A discussion of how and why it affects our weather and climate. I hope the
graaphics come through. they are impressive. Eric
In the Atlantic Ocean, Subtle Shifts Hint at Dramatic Dangers
The warming atmosphere is causing an arm of the powerful Gulf Stream to weaken,
some scientists fear.
By MOISES VELASQUEZ-MANOFF
and JEREMY WHITE
COLD BLOB
New York
Miami
SOUTH AMERICA
AFRICA
NORTH AMERICA
GREENLAND
Caracas
San Juan
Panama City
St. John's
Dublin
The Gulf Stream has shaped climate and history on four continents.
The Gulf Stream has shaped climate and history on four continents.
Currents swing west from Africa, ultimately influencing weather patterns from
Caracas to Miami to Europe.
Currents swing west from Africa, ultimately influencing weather patterns from
Caracas to Miami to Europe.
Warmer water sweeps past continents, slowly turning into cooler water farther
north.
Warmer water sweeps past continents, slowly turning into cooler water farther
north.
The Gulf Stream propels the heat of the Caribbean past Cape Hatteras, N.C.,
before bending toward the British Isles.
The Gulf Stream propels the heat of the Caribbean past Cape Hatteras, N.C.,
before bending toward the British Isles.
But now, in the North Atlantic, there is the “cold blob.”
But now, in the North Atlantic, there is the “cold blob.”
The fear: Melting Greenland ice will tip the delicate balance of hot and cold
that defines not only the North Atlantic, but life far and wide.
The fear: Melting Greenland ice will tip the delicate balance of hot and cold
that defines not only the North Atlantic, but life far and wide.
IT’S ONE OF THE MIGHTIEST RIVERS you will never see, carrying some 30 times
more water than all the world’s freshwater rivers combined. In the North
Atlantic, one arm of the Gulf Stream breaks toward Iceland, transporting vast
amounts of warmth far northward, by one estimate supplying Scandinavia with
heat equivalent to 78,000 times its current energy use. Without this current —
a heat pump on a planetary scale — scientists believe that great swaths of the
world might look quite different.
Now, a spate of studies, including one published last
week<https://www.nature.com/articles/s41561-021-00699-z>, suggests this
northern portion of the Gulf Stream and the deep ocean currents it’s connected
to may be slowing. Pushing the bounds of oceanography, scientists have slung
necklace-like sensor arrays across the Atlantic to better understand the
complex network of currents that the Gulf Stream belongs to, not only at the
surface, but hundreds of feet deep.
“We’re all wishing it’s not true,” Peter de Menocal, a paleoceanographer and
president and director of the Woods Hole Oceanographic Institution, said of the
changing ocean currents. “Because if that happens, it’s just a monstrous
change.”
The consequences could include faster sea level rise along parts of the Eastern
United States and parts of Europe, stronger hurricanes barreling into the
Southeastern United States, and perhaps most ominously, reduced rainfall across
the Sahel, a semi-arid swath of land running the width of Africa that is
already a geopolitical tinderbox.
The scientists’ concern stems from their understanding of thousands of years of
the prehistoric climate record. In the past, a great weakening or even shutdown
of this arm of the Gulf Stream seems to have triggered rapid changes in
temperatures and precipitation patterns around the North Atlantic and beyond.
The northern arm of the Gulf Stream is but one tentacle of a larger,
ocean-spanning tangle of currents called the Atlantic Meridional Overturning
Circulation, or AMOC. Scientists have strong evidence from ice and sediment
cores that the AMOC has weakened and shut down before in the past 13,000 years.
As a result, mean temperatures in parts of Europe may have rapidly dropped to
about 15 degrees Celsius below today’s averages, ushering in arctic like
conditions. Parts of northern Africa and northern South America became much
drier. Rainfall may even have declined as far away as what is now China. And
some of these changes may have occurred in a matter of decades, maybe less.
The AMOC is thus a poster child for the idea of climatic “tipping points” — of
hard-to-predict thresholds in Earth’s climate system that, once crossed, have
rapid, cascading effects far beyond the corner of the globe where they occur.
“It’s a switch,” said Dr. de Menocal, and one that can be thrown quickly.
Which brings us to the cold blob. Almost everywhere around the world, average
temperatures are rising — except southeast of Greenland where a large patch of
the North Atlantic has become colder in recent years.
