These researchers spent a winter trapped in Arctic ice to capture key climate data

Nature

There was nowhere to land amid all the broken ice. On 6 February, three scientists were in a helicopter flying high above the ocean roughly 250 kilometres from the North Pole, and all they could see was jumbled masses of fractured sea ice. The ice was clearly unfit to land on. Just a week before, it had been smooth, thick and flat.

They circled the region a few times and, when they spotted a seemingly solid section nearby, the pilot repeatedly tapped the helicopter against the ice to test whether the surface was stable. Then the scientists jumped out in search of a crucial research station: a sled laden with atmospheric instruments. For months, it had been carefully monitoring the frigid skies — gathering the first data ever about winter conditions near the top of the world, the fastest warming part of the planet. Now, the researchers feared that those precious data might be lost.

In their frantic search, the scientists jumped over cracks and blocks of ice — aware that the cracking noises beneath their feet meant that the ice was still moving, and that a wrong step could spill them into the Arctic Ocean below. When they finally spotted the sled, it was upside down and partly buried in a ridge of ice. So began a two-day rescue mission, as the scientists carefully pulled the station to safety and resurrected the instruments — all while working in the bitter-cold dark of Arctic winter.

Members of the MOSAiC expedition Leg 2 team stand on an overturned sled buried in snow during the Arctic polar night

A sled laden with instruments went missing and was discovered partially buried in snow.Credit: Michael Gallagher (CIRES)

It was one of the many challenges facing scientists who spent the winter months on a ship locked in the Arctic ice as part of a mission called the Multidisciplinary drifting Observatory for the Study of Arctic Climate, or MOSAiC. The year-long project began last October, when the Polarstern icebreaker cut its engines in the Arctic Ocean north of Siberia and became frozen in a giant slab of ice. It has been drifting ever since, with teams of scientists travelling to and from the vessel using icebreakers and aircraft. Over the mission, five separate groups of scientific personnel, totalling roughly 300 people, will join the Polarstern, with most stints lasting up to three months.

In darkness a group of people dressed in red cold weather clothing move up a gangway onto the Polarstern research vessel

The second scientific crew arrives for its rotation on the Polarstern on 13 December 2019.Credit: Alfred-Wegener-Institut/Esther Horvath (CC BY 4.0)

The COVID-19 pandemic upended those plans, delaying the arrival of the scientists currently on board the Polarstern. For the departing winter team, the trip home also faced delays as countries started to close their borders. But the scientists made it back safely — bringing daring tales and troves of data.

During the winter team’s time on the ship from mid-December to early March, the researchers faced some of the most dangerous conditions on Earth: continuous darkness, temperatures that can cause frostbite in minutes and the threat of polar bears. “You feel the power of nature,” says Giulia Castellani, a polar scientist at the Alfred Wegener Institute (AWI), Helmholtz Centre for Polar and Marine Research in Bremerhaven, Germany, the lead research institute for the mission. “You realize you won’t survive out there. It’s dark and it’s cold. It’s a big challenge.”

But the mission has yielded unprecedented measurements of what happens during the dark Arctic months to the atmosphere, the ocean, the ecosystem and sea ice. Although much of those data have yet to be analysed, early results reveal a wild atmosphere with temperature layers so extreme that the air at the top of the ship was sometimes 5 °C warmer than that nearest the ice. And in the ocean, researchers found a thriving ecosystem in dark waters they thought would be relatively quiet.

In darkness scientists collect samples from the ice sheet with the Polarstern research vessel in the distance

Crew members collect ice-core samples in December. Sea ice has been rapidly disappearing from the Arctic during summer, and one of the main focuses of the MOSAiC mission is to gather information about the wintertime conditions that affect sea ice.Credit: Alfred-Wegener-Institut/Esther Horvath (CC BY 4.0)

The data from the mission will be plugged into climate models that help scientists to understand how the Arctic will fare as it warms. It’s a major question, given that information from the high Arctic is almost non-existent, particularly in the middle of winter. And scientists need that information because the Arctic controls how quickly the rest of Earth heats up — even shaping the weather in New York City, London and Beijing.

“The whole overwintering effort was outstanding,” says Vishnu Nandan, a sea-ice physicist at the University of Manitoba in Winnipeg, Canada. “We got really good science.”

Sunless months

By the time the second group of mission scientists arrived at the floe on a separate icebreaker in mid-December, the Sun had long ago slipped below the horizon, and the world was truly dark. “In the beginning it was a bit scary, honestly, because you can’t really see farther than 100 metres from your nose,” Castellani says. “So you don’t really have an idea of the dimension of the world you’re in. There’s just this big black stage.”

