ON SEA ICE, Arctic Ocean — It's half-dark at 11 a.m. in the central Arctic. The brightest spots in the blue polar twilight are a cluster of scientists in crimson snowsuits and the enormous banana-yellow buoy they're assembling.
This is site "L3," a floating research station at the top of the world. The landscape could easily be mistaken for a vast expanse of snow-covered tundra — but it's actually a chunk of thick sea ice, drifting across the frozen Arctic Ocean. An enormous orange and white research ship looms in the background, waiting to collect the scientists when their day's work is finished.
At a small hole in the ice, about 2 feet in diameter, dark water ripples.
The scientists have spent the morning painstakingly unpacking and assembling the buoy's inner workings on the ice, manipulating small metal parts — sometimes with bare hands — in subfreezing temperatures. It's meticulous work. The buoy includes a variety of sensitive instruments that will measure everything from the water's temperature to the mixing and motion of the currents beneath the ice.
Now, after several hours of preparation, they're finally ready to set it loose.
"It only takes 15 minutes," says principal investigator Tim Stanton, an Arctic Ocean expert at the Naval Postgraduate School. There's still plenty of time to finish up and make it back to the ship for the 11:30 lunch.
Supervised by Stanton, a handful of red-clad scientists carefully roll the 70-pound, bell-shaped instrument into position near the hole in the ice. They make the final few attachments and give the equipment a last look-over. Then, with a "1 — 2 — 3!" they heave the buoy up on its end and slide it swiftly into the water.
The round top of the buoy rests on the ice, like a stopper in a bottle. It will freeze into place over the next few hours, locking the buoy in position on the ice. Over the course of the next year, the instrument will drift with the floe across the central Arctic Ocean, taking continuous measurements.
Stanton's yellow buoy is one of a large suite of scientific instruments installed on a cluster of ice floes in the central Arctic, managed by scientists from institutions around the world, floating together across the polar sea. Additional buoys will measure other characteristics of the ocean, such as salt content or the temperature at different depths. Meanwhile, equipment on the surface of the ice will monitor changes in the atmosphere.
The drifting research stations are all part of a major international science mission known as the MOSAiC Expedition, or the Multidisciplinary Drifting Observatory for the Study of Arctic Climate. Spearheaded by the Alfred Wegener Institute in Germany, the mission will include contributions from hundreds of scientists.
Central to the mission is the Polarstern, a German icebreaker moored to a large floe in the middle of the Arctic Ocean and frozen into place. Ship and floe will serve as MOSAiC's main research site as they drift together across the central Arctic, emerging next fall somewhere near the Fram Strait north of Greenland. MOSAiC researchers have also set up a wider network of unmanned drifting science stations around the Polarstern, outfitted with instruments like the yellow buoy, which collect additional data and send it home via satellite connection.
The measurements will provide a unique window into the central Arctic — a region where thick sea ice and months of winter darkness make it difficult for scientists to collect any data at all, let alone on a year-round basis. By this time next year, scientists will have a yearlong snapshot of conditions in the most remote parts of the Arctic Ocean.
These observations, in turn, may help them better predict how the central Arctic will respond to future climate change.
It's an essential question in the rapidly warming Arctic, where temperatures are rising faster than in just about any other part of the world. Climate change is already driving steady declines in the region's sea ice cover — since the 1970s, it's been shrinking by about 3% each decade.
As the ice disappears, research suggests the meltwater is physically altering certain parts of the ocean. And some scientists believe these changes may be worsening the effects of climate change, causing even more sea ice to melt.
Scientists have already observed some worrying changes. Certain parts of the Arctic Ocean are starting to resemble the vastly different Atlantic to the south — the waters are getting warmer, saltier and more mixed together, a fundamental shift for the isolated polar waters.
If that process continues, there could be a major shift in the Arctic ecosystem. Eventually, the region's unique plant and animal life could begin to resemble the much warmer Atlantic ecosystem, as cold-water species disappear and new ones move in to take their place.