COLD BLOB
New York
Miami
NORTH AMERICA
GREENLAND
San Juan
St. John's
Dublin
Cold water
circulation
COLD BLOB
Currents 1,000 feet
below surface
Currents 1,000 feet
below surface
Deep beneath the surface, scientists are searching for changes in the currents.
Deep beneath the surface, scientists are searching for changes in the currents.
Some fear that meltwater from Greenland is already inhibiting the northward
flow of the Gulf Stream.
Some fear that meltwater from Greenland is already inhibiting the northward
flow of the Gulf Stream.
Atlantic currents are a complex engine powered by wind, salinity and heat.
Atlantic currents are a complex engine powered by wind, salinity and heat.
Potential disruptions in this vast cycle of water, sinking as it cools, cut to
the heart of the scientific unease.
Potential disruptions in this vast cycle of water, sinking as it cools, cut to
the heart of the scientific unease.
In short, the cold blob may signal that the northern arm of the Gulf Stream no
longer arrives with the same strength to the North Atlantic. That a warming
atmosphere has, paradoxically, cooled one part of the world.
The science remains relatively new, and not everyone agrees the AMOC is
actually slowing. But in both scientific modeling of climate change and in the
prehistoric record, a North Atlantic cooling presages a shutdown of the
current. “One of the hallmarks of a shutdown is this cold blob,” says Dr. de
Menocal. “The cold blob is a big deal.”
IN 1513, THE SPANISH EXPLORER Juan Ponce de León noticed something bizarre off
the coast of today’s Florida: Relentless currents pushing his ships backward,
overpowering the winds blowing them forward. He became the first European to
describe the Gulf Stream. Benjamin Franklin finally mapped it in the late 1700s
— he named it the “Gulf Stream” — by measuring changes in water temperature on
a return trip from England.
Over the 20th century, oceanographers came to realize that the northern branch
of the Gulf Stream was part of a gigantic loop of water, with warm surface
water flowing north and colder water returning south, deep below the surface.
This was the network of currents that scientists now call the AMOC.
The system was driven by North Atlantic water that, as it lost heat to the
atmosphere and grew dense, sank to the ocean’s depths, pulling warmer surface
water northward. In the middle part of the century, oceanographer Henry Stommel
elucidated the physics of how the AMOC could change. His insight was that,
depending on the balance of heat and salinity, the sinking effect—called
“overturning”—could strengthen, or weaken, or maybe stop completely.
In the 1980s, Wallace Broecker, a geochemist at Columbia University’s
Lamont-Doherty Earth Observatory, pounced on that idea.
Colleagues studying ice cores from the Greenland ice sheet were seeing evidence
of strange climatic “flickers” in the past. As Earth warmed from the deep
freeze of the last ice age, which peaked around 22,000 years ago, temperatures
would rise, then abruptly fall, then rise again just as swiftly. Dr. Broecker
theorized this was caused by stops and starts in what he called the ocean’s
“great conveyor belt” — the AMOC.
The clearest example began about 12,800 years ago. Glaciers that had once
covered much of North America and Europe had retreated considerably, and the
world was almost out of the deep freeze. But then, in just a few decades,
Greenland and Western Europe plunged back into cold. Temperatures fell by
around 10 degrees Celsius, or 18 degrees Fahrenheit, in parts of Greenland.
Arctic-like conditions returned to parts of Europe.
The cold snap lasted perhaps 1,300 years — before reversing even more abruptly
than it began. Scientists have observed the sudden changes in the pollen
deposited at the bottom of European lakes and in changes in ocean sediments
near Bermuda.
This forced a paradigm shift in how scientists thought about climate change.
Earlier, they had tended to imagine creeping shifts occurring over many
millennia. But by the late 1990s, they accepted that abrupt transitions,
tipping points, could occur.
This didn’t bode well for humanity’s warming of the atmosphere. Dr. Broecker,
who died in 2019, famously warned: “The climate system is an angry
beast<https://www.nytimes.com/1998/03/17/science/scientist-at-work-wallace-s-broecker-iconoclastic-guru-of-the-climate-debate.html>
and we are poking it with sticks.”
