Significantly fewer pressure ridges in the Arctic sea ice
Photo: Alfred Wegener Institute / Andreas Preusser
The sea ice cover of the Arctic Ocean has not only shrunk and thinned due to global warming but has also become significantly flatter over the past three decades due to the decline in so-called pressure ridges. This may be good news for shipping, but not for the ecosystem.
The sea ice landscape in the Arctic (and Antarctic) is far from monotonous and flat. Instead, it features flat areas interspersed every few hundred meters with elevations reaching up to two meters high—known as pressure ridges. These ridges pose insurmountable obstacles for shipping.
Pressure ridges form when ice floes are pushed against each other and stacked by wind and ocean currents. Depending on the age and thickness of the floes, these ridges can become remarkably thick, particularly beneath the surface. The stacked floes can reach depths of up to 30 meters.
A research team from the Alfred Wegener Institute (AWI) and the Norwegian Polar Institute has now shown for the first time that the frequency and height of pressure ridges in the Arctic has decreased significantly: In areas such as north of Greenland, in the Fram Strait and in the Lincoln Sea north of Ellesmere Island (Canada) and northwest Greenland–the latter is a region where old sea ice is concentrated–the frequency decreases by 12 to 15 % per decade, and the height by 5 to 10 % per decade.
The results of the study, published on January 6 in the journal Nature Climate Change, are based on laser measurements collected over the Arctic sea ice for 30 years using AWI research aircraft. The researchers analyzed previously unpublished data from 52 survey flight campaigns conducted between 1993 and 2023, covering a total flight distance of 76,000 kilometers.
“Until now, it’s remained unclear how pressure ridges were changing,” says Dr. Thomas Krumpen, sea ice expert at the AWI and lead author of the study, in an AWI press release. “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.”
Professor Dr. Christian Haas, head of the Sea Ice Physics Section at the AWI, explains that the decline in pressure ridges has been greatest in regions where the age of the ice has decreased the most. This has been observed in the Beaufort Sea and the central Arctic, areas that are now partially ice-free during summer but were once dominated by ice five years old and older.
Essential for the ecosystem
From an ecological perspective, pressure ridges are a fundamental component of sea ice. The portion above the water surface, known as the sail, creates cavities and niches that polar bears use for hibernation and giving birth to their young. At the same time, the towering floes catch the wind, facilitating faster movement of the ice across the Arctic Ocean.
The underwater ice keel, on the other hand, serves as a habitat for ice algae and zooplankton, which attract polar cod—a key food source for seals, seabirds, and whales. The turbulence generated by the keel promotes mixing in the upper water layer, enhancing the availability of nutrients.
The effects of the observed changes on the Arctic ecosystem are still subject of research. Understanding the role the age of ice ridges plays for the numerous associated organisms is particularly crucial, as warming leads to fewer ice ridges surviving their first summer.
Faster drift despite smaller sails
During the study, the researchers also discovered that, despite the shrinking size of ice ridges, the speed at which ice is being driven across the Arctic Ocean is increasing.
Dr. Luisa Albedyll, sea-ice physicist at the AWI and co-author of the study, is looking for explanations for this contradiction: “Actually, the ice should drift more slowly when the sails shrink, since there’s less area for the transfer of momentum. This indicates that there are other changes producing just the opposite effect.”
“Stronger ocean currents or a smoother ice underside due to more intensive melting could be contributing factors. To answer these open questions and gain a better grasp of the complex interrelationships, we have made the entire dataset available in a public archive, ensuring that other researchers can use it and integrate it into their studies.”
The open questions, such as the biological and biogeochemical differences between ice floes and pressure ridges of different ages, are to be investigated this summer during an expedition with the German research icebreaker Polarstern.
“By combining ship-based and aerial observations, we hope to gain better insights into the complex interactions between the sea ice, climate and ecosystem – since we’ll only be able to devise effective strategies for the preservation and sustainable use of the Arctic once we better understand the region’s environmental system,” says Dr. Thomas Krumpen.
Julia Hager, Polar Journal AG
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