Global warming is causing the permafrost in the Arctic to melt. A study now shows how new river systems have emerged over the last 60 years, further accelerating the process. Based on field studies and historical aerial photographs, the researchers reconstructed the landscape development of a valley on Canada's Axel Heiberg Island. Physical models shed light on the complex interaction of various influencing factors. The results suggest that melting will continue to accelerate in the future - with serious impacts on local ecosystems and the global climate.
Permafrost soils are one of the most important carbon sinks on earth. While the upper layers thaw year after year in the summer months, the lower layers usually remain frozen and store the carbon they contain as well as numerous nutrients and minerals, sometimes for thousands of years. However, due to global warming, the permafrost continues to thaw. This process is intensified by the fact that rivers of meltwater dig into deeper layers of soil and remove parts of the soil in the process. However, exactly how these rivers arise and how they influence the landscape was previously unclear.
Networks of ice wedges
A team led by Shawn Chartrand from Simon Fraser University in Canada has now examined the formation and effects of these rivers in more detail. The team focused on an approximately eight-kilometer-long valley on Axel Heiberg Island in the Canadian Arctic. The seasonal thawing and freezing of the upper soil layers has led to a pattern typical of permafrost soils: deep cracks form in the ground in winter, which fill with meltwater in spring. When this freezes again, ice wedges are formed, which over time form networks. This creates a polygonal pattern, the so-called polygon fields.
“Our results show that the polygon fields influence how surface water runoff is channeled through the landscape,” the team reports. “They contribute to the creation of river systems that are changing the landscape due to global warming.” For the study, Chartrand and his team evaluated historical aerial photographs of the region and combined them with their own field studies and physical modeling. “The time series of aerial photographs since 1959 shows that polygon fields associated with freeze and thaw can form relatively quickly within a few decades,” they report.
Complex interaction of influencing factors
The formation of river systems follows different rules than in areas without permafrost. While rivers usually form depending on how easily the soil erodes and how strong the water flows are, in areas with permafrost other factors are important: “What is important is the timing, extent and duration of flood events and the question of whether the underlying sediment particles are completely or only partially frozen,” explains Chartrand. “By modeling water movement across the landscape, we found that floodwaters channeled through interconnected polygonal valleys increase the likelihood of erosion and channel formation.”
Depending on how much meltwater is produced in a year and at what point in the year it flows through the valley, the ice wedges of the polygon fields melt, creating river networks. This increases the area for heat exchange, which promotes further thawing and deepening of the riverbeds. “These cascading effects may increase the release of greenhouse gases in the Arctic as soil organic carbon thaws and permafrost retreats,” says co-author Mark Jellinek from the University of British Columbia in Vancouver.
Impact on local and global levels
The team's models show that higher air temperatures also play a role. “We predict that erosion and sediment transport depend on whether the floods occur before or after a period of increased air temperatures, as this affects the depth to which sediment particles are thawed and therefore whether the particles are transported by the flood waters “explains Chartrand.
In a commentary accompanying the study, also published in the journal Nature Communications, Joel Rowland of the Los Alamos National Laboratory in New Mexico points out that further studies are needed to understand more precisely how seasonal temperature fluctuations and global warming affect Influencing the landscape in permafrost areas. “Integrated models will enable predictions of how increasing erosion of permafrost will alter the release of sediment, carbon and nutrients in the Arctic,” he writes. This is relevant not only for the global climate, but also for local ecosystems. Increased nutrient inputs into fishing areas could have a significant impact on wildlife and thus also have an impact on the population of coastal areas.
Source: Shawn Chartrand (Simon Fraser University, Burnaby, Canada) et al., Nature Communications, doi: 10.1038/s41467-023-40795-9