WHY DID THE AMOC shut down? A leading theory is that meltwater from retreating
glaciers emptied into the North Atlantic or Arctic oceans. Freshwater is
lighter than saltwater, and the sudden influx of more buoyant water could have
impeded the sinking of denser, saltier water — that critical “overturning”
phase of the AMOC.
Today we don’t have massive glacial lakes threatening to disgorge into the
North Atlantic. But we do have the Greenland ice sheet, which is melting at the
upper end of projections, or about
<https://climate.nasa.gov/news/2958/greenland-antarctica-melting-six-times-faster-than-in-the-1990s/>
six times
faster<https://climate.nasa.gov/news/2958/greenland-antarctica-melting-six-times-faster-than-in-the-1990s/>
than in the 1990s. And according to
<https://www.nature.com/articles/s41467-020-14474-y> one
study<https://www.nature.com/articles/s41467-020-14474-y>, the subpolar North
Atlantic recently became less salty than at any time in the past 120 years.
There’s little agreement on cause. Changes in wind patterns or currents may be
contributing, as could greater rainfall. But Stefan Rahmstorf, a physical
oceanographer with the University of Potsdam in Germany, suspects that, similar
to what happened some 12,800 years ago, meltwater from Greenland is beginning
to slow the AMOC.
In 2014, a remarkable project launched in the North Atlantic. An array of
sophisticated sensors were moored to the ocean floor between Newfoundland,
Greenland and Scotland. They’re starting to provide an unprecedented view of
the currents that shape the Atlantic.
200feet below surface
400feet below surface
600feet below surface
800feet below surface
EAST
NORTHEAST
NORTH
100feet below surface
300feet below surface
500feet below surface
700feet below surface
Ocean currents
moving south
Current speeds are scaled
for illustration purposes.
Here off Labrador, we can see how waters move deep beneath the surface.
Here off Labrador, we can see how waters move deep beneath the surface.
The sensors reveal the hidden workings of ocean circulation, which many
consider a climate switch.
The sensors reveal the hidden workings of ocean circulation, which many
consider a climate switch.
Below the waves, scientists are watching for signs of weakening across the
North Atlantic.
Below the waves, scientists are watching for signs of weakening across the
North Atlantic.
In this location, cooler waters near the surface flow southward relatively
swiftly.
In this location, cooler waters near the surface flow southward relatively
swiftly.
Diving, we see the water begin sinking as it grows denser. This kind of sinking
is a key part of the vertical motion of the currents, the engine that drives
circulation.
Diving, we see the water begin sinking as it grows denser. This kind of sinking
is a key part of the vertical motion of the currents, the engine that drives
circulation.
Around 400 feet, the sinking becomes clearer as currents angle downward.
Around 400 feet, the sinking becomes clearer as currents angle downward.
The water grows colder and the current slower at a depth of 600 feet or so.
The water grows colder and the current slower at a depth of 600 feet or so.
It is the long-term change in the delicate balance of heat and cold, saltwater
and freshwater, that scientists are tracking.
It is the long-term change in the delicate balance of heat and cold, saltwater
and freshwater, that scientists are tracking.
At greater depths, we see currents shift direction, moving westward, eventually
joining an ocean-spanning round trip.
At greater depths, we see currents shift direction, moving westward, eventually
joining an ocean-spanning round trip.
In 2015, Dr. Rahmstorf and his colleagues published a seminal
paper<https://www.nature.com/articles/nclimate2554> arguing that the AMOC had
weakened by 15 percent in recent decades, a slowdown they said was
unprecedented in the past 1,000 years. He and his colleagues recently published
another paper<https://www.nature.com/articles/s41561-021-00699-z> that used
additional reconstructions of sea temperature around the North Atlantic, some
going back 1,600 years, to determine that the recent slowdown began with the
Industrial Revolution in the 19th century, then accelerated after 1950.
Other scientists have also presented different evidence of a slowdown. The
South Atlantic has become saltier in recent decades,
<https://www.nature.com/articles/s41558-020-0897-7> according
to<https://www.nature.com/articles/s41558-020-0897-7> a study by Chenyu Zhu at
Ocean University of China and Zhengyu Liu at Nanjing Normal University,
suggesting that more of the salt that once traveled north with the AMOC now
remains in the tropics, producing what they call a “salinity pile-up.”