Some found the darkness beautiful. That was the case when atmospheric scientists Michael Gallagher at the University of Colorado, Boulder, and Taneil Uttal at the National Oceanic and Atmospheric Administration (NOAA), also in Boulder, travelled some 30 kilometres north of the ship one week before their sled-retrieval rescue mission. It was a magical experience — complete with clear skies and even a faint aurora. “The thing that blew my mind is that the density of the stars went all the way down to the horizon all the way around you,” Uttal says. “It was like you were in a bowl full of stars.”

With the darkness came the cold. On some days, the thermometer read −55 °C, a temperature so frigid that any exposed skin could get frostbite in minutes. If you had to remove your gloves during that time (say, to connect cables or work on a laptop) it would feel like someone was driving nails through your fingertips.

A guard stands on a mound of snow overlooking the surrounding ice sheet as the beam of a head torch shines against a black sky

A guard stands watch looking for polar bears in early January.Credits: Alfred-Wegener-Institut/Lukas Piotrowski (CC BY 4.0)

“We were constantly reminding each other to jump around and move our bodies so we could increase circulation and at least try to stay a little warm,” says Ian Raphael, a master’s student in sea-ice geophysics at Dartmouth College in Hanover, New Hampshire.

That, coupled with complete isolation, compounds the risks of the winter leg. If a problem arises, help is a long way off. “The International Space Station is peanuts in comparison”, because astronauts on the station can return to Earth in three hours in an emergency, says Christian Haas, a sea-ice geophysicist at the AWI and cruise leader for the second part of the mission. During the winter leg of MOSAiC, he says, “it would have taken many days to mobilize aircraft or heavy-duty helicopters, which would have had to rely on fuel caches and good weather”.

To keep spirits high, Haas arranged a number of recreational activities. Scientists went skiing and hiking on the ice, played soccer, built bonfires and an igloo, and celebrated Christmas and New Year. They even spent a couple of nights camping — where eating chocolate could break a tooth and the tea had to be consumed before it froze, but where the starry skies and the sheer silence were stunning.

A polar bear and cub lit by a spotlight lean on research equipment on the ice around the Polarstern research vessel

Two polar bears visited the ice near the ship in October. During the second leg, researchers spotted fewer bears.Credit: Alfred-Wegener-Institut/Esther Horvath (CC BY 4.0)

On thick ice

It helped that the gruelling conditions were precisely what many scientists wanted to study — even the bitterly low temperatures.

During the winter, the atmosphere in the Arctic doesn’t get colder with height, but warmer. To study these temperature inversions, MOSAiC scientists released four weather balloons per day, allowing them to measure the temperature (among other parameters) at one-second intervals as the balloons climbed high into the atmosphere. They also built a meteorological tower that constantly monitored the atmosphere’s temperature at 2, 6, 10 and 23 metres above the surface.

Polar bears seen on the ice next to the research vessel Polarstern

Bears visited the Polarstern several times during leg 1 of the mission.Credit: Shannon Hall

The extreme layering surprised researchers. On several different occasions they saw inversions as big as 5 °C in the lowest 10 or 20 metres of the atmosphere. Yet scientists don’t know how often these inversions occur, or their typical strength. That’s a problem, because they have a crucial role in the amount of heat that is released from or confined within the Arctic, which influences the weather in other parts of the globe. They also help to further freeze the surface, therefore controlling the centrepiece of the Arctic system: the ice.

In 2018, a team including NOAA scientists discovered that the Arctic sea ice had declined by 95% in the past 33 years (see go.nature.com/3bkkpjt). That means that summer sea ice could disappear entirely, and relatively soon — a dramatic change that would further warm the Arctic and the rest of the world. Sea ice keeps the planet cool by reflecting sunlight. Without it, the Arctic would warm rapidly — thawing Greenland’s glaciers (which would push sea levels higher) and melting global permafrost (which would release huge volumes of heat-trapping gases into the atmosphere).

Gallagher is anxious to understand how often temperature inversions occur, given that the frigid air trapped near the surface allows more ice to form. And he is not alone. One of the main goals of MOSAiC is to better determine how the atmosphere and oceans affect the growth of ice in winter and its melting in summer.

Three scientists in stand in front of tents on the ice looking an aerial instrument equipped with atmospheric sensors

Geophysicist Christian Haas (left), leader of the second MOSAiC leg, and team examine a weather-balloon instrument package.Credit: Lukas Piotrowski

But to do that, the researchers have to keep a close eye on the ice. One of the early results from the winter expedition is that, by the time the scientists left for home, most of the ice they encountered was 1.5 metres thick. It didn’t matter whether the ice started off the winter as thick slabs left over from the previous winter (called multi-year ice) or whether it began as thin, relatively new ice (first-year ice). That finding suggests that first-year ice grows much more rapidly than its thicker, older counterpart.

