- The Arctic: In the first half of the month, there was only ice growth in the western part of the Arctic Ocean. The monthly mean value comes in at ninth place among the lowest monthly mean values for Arctic sea-ice extent.
- Monitoring the Arctic sea ice for an entire summer: AWI experts conclude comprehensive ice monitoring northeast of Greenland.
- In the Antarctic, satellites record the third-lowest October-sea ice extent since the beginning of satellite observation.
The Arctic: Clear signs of progressive Atlantification
In the Arctic, October is the first month with real new-ice formation when the summer ends. In many places, the frost returns, and new ice is formed when the ocean’s surface grows sufficiently cold. In October 2025, these conditions couldn’t be found in every region of the Arctic Ocean. Though the sea-ice extent increased from 5.5 million to 8.0 million square kilometres in the course of the month, this growth was limited to specific regions, as our time-lapse video of sea-ice concentration development shows (Video 1).
Video 1: Development of Arctic sea-ice concentration as a time-lapse video. The maps shown here cover the timeframe 1 – 31 October 2025.
In the first half of the month, new ice only formed in the Arctic Ocean’s Canadian Basin and the northern regions of the Canadian Arctic Archipelago. In the northern Barents Sea, in contrast, the ice margin actually retreated farther – most likely due to a combination of heat and winds. From mid-month, the satellite data shows the first patches of new ice in Siberia’s coastal waters. From this point on, the ice margin in the eastern part of the Arctic Ocean gradually shifted toward the coast and, in the final third of the month, closed the Northeast Passage in the East Siberian Sea.
In the same timeframe, very little new sea ice formed in the region northeast of Svalbard and Franz Josef Land.
Currently, the northern Barents and Kara Seas are virtually ice-free. This observation tracks with both the warm air temperatures and the high sea-surface temperatures in the region. There’s simply a great deal of heat in the ocean, which will prevent freezing any time soon,
Figure 1: Mean monthly sea-surface temperature anomalies in the Arctic in October 2025, compared to the monthly mean values for the reference period 1971 – 2000. As the data shows, surface temperatures were up to 3 degrees Celsius warmer than in the reference period in the waters northeast of Svalbard. This warmth most likely slowed the surface cooling, and with it, the formation of new sea ice.
Figure 2: Mean air temperature anomalies in October 2025, compared to the long-term October mean for the reference period 1971 – 2000. In the region north of the Franz Josef Land archipelago, the air was up to 8 degrees Celsius warmer than in the reference period at a mean altitude of 925 hPa (ca. 764 metres above the surface).
Figure 3: Mean pressure anomalies at sea level in the Arctic in October 2025, compared to the reference period 1971 – 2000. The arrows on the map indicate the wind’s direction; their length is proportional to the windspeed. Based on the map, it can be assumed that powerful offshore winds drove sea ice from the Kara and Barents Seas northward in October 2025.
The large ice-free areas northeast of Svalbard are indicative of the progressive Atlantification of the Arctic Ocean in this region. “If we compare the current development of the ice margin with the long-term mean, with last year, or with the data from the record-low year 2012, we can see that there is substantially less sea ice than before in the northern Barents and Kara Seas. Accordingly, we can assume that the interactions between the ice, ocean and atmosphere in these waters are increasingly changing,” says Klaus Grosfeld (Figures 4 & 5).
Figure 4: Difference in the mean position of the ice margin in October 2025, compared to the long-term mean for the years 2003 – 2014. The large red areas in the northern Barents and Kara Seas indicate where there is currently no sea ice due to rising air and water temperatures. In the northern East Siberian Sea, in contrast, there was more pack ice than the long-term mean (blue area) in October 2025.
Figure 5: Difference in the mean position of the ice margin in October 2025, compared to the record-low year 2012. Even in October 2012, there was more pack ice in the northern Barents and Kara Seas than in October 2025.
Generally speaking, the development of the Arctic sea ice in October 2025 was fairly normal. The monthly mean value extent, at 6.62 million square kilometres, came in above the long-term trend line for the month of October (Figure 6) and in ninth place among the lowest monthly mean values for Arctic sea-ice extent. Yet it only differed from the monthly mean value for October 2020, the lowest ever recorded, by roughly 1.16 million square kilometres – an area twice the size of Spain.
Figure 6: Development of mean sea-ice extent in the Arctic for the month of October. The light blue line represents the long-term trend.