And Christopher Piecuch of the Woods Hole Oceanographic Institution recently
<https://www.nature.com/articles/s41467-020-17761-w>
argued<https://www.nature.com/articles/s41467-020-17761-w> that the Gulf Stream
along Florida’s coast, also called the Florida current, has weakened. He found
this by measuring the differences in sea level across the Gulf Stream. Earth’s
rotation deflects flowing water to the right; this causes the two sides of the
current to have slightly different sea levels — and the faster the current, the
greater the difference. Tide gauge measurements going back 110 years indicate
that this contrast has declined, Dr. Piecuch found, particularly in the past
two decades. This suggests the current has slowed.
For Dr. Rahmstorf, these lines of evidence bolster the argument that the AMOC
is slowing. In his view, the change is occurring right on schedule. “The
long-term trend is exactly what was predicted by the models,” he said.
A <https://www.ipcc.ch/srocc/> 2019 report<https://www.ipcc.ch/srocc/> by the
United Nations’ Intergovernmental Panel on Climate Change, a synthesis of the
most significant climate research worldwide, says that while the AMOC will
“very likely” weaken later this century, collapse is “very unlikely.” Yet Dr.
Rahmstorf worries about the unknowns in a system that scientists understand can
rapidly shift between different states.
He points out that, in IPCC jargon, “very unlikely” translates to a probability
of less than 10 percent. But if a nuclear reactor in your neighborhood had a
less-than-10-percent likelihood of blowing up, he asked, “would you be
reassured?”
“We still don’t know how far away this threshold is where it could break down
altogether,” he said. If we limit warming to 1.5 degrees Celsius above
preindustrial times — a goal of the Paris agreement among nations to fight
climate change — a shutdown is unlikely, he thinks. “But for unmitigated
warming,” which is the world’s current trajectory, “I think there’s increasing
risk where we make AMOC so weak it goes over the edge and collapses.”
“There will be a lot of surprises if we disturb climate that much,” he said.
“It’s not at all predictable how bad things will be.”
Scientists also emphasize that the ultimate consequences of that weakening
remain unclear. That’s in part because the world is in such uncharted
territory. In the past, Europe became drastically cooler when the current shut
down, but today any cooling might ultimately be muted or possibly canceled out
by continued global heating.
But if past is prologue, a drastically altered AMOC could certainly shift
rainfall patterns, scientists said, making parts of Europe and Northern Africa
drier, and areas in the Southern hemisphere wetter. Changing ocean currents
might affect marine ecosystems that people rely on for food and livelihood.
A <https://www.nature.com/articles/ncomms7346> changing
<https://www.nature.com/articles/ncomms7346> Gulf
Stream<https://www.nature.com/articles/ncomms7346> could also accelerate
sea-level rise along parts of the Atlantic coast of the United States. In 2009
and 2010, when the stream inexplicably weakened by 30 percent, the Northeast
saw seas rise at a rate unprecedented in the entire roughly 100-year record of
tide gauges.
And if water in the tropical and subtropical Atlantic becomes warmer because
that heat is no longer shunted north, the expanding reservoir of energy could
strengthen hurricanes, something that scientists at the National Oceanography
Centre in the United Kingdom
<https://www.nature.com/articles/s41467-019-08496-4>
argue<https://www.nature.com/articles/s41467-019-08496-4> is already happening.
Hurricanes derive their energy from heat in the water.
Finally, in a perverse twist, a shutdown of the AMOC could exacerbate global
heating. The ocean absorbs nearly
one-third<https://science.sciencemag.org/content/363/6432/1193> of human carbon
dioxide emissions. But the sinking of salty, dense water — the overturning
portion of the AMOC — is critical to that absorption. So, if the AMOC stops or
greatly slows, and that water stops sinking, the accumulation of heat-trapping
gases in the atmosphere could accelerate.
THEN THERE ARE THOSE CONSEQUENCES that fall in the category of “global
weirding.”
Scientists at the U.K.’s National Oceanography Centre have somewhat
counterintuitively
<https://iopscience.iop.org/article/10.1088/1748-9326/11/7/074004/meta>
linked<https://iopscience.iop.org/article/10.1088/1748-9326/11/7/074004/meta>
the cold blob in the North Atlantic with summer heat waves in Europe. In 2015
and 2018, the jet stream, a river of wind that moves from west to east over
temperate latitudes in the northern hemisphere, made an unusual detour to the
south around the cold blob. The wrinkle in atmospheric flow brought
hotter-than-usual air into Europe, they contend, breaking temperature records.
