Why is an ocean current critical to world weather losing steam? Scientists search the Arctic for answers.

A conveyor belt of ocean water that loops the planet and regulates global temperatures could be heading for a tipping point.

By Cheryl Katz
Published 3 Dec 2019, 12:54 GMT
Sea ice over the Greenland Sea near northeast Greenland. Arctic sea ice this summer was the ...
Sea ice over the Greenland Sea near northeast Greenland. Arctic sea ice this summer was the second lowest on record, and ocean changes in the Arctic could dramatically change the climate for much of the rest of the planet.
Photograph by Lawrence Hislop

Summer sea ice has been shrinking so dramatically here in the Fram Strait, high in the Arctic between Norway and Greenland, that researchers who make this trip annually point out missing patches like memories of departed friends.

“The first time I was here, in 2008, you could walk on the ice,” says Norwegian Polar Institute (NPI) oceanographer Paul Dodd, gesturing from the deck of this research icebreaker toward the spot, near the Prime Meridian, where his team is about to take samples for temperature, salinity, dissolved carbon, and other chemical measurements of what is now open water. It’s dotted with only a few random, battered-looking ice drifts.

Temperatures are rising and ice is melting all over Earth. But this place is special: The ocean changes that are happening right here could dramatically alter the climate for much of the rest of the planet.

Fram Strait and the waters to the south, in the Greenland, Norwegian, and and Irmringer seas, make up the control room of a global “conveyor belt” of currents that stretches the length of the planet. Only in this region and one other, in the Antarctic, does water at the sea surface become heavy enough—dense with cold and salt—to sink all the way to the seafloor and race downhill along the deepening ocean bottom. That sinking powers the conveyor, known as the Atlantic meridional overturning circulation, or AMOC—which in turn regulates temperatures and weather around the world.

A new report warns that the AMOC is one of nine critical climate systems that greenhouse-gas-fuelled warming is actively pushing toward a tipping point. Crossing that threshold in one of these systems could trigger rapid and irreversible changes that drive other systems over the edge—leading to a global tipping cascade with catastrophic consequences for the planet. The analysis, released last week in Nature by an international group of leading climate scientists, says the tipping point risks are greater than most of us realise.

Two other new reports, by the World Meteorological Organisation and the United Nations Environmental Program, show that planet-warming emissions reached new highs in 2018—and continue to climb.

The AMOC conveyor belt may already be showing signs of sputtering as a result. A network of ocean probes across the mid-Atlantic, between the Bahamas and Africa, has recorded a 15 percent drop in the current’s flow over the past decade. A recent modelling study suggests that the slowdown began a half-century ago as planet-warming carbon emissions started to soar.

The “Special Report on the Ocean and Cryosphere in a Changing Climate,” released in September by the Intergovernmental Panel on Climate Change (IPCC), projects that the conveyor will weaken as much as a third by 2100 if emissions continue at their present rate. An enfeebled AMOC could trigger a host of changes, including floods, increased sea level rise, and disturbed weather systems.

That’s where the ice loss that Dodd was lamenting comes in. NPI scientists have been monitoring the Fram Strait since 1990. They’ve found that the waters east of Greenland are getting not only warmer but also less salty, says NPI oceanographer Laura de Steur, leader of this year’s expedition. Melting glaciers on Greenland, melting sea ice in the Arctic, and rivers swollen by increased precipitation in Siberia have all contributed to a large flush of fresh water into the Fram Strait—a 60 percent increase over the first half of this decade, de Steur says.

Whether those forces are the cause of the conveyor’s current sluggishness isn’t certain. But at some point, if the water here gets too fresh, or too warm, or especially both, it will become too light to sink, say de Steur and other ocean scientists—jamming the works of one of the most fundamental forces in the global climate system.

Other key components of Earth’s climate works that may be heading toward a tipping point include summer sea ice, which models predict will disappear as early as 2036, permafrost, now rapidly thawing across wide swathes of the Arctic, the vast Greenland ice sheet, the Amazon rain forest, and more.

Scientists collect an instrument array from the Fram Strait.
Photograph by Lawrence Hislop

All would have far-reaching impacts on the global environment. But when it comes to the oceans, a system that covers more than 70 percent of our planet and stores up to a third of the carbon dioxide humans have produced since the Industrial Age—and 90 percent of the resulting excess heat—one concern tops them all.

