In the Arctic, the old multiyear ice is melting at an alarming rate, leading to a marked decrease in both the number and size of pressure ridges. These ridges form when ice floes collide and stack upon one another, which is a distinctive characteristic of Arctic sea ice. While they pose challenges for shipping, they also play a vital role in the ecosystem. A new study published in the journal Nature Climate Change by researchers from the Alfred Wegener Institute details this trend and examines observational data collected over three decades from aerial surveys.
Data captured by satellites over the last thirty years reveals significant alterations in Arctic sea ice driven by climate change: the ice-covered area during summer is consistently shrinking, the ice floes are becoming thinner, and they are moving at increased speeds. Until recently, it was uncertain how pressure ridges were being impacted since reliable monitoring from space has only been possible in recent years.
Pressure ridges arise from lateral pressures acting on sea ice. Wind or ocean currents can pile up ice floes, forming ridges that can be several meters thick. The portion of the ridges that rises above the water, known as the sail, typically measures between one and two meters. Beneath the water, the keel can reach depths of up to 30 meters, posing a significant barrier for shipping. Pressure ridges are crucial as they influence the energy and mass balance of the sea ice, as well as the biogeochemical cycles and the ecosystem. When the sails catch the wind, they can drive the ice floes across the Arctic. Polar bears utilize pressure ridges for protection while they hibernate or give birth to their young. Moreover, these structures provide sanctuary for a variety of ice-associated organisms and enhance the mixing of water, promoting greater nutrient availability.
A team from the Alfred Wegener Institute and Helmholtz Centre for Polar and Marine Research (AWI) has now reanalyzed and assessed laser-generated data collected over thirty years of research flights across the Arctic ice. This survey, covering around 76,000 kilometers, reveals for the first time that the number of pressure ridges north of Greenland and in the Fram Strait is decreasing by 12.2%, with their height dropping by 5% each decade. In the Lincoln Sea, an area known for accumulating particularly old ice, the decline is even more pronounced, with a reduction of 14.9% in frequency and 10.4% in height per decade.
“It has been unclear until now how pressure ridges were changing,” explains Dr. Thomas Krumpen, a sea-ice specialist at AWI and the lead author of the study. “An increasing portion of the Arctic consists of ice that melts during the summer and is young, being less than a year old. This young, thinner ice is more susceptible to deformation, leading to the quicker formation of new pressure ridges. One might anticipate that their frequency would rise. However, the decline in pressure ridges is attributable to the significant melting of older ice. Ice that endures multiple summers typically exhibits a larger number of pressure ridges due to prolonged exposure to high pressures. The loss of this multiyear ice is so drastic that we are witnessing an overall decrease in pressure ridge frequency, despite the fact that the thin, young ice is easier to deform.”
To draw conclusions on Arctic-wide changes, the researchers compiled all observational data to create a metric. Then they used satellite data to apply this metric across the Arctic: “The greatest reductions in pressure ridges are seen in regions where the ice’s age has diminished the most,” summarizes Prof. Christian Haas, Head of Sea-ice Physics at AWI. “Notably, significant changes have occurred in the Beaufort Sea and the Central Arctic, areas that were once predominantly ice-covered with older ice, which are now partially ice-free during the summer months.”
During the study, researchers conducted detailed measurements and analyses of individual pressure ridges and their heights during survey flights. This was feasible due to low-level flights (less than 100 meters above the surface) and the rapid scanning capability of laser sensors, allowing for the creation of terrain models. The AWI commenced scientific expeditions over the sea ice in the early 1990s from Svalbard. Initially, two Dornier DO228 aircraft, Polar 2 and Polar 4, were used, which have now been replaced with two Basler BT-67s, Polar 5 and Polar 6. These aircraft are specially tailored for the extreme conditions found in polar regions and can be equipped with various sensors. Researchers survey the ice north of Greenland, Svalbard, and Canada biannually, and the onboard helicopters of the icebreaker Polarstern also contribute to the monitoring program.
To understand the direct implications of these observed changes on the Arctic ecosystem, models need to be created that can represent both the physical and biological processes in ice of varying ages. While it is known that pressure ridges host a range of organisms, there remains limited understanding of the significance of their age. This is particularly crucial as the proportion of ridges that do not survive their first summer is increasing. Another puzzle: although the size and frequency of ridge sails have decreased, the overall drift speed of Arctic ice has increased. Dr. Luisa von Albedyll, a sea-ice physicist at AWI who contributed to the study, explains: “Typically, you would expect the ice to drift more slowly as the sails shrink since there’s reduced area for momentum transfer. This suggests other changes are occurring that are producing the opposite effect. Potential factors could include stronger ocean currents or a more streamlined underside of the ice due to more extensive melting. To address these unanswered questions and gain a deeper understanding of the complex relationships at play, we’ve made the entire dataset available in a public archive, (Link to PANGAEA), allowing other researchers to access and integrate it into their studies.”
An expedition aboard the research vessel Polarstern is planned for next summer, focusing on examining the biological and biogeochemical differences among floes and pressure ridges of various ages and origins. Additionally, extensive aerial survey flights with research aircraft will be conducted. According to Thomas Krumpen: “By combining insights gained from ship-based and aerial observations, we aim to deepen our understanding of the intricate relationships between sea ice, climate, and the ecosystem. Only by comprehending the region’s environmental system better can we formulate effective strategies for preserving and sustainably utilizing the Arctic.”
