CLIMATE AND
ENVIRONMENT
A critical ocean system may be heading for
collapse due to climate change, study finds
‘The consequences of a collapse would likely be
far-reaching’
By Sarah
Kaplan
August 5,
2021 at 11:01 a.m. EDT
An aerial
view of Rhode Island shorelines. A weakening of the Atlantic Meridional
Overturning Circulation could bring extreme cold to Europe and parts of North
America, raise sea levels and disrupt seasonal monsoons. (Salwan Georges/The
Washington Post)
Human-caused
warming has led to an “almost complete loss of stability” in the system that
drives Atlantic Ocean currents, a new study has found — raising the worrying
prospect that this critical aquatic “conveyor belt” could be close to collapse.
In recent
years, scientists have warned about a weakening of the Atlantic Meridional
Overturning Circulation (AMOC), which transports warm, salty water from the
tropics to northern Europe and then sends colder water back south along the
ocean floor. Researchers who study ancient climate change have also uncovered
evidence that the AMOC can turn off abruptly, causing wild temperature swings
and other dramatic shifts in global weather systems.
Scientists
haven’t directly observed the AMOC slowing down. But the new analysis,
published Thursday in the journal Nature Climate Change, draws on more than a
century of ocean temperature and salinity data to show significant changes in
eight indirect measures of the circulation’s strength.
These
indicators suggest that the AMOC is running out of steam, making it more
susceptible to disruptions that might knock it out of equilibrium, said study
author Niklas Boers, a researcher at the Potsdam Institute for Climate Impact
Research in Germany.
If the
circulation shuts down, it could bring extreme cold to Europe and parts of
North America, raise sea levels along the U.S. East Coast and disrupt seasonal
monsoons that provide water to much of the world.
“This is an
increase in understanding … of how close to a tipping point the AMOC might
already be,” said Levke Caesar, a climate physicist at Maynooth University who
was not involved in the study.
Boers’s
analysis doesn’t suggest exactly when the switch might happen. But “the mere
possibility that the AMOC tipping point is close should be motivation enough
for us to take countermeasures,” Caesar said. “The consequences of a collapse
would likely be far-reaching.”
The AMOC is
the product of a gigantic, ocean-wide balancing act. It starts in the tropics,
where high temperatures not only warm up the seawater but also increase its
proportion of salt by boosting evaporation. This warm, salty water flows
northeast from the U.S. coastline toward Europe — creating the current we know
as the Gulf Stream.
But as the
current gains latitude it cools, adding density to waters already laden with
salt. By the time it hits Greenland, it is dense enough to sink deep beneath
the surface. It pushes other submerged water south toward Antarctica, where it
mixes with other ocean currents as part of a global system known as the
“thermohaline circulation.”
This
circulation is at the heart of Earth’s climate system, playing a critical role
in redistributing heat and regulating weather patterns around the world.
As long as
the necessary temperature and salinity gradients exist, AMOC is
self-sustaining, Boers explained. The predictable physics that make dense water
sink and lighter water “upwell” keep the circulation churning in an endless
loop.
Tens of
millions of people have been moving into flood zones, satellite imagery shows
But climate
change has shifted the balance. Higher temperatures make ocean waters warmer
and lighter. An influx of freshwater from melting ice sheets and glaciers
dilutes North Atlantic’s saltiness, reducing its density. If these waters
aren’t heavy enough to sink, the entire AMOC will shut down.
It’s
happened before. Studies suggest that toward the end of the last ice age, a
massive glacial lake burst through a declining North American ice sheet. The
flood of freshwater spilled into the Atlantic, halting the AMOC and plunging
much of the Northern Hemisphere — especially Europe — into deep cold. Gas
bubbles trapped in polar ice indicate the cold spell lasted 1,000 years.
Analyses of plant fossils and ancient artifacts suggest that the climate shift
transformed ecosystems and threw human societies into upheaval.
“The
phenomenon is intrinsically bi-stable,” Woods Hole Oceanographic Institution
President Peter de Menocal said of the AMOC. “It’s either on or it’s off.”
But is it
about to turn off now?
“That’s the
core question we’re all concerned about,” said de Menocal, who was not involved
in Boers’s research.
In its 2019
“Special Report on the Ocean and Cryosphere in a Changing Climate,” the U.N.
Intergovernmental Panel on Climate Change projected that the AMOC would weaken
during this century, but total collapse within the next 300 years was only
likely under the worst-case warming scenarios.
The new
analysis suggests “the critical threshold is most likely much closer than we
would have expected,” Boers said.
The
“restoring forces,” or feedback loops, that keep the AMOC churning are in
decline, he said. All the indicators analyzed in his study — including sea
surface temperature and salt concentrations — have become increasingly
variable.
It’s as
though the AMOC is a patient newly arrived in the emergency room, and Boers has
provided scientists with an assessment of its vital signs, de Menocal said.
“All the signs are consistent with the patient having a real mortal problem.”
Physical
oceanographers like him are also trying to confirm the AMOC slowdown through
direct observations. But the AMOC is so big and complex that it will probably
take years of careful monitoring and data collection before a definitive
measurement is possible.
“Yet
everyone also realizes the jeopardy of waiting for that proof,” de Menocal
said.
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After all,
there are plenty of other indications that Earth’s climate is in unprecedented
territory. This summer, the Pacific Northwest was blasted by a heat wave
scientists say was “virtually impossible” without human-caused warming. China,
Germany, Belgium, Uganda and India have all experienced massive, deadly floods.
Wildfires are raging from California to Turkey to the frozen forests of
Siberia.
The world
is more than 1 degree Celsius (1.8 degrees Fahrenheit) warmer than it was
before humans started burning fossil fuels, and it’s getting hotter all the
time.
And the
apparent consequences of the AMOC slowing are already being felt. A persistent
“cold blob” in the ocean south of Greenland is thought to result from less warm
water reaching that region. The lagging Gulf Stream has caused exceptionally
high sea level rise along the U.S. East Coast. Key fisheries have been upended
by the rapid temperature swings, and beloved species are struggling to cope
with the changes.
If the AMOC
does completely shut down, the change would be irreversible in human lifetimes,
Boers said. The “bi-stable” nature of the phenomenon means it will find new
equilibrium in its “off” state. Turning it back on would require a shift in the
climate far greater than the changes that triggered the shutdown.
“It’s one
of those events that should not happen, and we should try all that we can to
reduce greenhouse gas emissions as quickly as possible,” Boers said. “This is a
system we don’t want to mess with.”
What
questions do you have about climate change? Ask The Post.
Brady
Dennis contributed to this report.
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