Researchers don’t know why that happens. It could be that multi-year ice is too porous in the middle to grow quickly, so it has to freeze internally before it can thicken. Or it might be that multi-year ice is more likely to be covered by snow, which acts as an insulator from the extremely cold air. That would slow down the freezing of sea water. Indeed, MOSAiC scientists measured as much as 68 centimetres of snow on multi-year ice, whereas first-year ice gained only a few centimetres. (The reason is simple: snow tends to accumulate on the thick ridges of multi-year ice, but gets swept across smooth first-year ice.)

The answer is probably a combination of both processes. But researchers need to comb through their data to resolve such questions. They then hope to plug those details into models that can provide better forecasts of the changing Arctic.

Impossible life

For the most part, the scientists on board for the winter leg experienced a relatively quiet Arctic. They didn’t have to weather the kinds of ferocious storms that frequently fractured the ice at the beginning of the mission. They also avoided run-ins with polar bears. But for Robert Campbell, a marine research scientist at the University of Rhode Island in Narragansett, the world was anything but still. In fact, one of the most surprising findings was just how active the Arctic marine ecosystem seemed to be — even in the middle of polar night.

A tethered orange zeppelin used for making atmospheric measurements pictured against a starry sky with a green laser beam

A weather balloon floats above the Arctic ice, measuring air temperature and other variables.Credit: Lukas Piotrowski

For years, scientists assumed that marine life so far above the Arctic Circle was dormant during the long winter. Without sunlight, ice algae can’t grow. And because these algae form the base of the entire food web, most of the Arctic simply couldn’t be active — or so the thinking went. Then, studies at lower latitudes off the coast of Svalbard in Norway started to show the opposite: that life thrives in the darkness (for a review, see J. Berge et al. Prog. Oceanogr. 139, 258–271; 2015).

Still, no researchers had performed such studies so far north during polar winter — which is why Campbell was surprised when he dropped nets near the surface of the water and collected zooplankton, tiny organisms that drift freely in the ocean. The fact that they had food in their guts meant that they were feeding, although the menu item remains an open question (scientists will have to wait until the samples are sent back to land and they can analyse the contents). Some of the zooplankton were even reproducing. Campbell collected a number of females, only to find that when he incubated them on the ship, they released viable eggs — a finding that was so exciting he ran up to the researchers having breakfast and exclaimed: “I just became a father. I just had 300 babies.” “To see what was going on was an eye opener for me,” Campbell says.

Members of the MOSAiC expedition Leg 2 team stand around bonfires on the ice with the Polarstern ship in the background

The crew on leg 2 celebrates the winter solstice on 21 December with bonfires on the ice near the ship.Credit: Lukas Piotrowski

But it’s only a hint of what’s to come. Now that sunlight has returned to the Arctic, ice algae will bloom — providing a smorgasbord for animals throughout the Arctic food web. Scientists on the next leg will continue to monitor the plankton and sea-ice algae to determine how long it takes for the small creatures to start multiplying once the Sun warms the Arctic.

At least that was the plan. The original schedule called for the winter team to leave the Polarstern while darkness still gripped the region. But because the ship that transferred the scientists had to traverse through Arctic sea ice (a feat that’s never easy), it arrived late, giving the winter team more time on the ice and a chance to see the faintest layer of light along the horizon.

When the winter team returned to Norway on an icebreaker at the end of March, there were fears that the country’s COVID-19 lockdown would prevent the ship from docking. Luckily, the government made an exception and the team of scientists was able to reach land.

But the outbreak is wreaking more havoc with schedules. Travel restrictions imposed by Norway forced the cancellation of research flights bound for the Polarstern — leaving the current team (the third crew) stranded in the central Arctic. That has forced mission leaders to make the difficult decision to pull the Polarstern out of the ice. For three weeks, it will leave the research camp behind, travel to Svalbard to transfer scientists and then return to its place in the ice pack — temporarily disrupting some of the continuous data records that researchers have been taking at the site. Still, many scientists are amazed that the mission will go on amid the pandemic, with little disruption.

“I think it’s just remarkable at this stage that we’ll be able to continue the experiment,” says Donald Perovich, a geophysicist at Dartmouth College and a member of MOSAiC’s project board. “Part of it is just people’s dedication.”

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