永久凍土の融解が北極圏の河川浸食を加速させる可能性(Permafrost Thaw May Cause Arctic River Erosion to Speed Up)

2024-10-09 カリフォルニア工科大学(Caltech)

＜関連情報＞

• https://www.caltech.edu/about/news/permafrost-thaw-may-cause-arctic-river-erosion-to-speed-up
• https://www.nature.com/articles/s41586-024-07978-w

永久凍土が北極圏の河岸浸食を遅らせる Permafrost slows Arctic riverbank erosion

Emily C. Geyman,Madison M. Douglas,Jean-Philippe Avouac & Michael P. Lamb
Nature  Published:09 October 2024
DOI:https://doi.org/10.1038/s41586-024-07978-w

Abstract

The rate of river migration affects the stability of Arctic infrastructure and communities^1,2 and regulates the fluxes of carbon^3,4, nutrients⁵ and sediment^6,7 to the oceans. However, predicting how the pace of river migration will change in a warming Arctic⁸ has so far been stymied by conflicting observations about whether permafrost⁹ primarily acts to slow^10,11 or accelerate^12,13 river migration. Here we develop new computational methods that enable the detection of riverbank erosion at length scales 5–10 times smaller than the pixel size in satellite imagery, an innovation that unlocks the ability to quantify erosion at the sub-monthly timescales when rivers undergo their largest variations in water temperature and flow. We use these high-frequency observations to constrain the extent to which erosion is limited by the thermal condition of melting the pore ice that cements bank sediment¹⁴, a requirement that will disappear when permafrost thaws, versus the mechanical condition of having sufficient flow to transport the sediment comprising the riverbanks, a condition experienced by all rivers¹⁵. Analysis of high-resolution data from the Koyukuk River, Alaska, shows that the presence of permafrost reduces erosion rates by 47%. Using our observations, we calibrate and validate a numerical model that can be applied to diverse Arctic rivers. The model predicts that full permafrost thaw may lead to a 30–100% increase in the migration rates of Arctic rivers.