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When the polar bears visit for the first time, they show a magnetic curiosity towards the ship. In their almost featureless and frozen home, we have arrived as a vast, colourful and pungent intruder. Even to the human nose, the odorous fog of diesel, cigarettes, disinfectant and cooking can be overwhelming. To the bears – whose noses are so sensitive that they can smell a seal from a mile away – it must be extraordinary.

We are visiting their home on the sea ice of the Central Arctic as part of the Mosaic expedition, which intends to spend a year attached to an ice floe to study the constantly changing environment around the North Pole. The ship we arrived on has embedded itself deep inside the ice with the aim of drifting with it for the next 12 months. (Read more about why the ship is spending a year frozen in the Arctic ice.)

A mother and her cub approach the ship at the end of the second week of the expedition. They stop a couple of metres away from where the hull has broken the ice into shards and slush. The adult looks directly up at us and cranes her head back, nudging the air as she sniffs. Her cub does the same and then hides behind her, peeking around her back legs at us.

Even to the human nose, the odorous fog of diesel, cigarettes, disinfectant and cooking can be overwhelming

Several times the cub hurries away a little, growling, seemingly trying to draw its mother off. When she doesn’t move, the cub hesitates, waits and then runs back to burrow into the fur of her belly. The mother appears to be transfixed.

For many on this expedition to the Central Arctic, including me as one of the few journalists on board, it is our first time seeing a polar bear. Scientists line up on the bridge and the bow, pointing cameras like paparazzi. After about half an hour of watching the bears watching us, one of the ship’s helicopters takes off and flies low overhead to frighten them away. They lumber off with a fast but ungainly gait. We watch their retreat for another 10 minutes, until they become yellowish specks ambling on the horizon.

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Although all on board are very interested in the bears, the animal’s interest in us is not a good sign for the expedition. The scientists have found an unusual piece of ice, which they call the “fortress”, that seems capable of supporting their expedition over the next year. They have moored their ship, the German icebreaker Polarstern, to the ice and hope it will remain frozen in the floe as it drifts up towards the North Pole. With luck the floe will then carry them down towards the Fram Strait, emerging between Greenland and Svalbard in autumn 2020. (Read more about the hunt for the perfect ice floe.)

The job of a bear-watcher is to scan the horizon for bears, while scientists work with their instruments on the ice (Credit: Martha Henriques)

As the scientists start to move their equipment and build the camp where they will live over the next year on the ice, visiting polar bears will become a major risk. From the safety of the ship they might be a curious distraction from daily duties, down there within claws’ reach, they are extremely dangerous.

Protecting the expedition from bears will be a difficult task. Killing polar bears is strictly regulated in the Arctic. The powerful animals are listed as a vulnerable species by the International Union for the Conservation of Nature – they hunt seals on the Arctic sea ice, which is diminishing due to climate change. On expeditions like this, killing a bear should only be a last resort, if someone’s life is at risk.

Nevertheless, everyone on the expedition needs to be vigilant for approaching bears when people are on the ice. I take my turn on bear watch while on the bridge of the Akademik Fedorov, the Russian supply vessel that has accompanied Polarstern to the Arctic to assist in setting up the Mosaic camp. From my vantage point, it is possible to see up to 40km (25 miles) on a clear day.

Everyone on the expedition needs to be vigilant for approaching bears when people are on the ice

All around us there is solid ice, much of it flat and covered with a thin blanket of fresh snow, with a few crumbling ridges around a metre (3ft) or so high visible further away. We have moored the ship alongside a large floe about 10km (six miles) away from Polarstern to set up a remote sensing site, which will become one of the three main outposts of the camp.

Close to the ship on the starboard side, a polar bear guard stands on a flattened ridge. He wears a dark blue and fluorescent green ice suit with white reflective stripes, a rifle slung over his shoulder. A couple of hundred metres away another guard stands on look out. Next to the ship, scientists manoeuvre a large yellow buoy onto the ice from the bow by crane, before heaving it onto a sledge and pulling it away by skidoo. I try not to get distracted by the activity, or the twinkling lights of Polarstern just visible on the horizon.

Polar bears are uniquely well camouflaged for their icy environment, keeping bear watchers and guards on their toes (Credit: Martha Henriques)

The first quarter of my two-hour watch goes quickly. There is nothing but ice to see for miles around, but the closer you look, the more details appear. Between stretches of flat ice are areas where it has crumpled into long ridges. Some ridges are old, weathered and covered with snow – they look like perfect mountain ranges in miniature. Other ridges are freshly formed, with a more jagged, rocky look, like blocks of ice have been brushed into a long, chaotic heap.