August
2017
300 MILES
St. John's
The floats began their journeys in areas called the Iceland-Scotland and
Denmark Strait pathways, two places where meltwater originates.
To study deeper currents, in 2014 scientists began releasing floats that drift
1,800 to 2,800 meters underwater.
“That was not predicted,” said Joel Hirschi, principal scientist at the centre
and senior author of the research. It highlights how current seasonal
forecasting models are unable to predict these warm summers. And it underscores
the paradox that, far from ushering in a frigid future for, say, Paris, a
cooler North Atlantic might actually make France’s summers more like Morocco’s.
Even so, Dr. Hirschi takes a wait-and-see stance on whether the AMOC is
actually slowing. “I have great respect for what Dr. Rahmstorf is doing. And it
may well be spot on in the end,” he says. “But I’m afraid the data, the really
robust data, is not there.”
Susan Lozier, a physical oceanographer and dean at the College of Sciences at
Georgia Tech, also has her doubts about whether the AMOC is currently slowing.
At issue, she says, is how scientists infer changes in the AMOC. We can
directly measure many aspects of the ocean, such as temperature (it’s warming),
oxygen levels (they’re declining), even how stratified it has become (more so).
“There are very strong signals in the ocean of climate change,” she said.
But most studies on the AMOC don’t measure the “conveyor belt” directly.
Instead, they use proxies to infer that the overturning has changed.
Such inference can be problematic when considering changes that occur over
short time frames, says Dr. Lozier, because the changes observed could have
other causes. Consider that cold blob in the North Atlantic, she said. Dr.
Rahmstorf and others see it as evidence of a weakening Gulf Stream, but Dr.
Lozier notes that shifts in wind patterns or how storms move over the ocean
could also underlie the phenomenon. “There are other ways to explain it,” she
said. “A lot of our conceptual understanding of AMOC is in isolation of other
things going on in the ocean.”
Direct measurement of the AMOC only began relatively recently. A line of
sensors between the Bahamas and the Canary Islands, called Rapid, was installed
in 2004. A second sensor array, spanning the North Atlantic from Canada to
Greenland to Scotland and called Osnap, went live in 2014. (Dr. Lozier is the
international project lead for Osnap.)
Neither project has operated long enough to produce clear trends, in Dr.
Lozier’s view. What they have shown, though, is lots of natural variability. In
2009 and 2010, for example, the AMOC weakened — “people were like, ‘Oh my God,
this is happening,’” she said — only to pick right back up again over the
following years.
They’ve also revealed a system of currents that’s far more complex than once
envisioned.
Dr. Broecker’s old schematics of the AMOC posit a neat warm current flowing
north along the western edge of the Atlantic and an equally neat cold current
flowing back south below it. In fact, says Dr. Lozier,
<https://science.sciencemag.org/content/363/6426/516> that
deeper<https://science.sciencemag.org/content/363/6426/516> current is not
confined to the western edge of the Atlantic, but rather flows southward via a
number of “rivers” that are filled with eddies. The network of deep ocean
currents is much more complicated than once envisioned, in other words, and
figuring out how buoyant meltwater from Greenland might affect the formation of
cold deepwater has become more complicated as well.
This is the place scientists currently find themselves in. They suspect the
AMOC can work like a climate switch. They’re watching it closely. Some argue
that it’s already changing, others that it’s too soon to tell.
“There’s no consensus on whether it has slowed to date, or if it’s currently
slowing,” said Dr. Lozier. “But there is a consensus that if we continue to
warm the atmosphere, it will slow.”
Sources: Float and mooring data by the Overturning in the Subpolar North
Atlantic Program project. Sea surface current and temperature data provided by
Asia-Pacific Data Research Center, part of the International Pacific Research
Center at the University of Hawai?i at Manoa, funded in part by NOAA. Large
scale ocean circulation arrow from The Relationship Between U.S. East Coast Sea
Level and the Atlantic Meridional Overturning Circulation: A Review by
Christopher M. Little et al.
Designed and produced by Michael Beswetherick, Ruru Kuo, Jesse Pesta and Rumsey
Taylor.
