Vanishing Arctic Ice Ridges Signal Alarming Climate Shifts

Arctic sea ice is undergoing profound changes, with older, rugged ice diminishing and younger, smoother ice becoming more prevalent, affecting the ecosystem and ice dynamics in unexpected ways.

In the Arctic, the melting of old, multiyear ice is significantly reducing both the frequency and size of pressure ridges. These ridges, formed when ice floes collide and stack, are a defining feature of Arctic sea ice. While they present challenges for shipping, they are also vital to the region’s ecosystem. In a newly published study in Nature Climate Change, researchers from the Alfred Wegener Institute examine this trend using observational data gathered over 30 years of aerial surveys.

Arctic Changes

Satellite data collected over the past 30 years highlights dramatic changes in Arctic sea ice caused by climate change. Summer ice coverage is steadily shrinking, ice floes are becoming thinner, and their movement is accelerating. However, until recently, it was unclear how these changes have affected pressure ridges, as reliable monitoring of these features from space has only become possible in the past few years.

Nature and Impact of Pressure Ridges

Pressure ridges form when lateral forces, such as wind or ocean currents, push ice floes together, stacking them into ridges that can be meters thick. The part of a ridge that rises above the water, known as the sail, typically measures one to two meters in height. Below the waterline, the keel is even more striking, extending as deep as 30 meters and creating obstacles that are nearly impossible for ships to navigate.

These ridges play a crucial role in the Arctic environment. They influence the energy and mass balance of sea ice and impact the biogeochemical cycle and local ecosystems. When ridge sails catch the wind, they can drive ice floes across the Arctic. For polar bears, pressure ridges offer essential shelter for overwintering and birthing cubs. Additionally, these formations provide habitat and protection for ice-associated organisms across various trophic levels. They also enhance nutrient availability by promoting turbulent water mixing beneath the ice.

Recent Research Discoveries

A team of researchers from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), has now reprocessed and analyzed laser-based readings gathered in 30 years of research flights over the Arctic ice. The survey flights, which cover a total distance of roughly 76,000 kilometers, show for the first time that the frequency of pressure ridges north of Greenland and in Fram Strait is decreasing by 12.2%, and their height by 5%, per decade. Data from the Lincoln Sea, an area where particularly old ice is known to accumulate, paints a similar picture: here, the frequency is declining by 14.9% and the height by 10.4% per decade.

Surprising Findings on Ridge Dynamics

“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 characterized 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 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.”

Advances in Research Methodologies

For the study, individual pressure ridges and their heights were precisely measured and analyzed during survey flights. This was possible thanks to the low-level flights (less than 100 meters above the surface) and the laser sensors’ high scanning rate, which allowed terrain models to be created.

The AWI began scientific flights over the sea ice in the early 1990s, launching from Svalbard. Back then, the institute relied on two Dornier DO228s, Polar 2 and Polar 4; they have since been succeeded by two Basler BT-67s, Polar 5 and Polar 6. Specially equipped for flights under the extreme conditions found in the polar regions, they can be fitted with a range of sensors. Using these aircraft, researchers survey the ice north of Greenland, Svalbard, and Canada twice a year. But the icebreaker Polarstern’s onboard helicopters are also part of the monitoring program.

Implications for Arctic Ecosystems

In order to estimate the direct effects of the observed changes on the Arctic ecosystem, models need to be developed that can reflect both physical and biological processes in sea ice of various ages. Although we know that pressure ridges are home to a range of organisms, we still lack a deeper understanding of the role of pressure-ridge age. Yet this aspect is especially important, as the percentage of ridges that don’t survive their first summer is on the rise. Another riddle: although the size and frequency of ridge sails have decreased, the drift speed of Arctic ice has generally increased.

As AWI sea-ice physicist Dr. Luisa von Albedyll, who contributed to the study, explains: “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, (Link zu PANGAEA), ensuring that other researchers can use it and integrate it into their studies.”

An expedition with the research vessel Polarstern is planned for next summer, with a focus on investigating the biological and biogeochemical differences between floes and pressure ridges of different ages and provenances. At the same time, there will be extensive aerial survey flights with the research aircraft.

According to Thomas Krumpen: “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.”

Reference: “Smoother sea ice with fewer pressure ridges in a more dynamic Arctic” by Thomas Krumpen, Luisa von Albedyll, H. Jakob Bünger, Giulia Castellani, Jörg Hartmann, Veit Helm, Stefan Hendricks, Nils Hutter, Jack C. Landy, Simeon Lisovski, Christof Lüpkes, Jan Rohde, Mira Suhrhoff and Christian Haas, 6 January 2025, Nature Climate Change.
DOI: 10.1038/s41558-024-02199-5

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