Occasionally, it is also possible to spot lone, irregularly shaped lumps of ice. They are easiest to spot by their shadows in this bright, white landscape. It is these monoliths of ice that occupy me the most. Is that shadow just a low outcrop of ice, or is it the curve of a polar bear’s back? Is that trio of dark specks the shade cast by a jumble of ice, or are they a nose and a pair of eyes? I start second-guessing myself, seeing movement and bear faces where there are none, regularly raising and lowering my binoculars, turning away and then looking back to see if the shadows still seem bear-like.

If I do see a bear, my job as a bear-watcher is simple – alert the officer of the watch immediately, who can then radio down to the people on the ice. For a bear guard, who is out on the ice as the first line of defence, the first reaction is also to tell the bridge – but after that, it can go a number of ways.

Polar bear guards like Trude Hohle have to make life or death decisions to protect both the scientists and the Arctic predators they watch for (Credit: AWI/Esther Horvath)

“We cannot really have action cards for first you do that and then you do this,” says Trude Hohle, one of Mosaic’s dedicated polar bear guards. “You really have to see the situation and see how the polar bear is reacting, how far away it is, how is the weather, do we have bad visibility, all of these things.”

If the bear is far away, Hohle might not have to do anything. It could just be that it’s walking past, she says, and poses no danger to people. But if it does show an interest in the human activity, Hohle has a range of options. The first is flares that she can shoot into the air to frighten the bear away. But they are only useful over a close range.

Generally, I would draw an imaginary line and say to myself, if the polar bear crosses that line, then it is unfortunately a dead polar bear – Trude Hohle

“A bear won’t be scared when it is 500m (0.3 miles) away,” she says. “The flares will stop at 100 to 150m (330-490ft) and that won’t do anything.” If it is up close, then a flare is usually enough to scare away a bear, says Hohle. But it depends on the bear.

If the bear is close, then retreating to safety is the preferable option. But if that is not possible, it’s the bear guard’s job to ensure the group’s safety.

“What I do when I’m polar bear guarding in general is that I imagine all kinds of scenarios,” says Hohle. “It’s not to scare myself, it’s just to be prepared. So I think, ‘OK now I’m standing here, what if a polar bear comes from that direction? Do I have a good background if I need to shoot at it? Is it a safe background? If someone breaks their leg, where is the closest first aid kit or where is the sled that I can pull them back to? If someone falls in the water…?’ These scenarios go through my head all the time, so I’m prepared. It’s not to be paranoid, but it’s just that I’ve gone through it already and then it is for me easier to act.”

A polar bear and her cub took a particular interest in the icebreaker Polarstern, hanging around the ship for days (Credit: Martha Henriques)

The last recourse if there is no other option is a rifle. “The weapon is for killing the polar bear,” says Hohle. “Generally, I would draw an imaginary line and say to myself, if the polar bear crosses that line, then it is unfortunately a dead polar bear – which is what we want to avoid at all costs.”

There is a strong respect for the bears among the guards, Hohle says. There is acknowledgement that we are the ones intruding into the bears’ habitat, not the other way around. If scientific equipment is at risk from bears – which is very likely to happen at some point – then that’s an unfortunate loss for the scientists, but the animal is more important, Hohle says. “You know, things are things. They don’t have an intrinsic value as people and wildlife do.”

The ice camp is also to be protected by a tripwire that will be set up around the main perimeter. When a polar bear walks into this, the idea is that it will trigger a flare to scare the animal away. “I don’t trust the tripwire alone, but it’s a nice back up,” says Verena Mohaupt, logistics lead for the expedition. “When it comes to the whole safety issue, you just put up as many barriers as you can.”

Throughout my bear watch, I think about what Hohle has told me. I try to keep myself focused – a lot of people’s lives could depend on spotting a bear as early as possible. But no polar bears come to the floe while I am on watch.

When my two hours are up, I hand my binoculars over to the next person to keep a lookout. They will be the one watching over me as I go out onto the ice.

Guards stand on lookout at the perimeter of the camp to try to spot polar bears far off, while there is still time to react (Credit: Martha Henriques)

Stepping off the Fedorov’s wobbling metal gangway, it’s surprisingly easy to forget that there is only 30cm (1ft) of frozen water between the soles of my boots and bitterly cold, Arctic Ocean that is 4km (2.5 miles) deep. If you brush away the snow, the ice is rough and bumpy. It has a blue-black colour that is scuffed with white from the snow. I try giving it a cautious kick – it feels like kicking concrete.