Expedition’s end: Monitoring the sea ice for an entire summer
With the return of the research icebreaker Polarstern to her homeport in Bremerhaven, a one-of-a-kind Arctic expedition season came to an end for experts from the AWI’s Sea Ice Physics section. “For the first time, we were able to spend four consecutive months in the waters northeast of Greenland and observe how the sea ice’s behaviour changed in the course of the summer,” reports AWI sea-ice physicist Dr Marcel Nicolaus, who was on board for both cruises in this year’s Polarstern Arctic expedition.
Personally, he was especially impressed by the refreezing phase: “At first, there was only thin new ice. But it was thick enough for masses of snow to collect on top. The result was a slush of snow and ice that proved a challenge even for the research icebreaker Polarstern,” the sea-ice physicist recalls.
The problem: unlike pack ice, the slush couldn’t be broken through. “The slush that the ship parted with her bow simply reformed behind her shortly thereafter. Under these conditions, moving forward and backward proved difficult: there were plenty of times when we found ourselves stuck in the ice-and-snow slush,” Marcel Nicolaus reports.
From a scientific standpoint, the focus of the summer expedition was especially on how drifting motion determines the fate of sea ice. “For instance, I’m fairly certain that the large floe that was home to our monitoring station 3 from the CONTRASTS part of the expedition wouldn’t have disintegrated so badly if it hadn’t drifted so far to the east during the summer, which put it close enough to the Transpolar Drift to be influenced. As it was, the floe broke into seven main pieces, which then drifted about 140 kilometres apart over the next four weeks,” he recalls.
Video 2: AWI sea-ice physicist Marcel Nicolaus took part in both cruises of the 2025 Polarstern Arctic expedition, spending more than four months in the northern Fram Strait and in the waters northeast of Greenland. In this video, he shows how radically the Arctic sea ice’s appearance can change in the course of a single summer. Video: Marcel Nicolaus/Alfred Wegener Institute
Figure 7: View from the bridge of the Polarstern on the countless floe fragments in the marginal ice zone, which had previously constituted ice region 3 during the CONTRASTS expedition. Photo: Alfred Wegener Institute / Marcel Nicolaus
Figure 8: AWI sea-ice researchers hauling sledges with built-in ice-thickness-measuring instruments over the ice to the northeast of Greenland. Although the second cruise in the 2025 Arctic expedition focused on oceanographic research, sea-ice experts were also on board. They took every opportunity to gather additional data from the ice and the snow cover atop it. Photo: Alfred Wegener Institute / Marcel Nicolaus
Experts from the Sea Ice Portal contribute to the fourth edition of the book “Sea Ice”
Marcel Nicolaus isn’t just satisfied with the expedition’s outcomes; he’s also excited about the new fourth edition of the classic reference work “Sea Ice”, released this August (Figure 9). “This time, various Alfred Wegener Institute researchers from the fields of sea-ice physics and oceanography contributed their expertise as authors, and I was impressed to see how diverse and interdisciplinary the research and perspectives on sea ice have now become,” says the co-author. Apart from Marcel Nicolaus, Sea Ice Portal experts like Prof Stefanie Arndt, Dr Gunnar Spreen, Prof Christian Haas and Dr Luisa von Albedyll were contributing authors to the new edition (full book behind paywall).
Figure 9: The well-known reference work “Sea Ice” has now been released in a fourth, extensively revised edition. While the first edition from 2016 consisted of 11 chapters and 419 pages, the new edition is more than twice as long – weighing in at 24 chapters and 960 pages.
Arctic hooded seals now at greater risk because their habitat – the sea ice – is disappearing
Declining sea-ice extents and thicknesses in the Arctic pose growing threats to those fauna species that depend on pack ice for their survival – as also confirmed in the updated list of threatened species (Red List), released on 10 October 2025 at the International Union for Conservation of Nature and Natural Resources (IUCN) World Conservation Congress.
In the new IUCN list, the status of three Arctic seal species has worsened. The hooded seal (Cystophora cristata), formerly categorised as “vulnerable”, is now considered “endangered”, while the bearded seal (Erignathus barbatus) and harp seal (Pagophilus groenlandicus) are now listed as “near threatened”; in other words, they’re new additions to the list.
