weather — leading temperatures to plunge dramatically
in some regions and rise in others.
Using exceptionally complex and expensive computing
systems, scientists found a new way to detect an early
warning signal for the collapse of these currents,
according to the study published Friday in the journal
Science Advances. And as the planet warms, there are
already indications it is heading in this direction.
The Atlantic Meridional Overturning Circulation (the
AMOC) — of which the Gulf Stream is part — works like a
giant global conveyor belt, taking warm water from the
tropics toward the far North Atlantic, where the water
cools, becomes saltier and sinks deep into the ocean,
before spreading southward.
The currents carry heat and nutrients to different
areas of the globe and play a vital role in keeping the
climate of large parts of the Northern Hemisphere
relatively mild.
For decades, scientists have been sounding the alarm on
the circulation’s stability as climate change warms the
ocean and melts ice, disrupting the balance of heat and
salt that determines the currents’ strength.
While many scientists believe the AMOC will slow under
climate change, and could even grind to a halt, there
remains huge uncertainty over when and how fast this
could happen. The AMOC has only been monitored
continuously since 2004.
Scientists do know — from building a picture of the
past using things like ice cores and ocean sediments —
the AMOC shut down more than 12,000 years ago following
rapid glacier melt.
Now they are scrambling to work out if it could happen
again.
This new study provides an “important breakthrough,”
said René van Westen, a marine and atmospheric
researcher at the University of Utrecht in the
Netherlands and study co-author.
The scientists used a supercomputer to run complex
climate models over a period of three months,
simulating a gradual increase of freshwater to the AMOC
— representing ice melt as well as rainfall and river
runoff, which can dilute the ocean’s salinity and
weaken the currents.
As they slowly increased the freshwater in the model,
they saw the AMOC gradually weaken until it abruptly
collapsed. It’s the first time a collapse has been
detectable using these complex models, representing
“bad news for the climate system and humanity,” the
report says.
What the study doesn’t do, however, is give timeframes
for a potential collapse. More research is needed, van
Westen told CNN, including models which also mimic
climate change impacts, such as increasing levels of
planet-heating pollution, which this study did not.
“But we can at least say that we are heading in the
direction of the tipping point under climate change,”
van Westen said.
The impacts of the AMOC’s collapse could be
catastrophic. Some parts of Europe might see
temperatures plunge by up to 30 degrees Celsius over a
century, the study finds, leading to a completely
different climate over the course of just a decade or
two.
“No realistic adaptation measures can deal with such
rapid temperature changes,” the study authors write.
Countries in the Southern Hemisphere, on the other
hand, could see increased warming, while the Amazon’s
wet and dry seasons could flip, causing serious
disruption to the ecosystem.
The AMOC’s collapse could also cause sea levels to
surge by around 1 meter (3.3 feet), van Westen said.
Stefan Rahmstorf, a physical oceanographer at Potsdam
University in Germany, who was not involved with the
study, said it was “a major advance in AMOC stability
science.”
“It confirms that the AMOC has a tipping point beyond
which it breaks down if the Northern Atlantic Ocean is
diluted with freshwater,” he told CNN.
Previous studies finding the AMOC’s tipping point used
much simpler models, he said, giving hope to some
scientists that it might not be found under more
complex models.
This study crushes those hopes, Rahmstorf said.
Joel Hirschi, associate head of marine systems modeling
at the National Oceanography Centre in the UK, said the
study was the first to use complex climate models to
show the AMOC can flip from “on” to “off” in response
to relatively small amounts of freshwater entering the
ocean.
But there are reasons to be cautious, he added. Even
though the study used a complex model, it still has a
low resolution, he said, meaning there could be
limitations in representing some parts of the currents.
This study adds to the growing body of evidence that
the AMOC may be approaching a tipping point — and that
it could even be close.
A 2021 study found that the AMOC was weaker than any
other time in the past 1,000 years. And a particularly
alarming — and somewhat controversial — report
published in July last year, concluded that the AMOC
could be on course to collapse potentially as early as
2025.
Yet huge uncertainties remain. Jeffrey Kargel, senior
scientist at the Planetary Science Institute in
Arizona, said he suspected the theory of a potentially
imminent shutdown of the AMOC “will remain somewhat
controversial until, one year, we know that it is
happening.”
He likened its potential collapse to the “wild
gyrations of a stock market that precede a major crash”
— it’s nearly impossible to unpick which changes are
reversible, and which are a precursor to a disaster.
Modern data shows the AMOC’s strength fluctuates, but
there is no observed evidence yet of a decline, Hirschi
said. “Whether abrupt changes in the AMOC similar to
those seen in the past will occur as our climate
continues to warm is an important open question.”
This study is a piece of that puzzle, Rahmstorf said.
“(It) adds significantly to the rising concern about an
AMOC collapse in the not too distant future,” he said.
“We will ignore this risk at our peril.”
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