Reassured, I join the scores of people trudging around the floe, most wearing bright red insulated suits with reflective bands, black snow boots and layers of gloves, hats and scarves. It is already -18C (0F), with temperatures set to drop further in the next few days.

Today is the last time that the Sun will rise here this year, and the expedition party has a short window to stud this empty piece of ice with a complex array of remote sensing instruments that will measure the properties of the ice, ocean, atmosphere, ecosystems and biogeochemistry of this difficult-to-study region. The measurements taken during Mosaic – the most extensive Arctic expedition there has ever been – will provide the clearest picture to date of how the region around the North Pole is responding to climate change.

Finding suitable floes to set up the instruments on has been far from straightforward. On the satellite images that the team pored over throughout the summer, there were plenty of dark patches that they thought indicated a thick, stable platform of ice. The reality, however, has been worrying the scientists on board. Instead of firm, thick ice floes, they have encountered fragile thin and rotten ice. (Read more about Polarstern’s hunt for thick ice.)

“Honestly, when we just breezed through the slushy ice day after day I thought, ‘Woah,’” says Tim Stanton, who is leading the set-up of Mosaic’s outer network. “It was looking very, very grim.”

Working in -18C, scientists have to work quickly in just a few hours of light to set up an array of autonomous instruments (Credit: Martha Henriques)

The difficulty in finding thick floes was due to the extremely warm summer throughout much of the northern hemisphere this year, he says. These heatwaves were made more probable by climate change. “I think it’s unsurprising that the ice is so rotten,” says Stanton.

Even so, there are still some floes that are capable of supporting the expedition outposts. The one I have stepped onto is among the better ones. I watch two engineers drill a hole through it to deploy a large and complicated buoy called an ice-tethered profiler.

The instrument will dangle in the water to take detailed measurements of the conditions in the upper portion of the ocean between 5m (16ft) to around 750m (2,460ft). They will help to assess the ocean currents, temperature and salinity. The buoy’s GPS position will also help to track its drift. Understanding how eddies and currents bring heat to the ice and influence melting is an essential part of the energy balance in the Arctic. Knowing this can help to unravel how and when Arctic sea ice melts.

Instrument-laden buoys like these are being deployed in a wide area around Polarstern, tens of kilometres away in some cases. But putting an ice-tethered profiler, which weigh 700kg (1,543lbs) each, into the ocean is a particularly delicate operation.

Engineers have to work quickly to drill large holes through the ice using specially adapted equipment (Credit: Martha Henriques)

The engineers use a 24-inch drill, suspended from a tripod, to cut down into the ice. In little more than a minute, freezing water rushes up through the hole and over their ankles. It’s quite common to end up with wet feet. Another scientist uses a metal ladle to fish out the slush at the top of the hole so they can see if there’s anything down there that could block a deployment. It’s not always easy to see, but unexpected outcrops of ice below the surface – known as false bottoms – can wreck a buoy.

On one of the other sites, Stanton comes across just this problem. After large holes are drilled to deploy the buoys, Stanton checks them for a false bottom – but they all appear to be fine. But then he lowers a buoy to measure upper-ocean properties into the hole. He can tell it starts to bump into something. The team pulls it back up quickly and the instrument – which costs close to $100,000 (£77,340) and weighs 480kg (1,058lbs) along with its deployment apparatus – isn’t damaged, but it was a very close call.

“Thank god we caught it, because otherwise everything would have gone clunk, clunk, clunk over that edge and it would have damaged stuff for sure,” says Stanton.

The array of autonomous instruments being set up here – measuring a range of environmental physical and biological processes – will transmit data via a satellite to teams back on land thousands of miles away as well as to Polarstern. Unlike much of Mosaic’s central camp, the scientists won’t be able to reach these outposts easily to check up on the instruments – perhaps just visiting once a month to resupply their fuel. This means that the set-up phase at these outposts is even more crucial – if one of the instruments isn’t working now, it will be much harder to fix after the Fedorov has left.

The Arctic ice is deceptive as outcrops called “false bottoms” can lurk out of sight beneath the surface (Credit: AWI/Stefan Hendricks)

We’re at a new floe a few days later when cracks begin to appear. I’m in the mess hall of the Fedorov when it happens, a long room with rows of tables bolted to the floor and small round windows along one wall. Jari Haapala of the Finnish Meteorological Institute, the leader of the Mosaic ice team, comes in and points out of one of the windows. “Have you seen the crack?”

I hurry onto the deck – there is a long fissure in the ice, about 30cm (1ft) wide, extending from the hull of the ship out as far as I can see. Haapala was outside bouncing electromagnetic pulses off the boundary layers between ice and water to measure its thickness when it opened. Two scientists were stuck on the far side of the crack and quickly crossed in case it opened up further. Several groups, including Haapala’s, had to turn around and come back on board.