And the warming could also have profound consequences for the entire Arctic climate system. As sea ice disappears, it could accelerate the already breakneck pace of Arctic warming.
But to understand what the future might hold, and how fast these changes might happen, scientists first need a lot more information about how the mysterious Arctic Ocean works.
"For some things, we are aware that we have no clue how things are happening — how the various layers in the ocean are changing, what is the actual real relationship between the ocean, the ice and the atmosphere," said Céline Heuzé, a physical oceanographer at the University of Gothenburg, Sweden, and co-lead of MOSAiC's team of ocean researchers. "So these are the big questions that we are there to answer."
A larger Atlantic
The Arctic Ocean has a unusual structure, different from most other seas around the world. The coldest water is found closest to the surface, with layers of warmer water underneath.
Sea ice is a big part of the reason.
Across much of the Arctic, sea ice operates on a seasonal cycle — it grows and thickens during the cold winter months, and it melts, at least partially, during the warmer summer. When ocean water freezes into sea ice, it expels most of its salt into the water below. When it melts again, an influx of cold, fresh water pours back into the sea.
Fresh water is less dense than salt water, so the meltwater tends to form its own layer at the surface of the ocean. Underneath, there's a saltier layer of cold water. And finally, a thick layer of warmer salty water rests below.
Sea ice caps off the whole system. It forms an additional barrier at the top of the water that protects the cold layer from winds and waves and keeps the waters from mixing up.
But in some parts of the Arctic, scientists have noticed that the layers are starting to churn together. When that happens, warmer water can rise to the surface — and it can melt sea ice from the bottom up.
Scientists are most concerned about a part of the Arctic known as the eastern Eurasian Basin, an area off the coast of Siberia including parts of the Barents, Kara and Laptev seas.
It's a kind of gateway region, where warm Atlantic water flows into the chilly Arctic from the south. The warm currents enter through the Barents Sea and gradually sink beneath the top layer of cold water as it flows through the neighboring Kara and Laptev seas and into eastern Siberia.
Sea ice in these regions has steadily declined over the last few decades. And as the ice cover has thinned out, the waters have begun to mix up. Multiple studies in the past few years have found that the warm Atlantic layer is creeping closer to the surface of the sea.
There are a few reasons this may be happening — all linked to the decline of the ice.
In the Barents Sea, where the Atlantic current first enters the Arctic, much of the ice cover drifts in from other parts of the ocean. But since sea ice is declining all over the Arctic, less and less ice is getting transported in.
A 2018 paper published in Nature Climate Change spelled out the consequences. With less ice drifting into the Barents Sea, there's also less meltwater produced in the summer. That means the cold water layer is growing weaker.
As a result, it's getting easier for the warm Atlantic layer to mix upward toward the surface, meaning the sea is growing warmer as a whole. It's a process scientists have dubbed "Atlantification," and it has the potential to profoundly alter the area's unique character and ecology.
In fact, the Barents Sea is considered an Arctic "hot spot" — it has experienced some of the strongest warming in the entire region.
Scientists have observed a similar Atlantification pattern throughout the rest of the eastern Eurasian Basin. A 2017 paper in Science, led by Igor Polyakov, found that parts of the Kara, Laptev and East Siberian seas are also experiencing increased mixing and warming of the waters.
The warmer waters are now causing sea ice to melt from the bottom up, the researchers found. In fact, they suggest that the ocean's influence in these regions is about as strong as that of the warming air temperatures.
Some researchers suggest other factors could be worsening the mixing process as sea ice continues to decline.
Winds and waves are one major potential influence. As the ice cover shrinks, it exposes more of the liquid ocean to the atmosphere. Some scientists believe this makes the sea surface more vulnerable to the weather, allowing winds to churn the water and mix up the layers.
