In the Arctic, the old, multiyear ice is increasingly melting, dramatically reducing the frequency and size of pressure ridges. These ridges are created when ice floes press against each other and become stacked, and are a characteristic feature of Arctic sea ice, an obstacle for shipping, but also an essential component of the ecosystem. In a recently released study in the journal Nature Climate Change, experts from the Alfred Wegener Institute report on this trend and analyse observational data from three decades of aerial surveys.
Satellite data from the last three decades documents the dramatic changes in Arctic sea ice due to climate change: the area covered in ice in summer is declining steadily, the floes are becoming thinner and moving faster. Until recently, it was unclear how the characteristic pressure ridges had been affected, since it's only been possible to reliably monitor them from space for the past few years.
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. In addition, the structures offer ice-associated organisms at various trophic levels protection and promote the turbulent mixing of water, which increases nutrient availability.
A team of researchers from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), has now reprocessed and analysed 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 kilometres, 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.
"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 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."
For the study, individual pressure ridges and their heights were precisely measured and analysed during survey flights. This was possible thanks to the low-level flights (less than 100 metres 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 programme.
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."