“This is the natural behaviour of the ice,” says Haapala. Even in the depths of the Arctic winter, the ice will crack and open up leads of water. “Of course, thinner and warmer ice cracks more, but cracks always exist.”

In the next year, the expedition will have to deal with many more cracks like these. It’s to be expected that in some cases cracks will cause damage. “It’s bad luck,” says Haapala. “There is a high probability that there are cracks, so we have to have several stations and we know some of our instruments will be destroyed.”

That afternoon I go up to the bridge to do another shift on bear watch. The ship’s first mate gives me the additional task of looking out for more cracks. In between scanning the horizon I check back down by the hull of the ship. From up here – on the eighth deck – the cracks are delicate dark narrow lines that seem to run on for miles. If the floe starts to disintegrate any further, then all the remaining scientists might have to be pulled off the ice.

Scientists have been establishing the central camp by Polarstern despite cracks appearing and constant attention from bears (Credit: AWI/Stefan Hendricks)

Over on the central floe by Polarstern, the scientists there have been having their fair share of both cracks and bears. The curious female polar bear and her cub have revisited both ships, choosing to hang around Polarstern for several days. Chasing them away by helicopter is only an option for so long. Chase them too hard or too fast and they can die from exhaustion – particularly the young ones. Work on the ice has also been delayed by the appearance of several cracks.

“This is the way things go out here and it is easy to give into the forces of nature,” says Matt Shupe of the University of Colorado, leader of Mosaic’s atmosphere team. One of the cracks opened right beneath Shupe’s feet as he was working. One of the other expedition sites, or “cities” as the scientists call them, was affected more seriously. The hub for remote operated underwater vehicles was also hit by a crack but is “still alive”, he says. Shupe, who has many years of experience of work on the ice, remains unflappable. “Things are still pretty stable in the fortress and in our main cities.”

That week, peering over the side of the Fedorov I see a small patch of what looks like blood on the ice, and wonder if a bear made a kill close to us. But soon there is an announcement to the expedition party that someone was seen feeding a bear from the side of the ship. If people are tempting the bears closer, then they are more likely to pose a risk to people on the ice, and the bears themselves are more likely to end up being shot.

But soon both bears and cracks take a back seat as the main concern on the Fedorov. One night when the ship is stationary, I hear an enormous thunk, as something bashes hard into the side of the ship. Again, a few seconds later, there is another thunk. It is far louder than any icebreaking I have heard so far – and besides, the ship is stationary.

In the corridor, people are hurriedly putting on their ice suits and rushing out on deck. “Jari went that way, he said something about ice compression,” says Anne Gold of the University of Colorado. I run to my cabin and climb into my own ice suit. The ship starts to shake again and there is a sound like low thunder outside, and then a scraping noise like metal on metal.

The Akademik Fedorov is capable of manoeuvring in moderate sea ice, but it can get stuck in more extreme ice conditions (Credit: Martha Henriques)

Out on the deck, Jari Haapala is looking over the edge of the ship. Below us, there is a trail of footprints where someone had walked alongside the ship earlier. But the line of footprints is broken by a mound of ice, forced upwards into a small hill of shards. The footprints are visible across the mound, but it is obvious that no one could have walked across it. Behind us, the wake of the ship, which is almost 25m (82ft) wide, has enclosed entirely. A neat seam zig-zags away where the ice has been forced back together.

The ship is caught between two stretches of ice that are being pushed together by ocean currents, Haapala tells me. The ice is crushing in on us, building up pressure around the hull until the ice cracks and breaks. The ship’s reinforced hull won’t collapse under the pressure, Haapala says, but there is a risk that we might get trapped in the ice.

If we do, we might be forced to begin our own unplanned Arctic drift experiment.

* Martha Henriques is a senior journalist at BBC Future. She is spending six weeks on board Polarstern and Akademik Fedorov as they embark on their mission. You can follow her progress on Twitter @Martha_Rosamund.

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Frozen North

This article is part of our Frozen North series. Climate change is already transforming the Arctic. In many areas, what was ice is now open water. But in the most inaccessible reaches of the far north, how much has changed? And what will the knock-on effects be for the global climate?

The world’s largest polar expedition has just set off to answer those questions – and BBC Future’s Martha Henriques is one of the lucky handful of journalists onboard. In our series Frozen North, she reports from the Arctic’s floating sea ice as scientists seek to find out how this shifting environment will affect all of us.

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