At the same time, some measurements suggest the flow of Atlantic water into the Arctic may also be growing warmer. And these temperatures may continue to rise as the rest of the globe heats up.
"As the waters in the midlatitude ice-free regions start to warm up, then we can get more heat into the ocean," said Mary-Louise Timmermans, a physical oceanographer at Yale University, in an interview with E&E News. "And that's not good for the longevity of sea ice."
Ocean, ice and Arctic feedbacks
On the other side of the Arctic Ocean, the water is also heating up. Some of the reasons may be different — but scientists believe that melting sea ice is also partly the culprit.
The Beaufort Sea "has really responded the most dramatically, without question," said Stanton, who has participated in some recent expeditions there. Research suggests there's more heat closer to the surface of the sea, he says, meaning it's getting warmer at the base of the sea ice.
In this case, much of the warming may be caused by the simple influence of the sun.
"Ice is a great insulator, obviously," Stanton explained. "It is this incredible barrier to solar radiation."
But as the sea ice declines, more sunlight is able to get into the ocean and warm up the water. At the same time, winds can help to stir up the open water and spread the heat around. In turn, more heat stored in the ocean can delay the winter freeze-up or cause more ice to melt, Stanton says.
For now, rising air temperatures are still the biggest influence on melting Arctic sea ice. But as the region continues to warm, and more ice melts away, scientists worry that some of these ocean processes could begin to feed on themselves. The more ice that disappears, the more open water is exposed to the sun and the wind, which can potentially drive more mixing, more warming and more melting — a self-perpetuating cycle.
And as sea ice melts, and the ocean absorbs more and more heat, it could also speed up the overall rate of Arctic warming. In fact, many scientists believe that declining ice cover is one reason the Arctic is already warming so much faster than the rest of the world.
"So basically, the ocean is a key element of Arctic climate via its influence on the sea ice cover," Timmermans said.
There's still some debate about exactly how the ocean will respond as the climate continues to shift.
Rapid ice melt can dump large quantities of cold, fresh water into the sea at once. That could potentially strengthen the cold top layer — at least temporarily. Climate models also suggest there may be more rainfall in a warmer Arctic. That could increase the flow of fresh water from rivers into the ocean, Timmermans added.
But as the total ice cover declines, less and less meltwater may be available to feed this cold layer, and the layers could begin to weaken again — the same process scientists are currently observing in the Barents Sea. At the same time, the growing influence of sun and wind on the open water could add to the mixing and warming process.
For now, it appears that an increase in mixing, rather than layering, is "the scenario we're seeing now over much of the Arctic Ocean," Timmermans said.
But there are many parts of the puzzle that researchers still need to put together.
While scientists know the general pathway that warm Atlantic currents take into the Arctic Ocean, there are some details about the physical flow that are still not well understood. That means it's difficult to say how those currents will change — whether they'll warm, cool, strengthen, weaken or change course — under future climate conditions.
"We don't know very well the dynamics of how that happens, how the Atlantic water moves into the Arctic," Timmermans said. "So it's difficult to make predictions about the future fate of Atlantic water inflow into the Arctic."
Much of the remote central Arctic remains a mystery. Data from the northernmost reaches of the Arctic Ocean is sparse compared with other parts of the world — and without a good picture of what the region looks like now, it's hard to predict how it will change in the future.
"Obviously the Arctic is changing, and we don't really know what is the baseline," said Heuzé, the co-leader of MOSAiC's ocean team.
That's why projects like MOSAiC are focused on observing "what are the ocean currents doing, what are the properties of the ocean at different depth levels," she added. "So we can actually measure the changes."
Stanton's bright-yellow buoys, along with the rest of the MOSAiC array, will begin to fill in some of those gaps as they drift across the central Arctic. The information they beam back to land will contribute to a "canonical sort of data set," according to Stanton — one that will be shared among climate scientists all over the world.
"I consider it to be a really important yearlong sample, which is so rare," he said.