According to an official press release from the IUCN: “Arctic seals rely on sea ice for breeding and raising their pups as well as for moulting, resting, and accessing foraging areas. Thinning and disappearing sea ice also affects Arctic seals’ feeding habits, and makes the Arctic more accessible to humans, further increasing the overall risk to these species.”
Figure 10: Stocks of the up to 3-metre-long Arctic hooded seal (Cystophora cristata) are now considered to be “endangered” – in part because the species needs pack ice to breed and raise its young. Photo: Hans Verdaat
How ice algae remain mobile despite extreme temperatures in the sea ice
Diatoms that live in sea ice have developed unique adaptation mechanisms which allow them to move through the ice even at extremely low ambient temperatures. In a new study, researchers at US-based Stanford University were able to show that the ice algae not only produce ice-binding proteins that keep them from freezing solid or becoming trapped in the sea ice; the organisms also secrete a unique form of mucilage that allows them to glide through the tiny brine channels, even at temperatures down to minus 15 degrees Celsius (Figure 11). Diatoms from warmer waters have no such luck. If they come into contact with sea ice, they can’t move under their own power, as they can’t gain purchase on the slippery surface.
But ice algae can wander – or better said, glide – through the sea ice to find those spots with the ideal conditions for photosynthesis. For this essential process, the algae above all need an optimal supply of light and nutrients, together with salinities that are as low as possible. According to reports, the single-celled organisms can cover 1.5 centimetres or more per day. What this looks like can be seen in a variety of videos prepared by the researchers.
Figure 11: This diagram illustrates the gliding motion of diatoms living in sea ice. It is powered in part by a mucilage filament-based locking mechanism. Graphic: Q. Zhang et al. (2025), https://doi.org/10.1073/pnas.2423725122, CC BY-NC-ND.
The Antarctic: Third-lowest October sea-ice extent since the beginning of observations
In October 2025, the sea-ice development in the Antarctic held few surprises. After reaching a comparatively low winter maximum in September, the sea-ice extent curve largely remained below the span of minima and maxima in the period 1981 – 2010 (Figure 12) in the first month of spring. The monthly mean sea-ice extent was 17.09 million square kilometres, making it the third-lowest ever recorded in October and placing it well below the trend line for the month (Figure 13).
Figure 12: Development of Antarctic sea-ice extent in comparison; the blue line is the curve for 2025. For the majority of October, the curve was below the turquoise band, which indicates the span of minima and maxima in the period 1981 – 2010. The Antarctic sea-ice extents on 31 October of selected years are also provided as figures.
Figure 13: Development of mean sea-ice extent in the Antarctic for the month of October. The light blue line represents the long-term trend.
“Our ice-concentration maps for the Antarctic show significantly less sea ice than in the past, especially off the coast of East Antarctica. The same can be said of the Bellingshausen Sea,” says Klaus Grosfeld (Figures 14 & 15).
It’s difficult to make further-reaching statements on the basis of ice-concentration maps alone; particularly in the marginal zone of the Antarctic sea ice, its distribution and extent are largely shaped by the wind. In other words, they are subject to a range of natural fluctuations, which also influence e.g. how the pack ice is drifting. “Off the Amery Ice Shelf in East Antarctica, for instance, the winds chiefly blew landward in October. This wind direction may have contributed to the fact that here, the pack ice was densely compressed against the coast. In the eastern part of the Weddell Sea, in contrast, offshore winds scattered the floes,” explains Klaus Grosfeld (Figure 16).
Figure 14: Difference in the mean position of the ice margin in October 2025, compared to the long-term mean for the years 2003 – 2014. Regions marked in blue had more Antarctic sea ice in October 2025 than in the reference period; those marked in red had less.
Figure 15: Difference in the mean position of the ice margin in October 2025, compared to the mean position in October 2024. Regions marked in blue had more Antarctic sea ice in October 2025 than in the reference period; those marked in red had less. It can be clearly seen that in e.g. the eastern Weddell Sea and the sector of the Ross Sea, the ice margin extended farther north this October than last October.
Figure 16: Mean pressure anomalies at sea level in the Antarctic in October 2025, compared to the reference period 1971 – 2000. The arrows on the map indicate the wind’s direction; their length is proportional to the windspeed. As the map indicates, powerful onshore winds drove the sea ice against the coast off the Amery Ice Shelf in East Antarctica. Conversely, in the eastern Weddell Sea, offshore winds pushed the ice far into the open ocean.
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