“There's one particular tipping point that people are afraid of,” says Henk Dijkstra, an oceanographer with Utrecht University in the Netherlands. “That's basically the collapse of the Atlantic circulation due to the input of fresh water."

A summer of intense heat

When the Kronprins Haakon departed Svalbard, Norway, in early September for this year’s research expedition, a summer of extreme heat and weather was drawing to a close. Temperatures in parts of Greenland soared 22 degrees Celsius above normal in June. In July, the hottest month ever recorded on Earth, the Greenland ice sheet shed more than 30 billion metric tons of ice in three days. This year’s runoff from Greenland flushed nearly 330 billion tons of fresh water into the coastal seas. Arctic sea ice this summer shrank to its second lowest extent since satellite measurements began in 1979—more than 800,000 square miles smaller than normal, this animation shows.

The scant ice stymies researchers from the start. The Kronprins Haakon steams more than 300 nautical miles west across the Fram Strait before they find a patch large and solid enough for the scientists to walk on and make chemical and physical measurements. It’s the third year in a row of extremely low ice, and much of what’s here has been broken up by recent storms and swells.

“In 2016, we had 19 sea ice stations during the cruise. We had ice stations all over the area,” says a frustrated Dmitry Divine, an NPI ice scientist, who spends much of the cruise scanning for ice from the ship’s bridge. “There was ice which was quite decent, comfortable to work on. That is not the case these days.”

When we finally do reach ample ice, we’re all the way over by the northeast coast of Greenland. The captain wedges the ship into a field of “fast ice” stuck to icebergs grounded in the shallow water here. We don bulky “survival suits”—uncomfortable, but a lifesaver if the ice were to give way—and head out onto the pocky white crust over the Greenland Sea. Here, Divine drills ice cores to be analysed for salinity, thickness, age, and other vital signs of the solid water flowing south from the Arctic Ocean.

Meanwhile, de Steur and colleagues are taking the vitals of the liquid water. On our way west, de Steur retrieves and uploads data from oceanographic instruments that have spent the past two years under water monitoring temperature, salinity, and currents for changes that could affect how deep water forms.

That deep water fuels the ocean conveyor belt. And it primarily happens here in the Fram Strait—the main gateway between the Arctic and the North Atlantic oceans. Here warm, salty Atlantic water ferried north from the tropics on the Gulf Stream meets the colder, fresher Arctic water. The mixture cools, and begins to sink and head back south. Churned by density differences between the two flows, and stirred at the surface by winds, the ocean’s circulation is set in motion.

So far, de Steur says, there’s been no change in the deepwater formation. But the warming and freshening she has observed here on past voyages are worrisome.

“It shows how important it is to monitor this gateway to the Arctic,” says de Steur. “We see changes... . Those changes are going to be transported to the Atlantic, and that’s going to have an impact. I can’t say it’s going to be tomorrow, or next year, but if things continue as they are, at some point it’s going to have an impact.”

Huge puzzle, missing pieces

A big limitation in scientists’ ability to pinpoint precisely what’s going on with the AMOC is the lack of long-term observations throughout the current’s route. The NPI’s studies are among the longest running, and they give a critical heads-up about changes near the current’s headwaters. But they can’t tell what’s happening with the AMOC further south, how long the changes in the Fram Strait could take to show up there, and with what effect.

An array that has measured a drop further south is the RAPID-MOCHA, a string of moorings between the Canary Islands and the Bahamas that detected the 15 percent AMOC slowdown over the past decade. Launched in 2004, it began seeing the current weaken in 2008, including a whopping 30 percent drop in 2009-10, says University of Miami oceanographer Bill Johns, a principal investigator with the project. But the signal is “noisy,” says Johns, making it hard to sort out natural variability from climate change impacts so far.

An important new array called OSNAP (Overturning in the Subpolar North Atlantic) began installment in 2014 between the southern tip of Greenland and Scotland near where the current makes its deepest dive and picks up southward speed. But it’s too soon to see ongoing trends in the AMOC’s strength, says program director Susan Lozier, a Georgia Tech oceanographer.

The system of currents is a huge puzzle, says Femke de Jong, an oceanographer with the Royal Netherlands Institute for Sea Research who is out on the deck of the Kronprins Haakon tossing “drifters” that will show whether the fresh water flowing into the Fram Strait ends up in regions that are critical for the AMOC’s formation.

“And we don’t have half the pieces,” she says.

