Nov 2021

The Ship that was Trapped in the Arctic Ice

In 2019 - 2020, the German research icebreaker Polarstern spent a year drifting 3,500km through the Arctic Ocean, deliberately trapped in the ice. Following in the footsteps of Fridtjof Nansen's expedition with Fram in 1893-1896, the Alfred Wegener Institute-led, year-long, MOSAiC expedition brought a modern research icebreaker close to the North Pole, for the first time, in the polar winter when temperatures dropped to -42 degrees Centigrade. Halfway through the ship’s voyage, the covid-19 pandemic swept across the world, threatening an abrupt halt to the €140 million expedition that had been 10 years in the making.

Dr Markus Frey was one of 240 scientists from 35 nations that took part in the expedition. He joined the ship on its third resupply and crew rotation, staying onboard from March – June 2020. Dr Frey’s research looked at whether sea ice emissions of small particles can affect the formation of clouds, and therefore influence the climate in the Arctic. He said of the covid-19 disruption:

“We were on the ship when the pandemic broke out, and it seemed quite surreal to us. We couldn’t fly out at the end of our original two-month rotation, because there were no planes, crew or ice breaker to come and get us. There were a few weeks when we didn’t know when they were going to return or when we would get home, which was quite stressful. Fortunately, we had enough food and fuel to steam to safety if we had to, but there was a lot of uncertainty at that time.”

One aim of the multi-disciplinary MOSAiC expedition was to investigate the Arctic as a whole, complex system of ocean, sea ice, snow and the atmosphere above. Some of the linkages are already well known, such as the sea ice albedo feedback where the white surface of the ice reflects sunlight back into space, but without the ice, the darker ocean retains more heat and accelerates the warming process. However, Dr Frey thinks that other mechanisms between the sea ice and the atmosphere may be influencing the climate.

Whereas in the past, the majority of sea ice was multi-year (surviving at least one summer without melting), it is now first year ice that dominates. The first-year sea ice is more saline, which could be causing increased emissions of sea salt particles into the atmosphere. Dr Frey’s research at the British Antarctic Survey aims to understand how sea salt aerosol particles emitted from the snow on sea ice react with sunlight and could also act as a cloud forming particle. This climate feedback could be important in the Arctic region, as well as the lower latitudes.

As there is very little available data on this, in situ measurements were essential. Fieldwork in the high Arctic can be very challenging, with leads (channels of open water) opening up without warning and then refreezing within 48 hours, and with pressure ridges forming which would crush the scientific equipment and disrupt its power supplies. The risk of polar bears was another factor, with scientists providing a constant watch from the bridge and the ice. If a bear was spotted, the “bear guards” alerted the field teams via radio to return immediately to the safety of the ship.

A year on from the expedition, many scientists are still working on the unprecedented data sets which document the full life cycle of sea ice in one year. But were the weather and ice conditions in 2019 – 2020 unusual? It was the second warmest year on record in the Arctic and, due to the weather, the ice drift was 20% faster than average in the past 10 years. Sea ice thickness was lower and the ice floe (large, floating ice platform) that the crew and scientists established their camp on was thinner than in previous years. Further research will show how this year relates to the ongoing warming in the Arctic and the data will be fed into future climate forecasts.

Elise Droste joined the expedition to investigate the role of sea ice in carbon dioxide (CO2) uptake in the Central Arctic Ocean. She was part of the last rotation of scientists, onboard from August to October 2020, and was surprised how many melt ponds and leads there were, even at the North Pole, in the summer.

The world’s oceans absorb heat and carbon dioxide that we emit into the atmosphere, and the Arctic Ocean carries a significant burden. Cold water is capable of holding more CO2 than warm water so as the warm Atlantic water encroaches into the Arctic and cools down, its capacity to take up CO2 is enhanced.

The growth and melt of the sea ice every year changes the properties of the sea water, which affects its capacity to take up and hold CO2. It is the difference in CO2 content between the sea water and the atmosphere which decides whether the ocean will take up or release CO2. The CO2 content of the water also changes seasonally, which is why the collection of year-round observations is necessary.

Certain processes cannot be studied fully via satellite or remote sensing data, so it’s necessary for scientists to collect samples directly from the Arctic Ocean. For oceanography, this is done using a CTD Rosette, which takes water samples and measures temperature and salinity, from the surface to the ocean floor 4km below. Many scientists used the water samples for numerous different studies and the samples were either analysed onboard Polarstern, or stored for transportation to labs all over the world.

Polarstern navigating the Arctic Ocean in summer, 25 August 2020. Photo: Charles Finkbeiner & Steffen Graupner.

Ms Droste was onboard for the Arctic Ocean’s freeze-up and she studied the transition period, when the surface starts to reflect more radiation than it absorbs, which is the cue for the ocean to plunge into winter. She analysed how the sea water’s capacity to take-up CO2 changed, determining whether the ocean was taking-up or emitting CO2. Her data is still being analysed, but she hopes to understand how the future loss of sea ice will affect the ocean’s ability to take up CO2.

Polarstern navigating the sea ice after freeze-up, 18 September 2020. Photo: Charles Finkbeiner & Steffen Graupner.

Dr Katrin Schmidt was also onboard from August to October and was studying the changes in the Arctic food web. She used nets, the CTD Rosette and an echosounder to sample zooplankton and sea ice fauna to depths of 2km.

