- On 1 January 2024, the sea-ice extent in the Arctic was 12.54 million square kilometres – just 70,000 square kilometres more than on the last day of December in 2016.
- New AWI study: The more the percentage of older, thicker ice in the Arctic declines, the less frequently pressure ridges are formed.
- Antarctic: Offshore winds largely dispersed the pack ice. In the course of December, the sea-ice extent declined more slowly than in the past three summers.
The Arctic: Large ice-free areas in Hudson Bay
In the Arctic, 2024 nearly ended with a new daily record low sea-ice extent: on 1 January 2025, satellites observed a total area of 12.54 million square kilometres with the minimum ice concentration of 15 percent. According to our statistics, this extent was only 70,000 square kilometres more than on 31 December 2016, the record holder for the lowest sea-ice extent on the last day of December.
In terms of the monthly mean value, December 2024, with a sea-ice extent of 11.56 million square kilometres, was the second-lowest to date: the only December with a lower sea-ice extent in the Arctic Ocean was in 2016, with 11.44 million square kilometres (Figure 1 and interactive graphic).
However, a comparison of the sea-ice maps from December 2016 and 2024 reveals clearly recognisable differences in the distribution of sea ice in the Arctic Ocean. Whereas in December 2016 there were large ice-free areas particularly in the Bering Sea and northern Barents Sea, in December 2024 these areas were most striking in Canada’s Hudson Bay. Roughly two-thirds of this marginal sea of the Arctic Ocean, with an area of 1.23 million square kilometres, was frozen over at the end of the month; its eastern waters and much of the Hudson Strait remained ice-free – for the first time in the history of sea-ice observation. Normally, both Hudson Bay and the Hudson Strait freeze over in the first half of winter (Figure 2).
In the last three months of 2024, most likely the higher air and water temperatures in and above Hudson Bay are what kept the surface water from freezing. According to data from the Copernicus Climate Change Service (C3S), in November 2024 the surface water in the eastern part of the Bay and in the Hudson Strait was the warmest it had ever been in the monitoring period. In December, the surface temperature in the eastern Hudson Bay and the Hudson Strait was in some places up to 2.5 degrees Celsius above the monthly mean for the reference period 1971 – 2000 (Figure 3).
In November and December, air temperatures over northeast Canada were also exceptionally high. Our graphic of the temperature anomalies at an altitude of ca. 760 metres (925 hPa pressure altitude) shows that over Hudson Bay, the monthly mean temperature in December 2024 was up to 6 degrees Celsius above the long-term mean temperature values from the monitoring period 1971 – 2000. And in many other parts of the Arctic, it was much warmer in December than it had been 25 to 50 years earlier (Figure 4).
Our anomaly map also shows extremely high sea-surface temperatures in other parts of the Arctic Ocean – like in Fram Strait and in the Barents Sea. Warm Atlantic water flows through both regions on its way to the Arctic Ocean. As such, this prominent heat signal could be a further indication of the increasing “atlantification” of the Arctic Ocean (for more information, please see this article) – mutually amplifying changes in the sea-ice cover and water layering in the northern Barents Sea, which are triggered by the influx of increasingly warm Atlantic water.
New AWI sea-ice study: Significant decline in pressure ridges on the Arctic Ocean
Due to the decline in older, thicker sea ice in the Arctic Ocean, over the past 30 years the surface structure and coarseness of the ice cover have changed as well. In particular, the frequency and size of pressure ridges (Figure 5) have significantly declined – a conclusion reached by experts from the Alfred Wegener Institute in a new study published today in the journal Nature Climate Change. In the study, the researchers reprocessed and analysed the laser-based readings taken by the AWI’s sea-ice physics team for the past 30 years in aerial campaigns over the Arctic.
Pressure ridges are produced by lateral pressures on sea ice. Wind or ocean currents can stack floes up, forming metre-thick ridges. The part of the ridges – which break up the otherwise smooth surface of the ice every few hundred metres – extending above the water is called the sail and measures between one and two metres. Even more impressive is the keel below the water line, which can extend down to 30 metres and create an impassable obstacle for shipping.
Pressure ridges affect not only the energy and mass balance of the sea ice, but also the biogeochemical cycle and the ecosystem: when their sails catch the wind, floes can be driven all across the Arctic. Polar bears use pressure ridges as a source of protection for overwintering or birthing their young. Below the water line, the jumble of ice wedges offers protection for algae, small crustaceans, and various other ice-related organisms and promotes the turbulent mixing of water, which increases nutrient availability.
