From the deck of a ship off eastern Greenland, the most majestic presence isn’t the whales, the icebergs or even the towering, glacier-wrapped mountains.
It’s the parade of frigid, midnight-blue water, 75 miles wide, that streams down the coast from the Arctic Ocean. Farther south, these currents mingle with tropical water swinging up through the Gulf Stream, and together they set ocean temperatures throughout the North Atlantic, like hot and cold taps on a giant bath.
Now, though, mankind is fiddling with the taps. Rapid warming caused by greenhouse gas emissions is making the Arctic rainier and melting its ice, both on land and at sea.
If too much of this excess water makes its way into the North Atlantic, scientists fear it could disrupt the processes that draw warm water up from the tropics. The consequences for the climate would be far-reaching: frostier winters in Britain, stronger hurricanes in the Eastern United States and, perhaps most troublingly, shifts in the rain belts that feed people across Africa, South America and Asia.
To better understand this threat, a team of researchers sailed from Iceland to Greenland’s east coast this summer with a shipload of data-gathering equipment. These waters are unforgiving to delicate instruments: Run-ins with icebergs are a constant risk. Sea ice makes the area impassable for much of the year.
“There’s just very little data here,” said Nick Foukal, an oceanographer at the University of Georgia, who led the two-week expedition. “So any data is going to be really, really important.”
Observing ocean currents as they change is critical to fleshing out scientists’ understanding of how and when they might reach critical tipping points. The problem, as humans alter Earth’s climate, is that sweeping change might not wait for researchers to feel certain in their understanding before it gets underway.
A Remarkable Loop
What makes the seas around Greenland so important for the planet? The answer has to do with an immense loop of water that winds through the world’s oceans, linking changes in the climate far and wide.
You might start following this loop in the tropical Atlantic, where strong sunshine warms the sea and makes it extra-salty through evaporation. This water travels up past the East Coast of North America until, in the vicinity of Iceland and Greenland, it starts to transform.
The warm water meets cold air and surrenders some of its heat, becoming colder and denser. This causes it to sink into the depths. It flows through the abyss past South America and all around Antarctica before resurfacing centuries later, ready to begin the journey once more.
Scientists have given the Atlantic portion of this oceanic conveyor belt an unlovely name: the Atlantic Meridional Overturning Circulation, or AMOC (pronounced “AY-mock”). And they fear it will someday grind to a halt.
What worries them are those growing quantities of fresh water sloshing around the North Atlantic. They dilute the warm, salty currents that travel up from the tropics, causing them to sink less. But this sinking is what propels that warm water north in the first place. When one end of the conveyor belt grows sluggish, the other does, too. Little by little, the whole belt slows until, at a certain point, it stops.
If that happens, less tropical heat would make its way into the northern reaches of the Atlantic, making Britain and the Nordic countries colder. More of that heat might stay near the Caribbean instead, providing extra fuel for hurricanes and warping the atmospheric patterns that distribute rainfall on both sides of the Atlantic and beyond.
Geological evidence tells us all this has occurred several times before, most recently around 12,800 years ago. Today there are signs a slowdown is underway, and scientists’ models predict it will continue for decades. The tricky part is anticipating when it might lead to another shutdown: Next century? Next decade? Next year?
“In the real world, the tipping point looks much closer than models are suggesting,” said Stefan Rahmstorf, a professor of ocean physics at the University of Potsdam in Germany. That means the risk it could be upon us before the century’s end is too significant to ignore, Dr. Rahmstorf said.
What would help narrow the uncertainties are more sustained, long-term measurements of how water all across the Atlantic is changing, said Thomas Haine, a professor of earth and planetary sciences at Johns Hopkins University.
For the past two decades, scientists have been measuring the AMOC along two key cross-sections in the Northern Hemisphere, one from Labrador to Greenland to Scotland, the other between the Bahamas and the Canary Islands. But they have much less visibility into the fresh water flowing out of the Arctic, Dr. Haine said.
“We think the system is going to have this big shift, and we know ahead of time,” he said. “We should go out and try and watch that happen.”
Where It Happens
After leaving port in Iceland and steaming past Snaefellsjokull, the volcano where Jules Verne’s “Journey to the Center of the Earth” begins, Dr. Foukal and his colleagues were greeted on Greenland’s coast by glorious sunshine and glass-flat seas. It would be days before they saw either of those things again.
The key cargo on their expedition was an arsenal of moorings, each one fitted with sonar-like instruments to measure ocean currents. Each mooring was also connected to a series of floats, buoys and smaller instruments measuring water temperature and salinity, like a daisy chain hundreds of feet long.
Five of the moorings resembled yellow lunar landers. Another looked like a squat, orange torpedo, and it was connected to a barrel-shaped buoy designed to dodge icebergs. The buoy would lurk deep underwater, scan the waters above it for passing ice, then ascend to a safe depth to take measurements before plunging once again.
The scientists planned to place the moorings along a 35-mile-long stretch of the seafloor, at 71 degrees north latitude. They would leave them there until next year, allowing them to gather months of measurements of the water coursing past Greenland’s eastern shore.
Collecting data this way reminds Dr. Foukal of the lobster fishing he did growing up in Massachusetts: You don’t know what you’ve got until you pull up the cages. “It’s rolling the dice,” he said.
First, though, the scientists and crew had to get the heavy moorings safely off the moving ship and into the sea at precisely the right spots — a highly choreographed production of winches, pulleys and ropes that James Dunn, a senior engineering assistant on the expedition, compared to a ballet. “It has to all fall into place all at once,” said Mr. Dunn, who works for the Woods Hole Oceanographic Institution in Massachusetts.
Below deck, Manuel Colombo, a chemical oceanographer at the Virginia Institute of Marine Science, was conducting a more delicate type of field work. Every drop of water on the planet carries chemical traces of where it’s been, whether that’s a river, a melting glacier or a cloud. By analyzing those traces, Dr. Colombo hopes to better understand the complex paths that water takes between the Arctic Ocean and the North Atlantic.
Collecting samples of the seawater without contaminating them required Dr. Colombo and a graduate student, Victoria Grisson, to work obsessively to keep their lab on the ship spotless. “You can imagine how clean my house is,” he said.
In the early days of the voyage, clear skies and calm seas helped the scientists and crew to install the moorings smoothly. Soon, though, the wind picked up and things got harder.
The team’s cigar-shaped underwater robot stopped working and had to be fished out. An important piece of scientific equipment on deck blew a fuse, and a replacement was flown from Iceland to the seal-hunting hamlet of Ittoqqortoormiit and brought aboard. To shelter from storms offshore, the scientists took a multiday detour in Scoresby Sound, one of the world’s largest fjords, where they gathered measurements of the waters swirling in and out of the deep inlet.
Then the iceberg-dodging buoy started acting up. The scientists hatched a risky plan: They would sail back, pull the mooring out of the water, try to fix the buoy and redeploy it.
The plan was the researchers’ best hope for gathering all the data they wanted before next July, when they plan to recover their instruments and begin drawing conclusions about how the coastal currents are moving and changing.
On a crisp gray morning, the torpedo-like mooring drifted into view. Andrew Davies, a Woods Hole engineering assistant, leaned out over the starboard railing and grabbed it with a hook on a pole. Nikiforos Delatolas, an engineer, spent the afternoon and evening hunched over a laptop, troubleshooting the misbehaving buoy.
It was midnight, cold and drizzly, by the time all the gear was back beneath the waves. For the first time in hours, the scientists smiled. “That was fun,” Mr. Dunn said. “Let’s do it again.”
Raymond Zhong reports on climate and environmental issues for The Times.
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