Zooplankton (such as copepods and krill) are a vital link in marine food webs and they are able to sequester large amounts of our carbon emissions. When freeze-up occurs in the polar regions, the biggest animal migration on the planet begins. Billions of copepods migrate to the deep ocean to hibernate for up to 8 months. In the summer, the copepods feed on algae at the surface and build up their fat reserves then, while overwintering at depth, they use up these reserves and respire CO2 which is sequestered in the deep ocean for 1,000 years.

So how will an ice-free Arctic affect the food web? Global warming is prompting the “Atlantification” of the Arctic Ocean, as more warm Atlantic water is passing through the Fram Strait into the Arctic. Larger areas of permanently open water and longer ice-free seasons have resulted in a 50% increase in algae production in the last 10 years. But the algae are consumed by the smaller Atlantic copepods who are starting to out-compete their Arctic counterparts, directly impacting predators such as the ringed seal, which are showing a long-term reduction in blubber.

Moving up the food chain, 50% of a polar bear’s body mass is fat reserves, allowing them to starve for up to 4 months. These fat reserves provide insulation, energy for their offspring, assist with buoyancy when swimming and even provide water (similar to camels). Seal blubber is their richest food source and if there is no ice from which to hunt seals, the polar bears have few alternatives. Unlike brown bears, they are purely carnivorous and cannot eat berries, and as ambush hunters they’re not adapted to hunt caribou.

Dr Schmidt’s research has concluded that the Arctic food web is changing due to the loss of sea ice as a habitat, the increase in algae production and the increased infiltration of Atlantic species into the Arctic. As the Arctic gets “greener” (or more productive), her research questions whether the “benefits” are actually transferred to the higher levels of the food chain. Traditional Arctic species such as the polar bear and Arctic copepods need high quality food to produce sufficient fat reserves to see them through the long winter starvation period. An increasingly warm, green Arctic is a warning of a potentially lost ecosystem where species that are adapted to live around sea ice will soon be out-competed.


James Gray MP – “You’re studying closely what’s happening in the Arctic now, but how can that information be used to understand what will happen in the future?”

MF – The field measurements from MOSAiC will be used to validate the data from satellite sensors which will give more accurate measurements of sea ice thickness, how much snow there is on the sea ice etc. This data will inform our global climate models which are our best possible understanding of the system.

ED – MOSAiC allowed us to study smaller processes on different time scales from daily, to seasonal, or decades and those smaller processes can now be fed into the models to improve their accuracy.

Baroness Neville-Jones – “Did you discover anything that surprised you?”

KS – We repeatedly found some ice amphipods that are known to live among the sea ice, at a depth of 2,000m. It’s not clear how they get there and we’d like to explore whether it’s an adaptation mechanism that would allow them to exist in a warming Arctic.

MF – Theories have suggested that big storms could lift salty snow off the sea ice and release small particles through sublimation, but we found the first evidence that this is happening. It shows that the sea ice is not a lid that seals the Arctic Ocean in winter, it’s an active surface that may even influence the formation of clouds.

Henry Burgess, NERC – “You all said that it was important to come back and take these measurements again in the future. Realistically, how can this happen because MOSAiC was 10 – 15 years in the planning? Are there ways of gathering this data remotely or autonomously so that we can build up that record in the future?”

KS – We left an echosounder on the ice which drifted for another 7 months to monitor the abundance and migration of zooplankton. Traditionally it was always sampled by net, but the advantage now is that the data is generated continuously and remotely.

MF – autonomous sensors is one cost-effective solution and we have deployed buoys that take observations down into the ocean and up into the atmosphere. We’re also using satellite data, and from MOSAiC we now have a ground truth to better understand exactly what they’re measuring. But occasionally we need in situ measurement and sample collection, not always on the scale of MOSAiC, but winter, spring and autumn seasons are notoriously under-sampled.

ED – Ocean robots float with the currents and, every 5 or 10 days, dive to a depth of 2,000m and come back up, taking measurement profiles along the way. These are being deployed throughout the world’s oceans and can operate throughout the year. The problem is that there’s a risk in sea ice areas that the floats will get damaged or lost because they cannot come to the surface and send the data back. But even these autonomous vehicles need to be calibrated to make sure that the sensors are not drifting, and are giving accurate data.

James Gray MP – “Were you concerned that the ship might have been crushed under the strain of the ice?”

KS – The Polarstern is 35 years old and has travelled in these regions a lot so I felt very safe there. There are plans to replace her in 2026 with a more environmentally friendly ship.

MF – Having been onboard Polarstern in the Weddell Sea in the Antarctic winter, I was confident that she would be able to cope with the ice conditions on this expedition.

Links to further reading and a Channel 4 documentary about the expedition:

Channel 4, A Year in the ice: The Arctic drift

Book, Into the Arctic Ice: the Largest Polar Expedition of all Time, by Esther Horvath

MOSAiC Expedition website

UEA Stories: Elise Droste’s report on life onboard during the MOSAiC expedition

MOSAiC research

This report was prepared by Sophie Montagne (Director, APPG for the Polar Regions Secretariat), and endorsed by James Gray MP (Chairman, APPG for the Polar Regions).

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This is not an official publication of the House of Commons or the House of Lords. It has not been approved by either House or its committees. The views expressed here are the author’s own and do not represent those of the All-Party Parliamentary Group for the Polar Regions