The reprocessed laser-based readings show for the first time that the frequency of pressure ridges north of Greenland and in Fram Strait has dropped by 12.2 percent, and their height by 5 percent, per decade. Readings taken on the Lincoln Sea, a region where particularly old ice accumulates, paint a similar picture: here, the frequency has decreased by 14.9 percent and the height by 10.4 percent per decade (Figures 6 through 9).
“Until now, it’s remained unclear how pressure ridges were changing,” says Dr Thomas Krumpen, a sea-ice expert at the AWI and the study’s main author. “More and more of the Arctic consists of ice that melts in the summer and is no more than a year old. This young, thin ice can more readily be deformed and more rapidly forms new pressure ridges. So you might expect their frequency to increase. The fact that pressure ridges are nonetheless in decline is due to the dramatic melting of older floes. Ice that has survived several summers is characterised by a particularly high number of pressure ridges, since it has been subjected to high pressures over a longer timeframe. The loss of this multiyear ice is so severe that we’re observing an overall decline in pressure-ridge frequency, even though the thin young ice is easier to deform.”
In order to draw conclusions regarding Arctic-wide changes, the researchers combined all observational data to develop a metric. Then, with the aid of satellite data, they applied it to the Arctic Ocean as a whole: “We tend to see the greatest decline in pressure ridges in those places where the ice’s age has decreased most,” summarises Prof Christian Haas, Head of Sea-ice Physics at the AWI. “Major changes can be seen in the Beaufort Sea, but also in the Central Arctic. Both regions are now partly ice-free in summer, though they were once dominated by ice that was at least five years old.”
In order to gauge the direct effects of these observed changes on the Arctic ecosystem, new models have to be developed that reflect both the physical and biological processes at work in sea ice of varying ages. Although we know that pressure ridges are home to a range of organisms, we still lack a firm grasp of the role played by their age. But this aspect is especially important, since the percentage of pressure ridges that don’t survive their first summer is on the rise. Another riddle: although the ridges now tend to have smaller and fewer sails, the drift speed of the Arctic ice has increased on the whole. As AWI sea-ice physicist Dr Luisa von Albedyll, who contributed to the study, explains: “Actually the ice should drift through the Arctic more slowly when the sail area declines, as this reduces the transfer of momentum. This indicates that there are other changes taking place that produce just the opposite effect. More powerful ocean currents or the smoothing of the ice’s underside due to intensified melting processes could be relevant in this regard.”
The Antarctic: Offshore winds largely disperse the remaining sea ice
On the Southern Ocean, in December 2024 winds had dispersed the sea ice to such an extent that in the course of the month, the sea-ice extent declined more slowly than in the past three summers in the Southern Hemisphere. At the end of the year, the sea-ice extent was 7 million square kilometres and the sea-ice concentration in regions like the northern Weddell Sea, the Ross Sea, and off the coast of West Antarctica was very low in some places. In other words, there were only scattered floes in these regions (Figure 10). In terms of the monthly mean value, the sea-ice extent was 9.29 million square kilometres – the eleventh-lowest mean value for December since the beginning of satellite observations.
In December, offshore winds especially blew over the Weddell Sea and Ross Sea (Figure 11). In the Weddell Sea, southerly winds drove the pack ice to the northwest; as a result, monitoring satellites detected above-average ice levels compared to the long-term mean off the tip of the Antarctic Peninsula throughout the month (Figure 12). At the same time, comparatively low temperatures dominated in the northern Weddell Sea, slowing the melting of the sea ice. Both the air temperature and sea-surface temperature were below the long-term mean for the time of year (Figures13 & 14).
Conversely, winds in the Ross Sea delayed the formation of large, regularly occurring summer polynyas in December. To the west of the Antarctic Peninsula, in the Bellingshausen Sea, northerly winds compressed the pack ice to such a degree that the remaining sea-ice extent there is below the long-term mean for 2003 – 2014.
“So far, the summer sea-ice melting in the Antarctic has held few surprises,” says Prof Stefanie Arndt, a sea-ice physicist at the Alfred Wegener Institute. “However, the developments in December confirm that the combination of air and water temperatures, wind fields and resultant ice drift has a major influence on how the Antarctic sea ice is distributed and how thick it is,” she reports from on board the research icebreaker Polarstern, which is currently bound for the Antarctic.
During the research cruise in the Weddell Sea, the AWI sea-ice physicists will take a closer look at the interactions between the Antarctic sea ice, ocean, atmosphere and wind, as our understanding of the processes involved remains incomplete. Though the ship will have to travel several hundred more nautical miles before reaching the sea-ice edge, the first iceberg has already been spotted on the horizon (Figure 15). Over the weeks to come, we’ll provide extensive coverage on the latest findings made by Stefanie Arndt and her team on site.
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Sina Löschke (science writer)
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