[D66] [JD: 14] In the Atlantic Ocean, Subtle Shifts Hint at Dramatic DANGERS

Mon Mar 8 18:52:25 CET 2021

https://www.nytimes.com/interactive/2021/03/02/climate/atlantic-ocean-climate-change.html

In the Atlantic Ocean, Subtle Shifts Hint at Dramatic Dangers
By
JEREMY WHITE
The New York Times
15 min
View Original

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, 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.

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 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 six times faster than 
in the 1990s. And according to one study, 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.

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.

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.

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.

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.

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 
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 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, according to 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 argued 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 2019 report 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 changing Gulf Stream 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 argue 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 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 linked 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.

“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, that 
deeper 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.”