Historical climate models, however, find that the conveyor belt has slowed significantly in recent decades. And palaeoclimate evidence shows the AMOC is currently the weakest it’s been in at least a millennium, says Stefan Rahmstorf, an oceanographer and climatologist at the Potsdam Institute for Climate Impact Research in Germany. His own research concludes that the system has been slowing for at least 50 years—in line with the rise in humans’ carbon output.

The mechanism is clear, Rahmstorf says: Man-made warming slows down the AMOC by reducing the density of surface waters in the North Atlantic, “And that is what is being observed.”

Ice floes float in the Fram Strait between Svalbard and Greenland.
Photograph by Lawrence Hislop

If the conveyor jams...

Is the conveyor belt headed for a tipping point?

“That’s the million-dollar question; I don't think anybody can answer that,” Rahmstorf says. “I think we have robust evidence that there is a threshold somewhere out there and we have increasing evidence that the AMOC is actually weakening. Which means it’s moving in the direction of where that threshold is.”

The recent IPCC report on climate change in oceans projects that, while the AMOC will weaken substantially during this century, a collapse by 2100 is unlikely. However, at our current rate of carbon output, its models give even odds of a shutdown by 2300.

Dijkstra, the University of Utrecht oceanographer, cautions that those models may actually be too optimistic. They ignore impacts of the melting Greenland Ice Sheet and overlook possibilities, such as a few especially rainy years over the North Atlantic, that could flood the system with fresh water and tip its balance.

“This is a dangerous thing,” he says. “People underestimate this, I think.”

The consequences of a conveyor belt jam would be severe.

Fingerprints of AMOC shutdowns appear in massive climate swings of the past.

An abrupt brake on the current around 950,000 years ago sent the planet into a long series of ice ages. More recently, Europe was plunged into a 2,000-year cold spell known as the Younger Dryas around 13,000 years ago, after the current sharply weakened. Although it’s not certain what caused those conveyor breakdowns, melting ice sheets are believed to have played a major role.

Even short of a shutdown, the effects of weakening ocean circulation would be felt around the globe. Because the Gulf Stream warms Northern Europe by as much as 10 degrees F, a drop in the heat flowing north would make European winters colder. An exaggerated version of this scenario was the subject of the 2004 disaster film “The Day After Tomorrow.”

Scientists say it won’t be anything like the overnight ice age depicted in the film. Effects of an AMOC slowdown—while rapid on the geological time scale—would take decades or longer to appear. But the changes in the ocean’s heat uptake and transport would make the South Atlantic hotter, shifting the bulk of the planet’s heat southward and disrupting monsoon cycles vital to Asian and South American crops, according to the IPCC.

Floods and droughts would increase on both sides of the Atlantic, along with more frequent hurricanes along the southeastern United States and Gulf of Mexico. A backed-up Gulf Stream could raise sea levels along the U.S. East Coast, driving more warm water—and possibly steamier temperatures—ashore.

Marine ecosystems and fisheries would suffer. On top of that, muddled ocean circulation could knock the already wobbly jet stream further off kilter, triggering more summer heatwaves and winter cold snaps across North America and Europe.

Unstoppable changes?

The autumn freeze should be about to begin as we sail back east across the Fram Strait in mid-September. A thin skin of “grease ice” has formed over patches of the ocean, while dots of “pancake ice” float in others.

But the warming waters are slow to freeze again this year. September ended up with the third lowest ice extent in the satellite record and October followed with another new low. The less ice there is to reflect the sun’s heat, the more the Arctic will warm up, in an escalating feedback cycle.

Back at dock in Svalbard’s colourful port town of Longyearbyen, de Steur shares with me some of the latest data from her ocean probes. The Arctic freshwater flood likely peaked in 2017, she says, and appears to have let up since then. That’s good news, but it may simply mean that winds are keeping all that freshwater up in the Arctic—for now. When the wind patterns shift again, as they did earlier this decade, it could once again gush south.

Meanwhile, the warming and ice loss spiral continues. De Steur’s moorings find temperatures in the polar water flowing into the Fram Strait have climbed nearly 1 degree F over the past 17 years, while the Atlantic water has warmed nearly half a degree. If the trend continues, it could dampen deepwater formation and throttle the conveyor belt’s engine just like freshening will, she says.

If our planet-warming emissions aren’t seriously cut, and soon, she says, "I am worried that these changes are unstoppable and cannot be reversed.”


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