Thermal Erosion of an Arctic Coastline: Field Observations and Model Calibration

Coastal erosion rates locally exceeding 30 meters per year have been documented along portions of Alaska's Beaufort Sea coastline, and a number of studies suggest that these erosion rates have accelerated as a result of climate change. However, a lack of direct observational evidence has limited our progress in quantifying the role of climate change on coastal erosion rates in the Arctic. In particular, while longer ice-free periods are likely to lead to both warmer surface waters and longer fetch, the relative roles of thermal and mechanical (wave) erosion in driving coastal retreat have not been comprehensively quantified. We focus on the potential magnitude of thermal erosion along a permafrost coastline in the northern National Petroleum Reserve-Alaska (NPR-A), where erosion rates have averaged 10-15 meters/year over two years of direct monitoring. We take advantage of these extraordinary rates of coastal erosion in the Arctic to observe coastal erosion directly via time-lapse photography, and to use these observations to calibrate simple models of thermal erosion. Our observations suggest that virtually all of the erosion in this setting can be explained as a thermal process: the high ice content and the fine grain size of the bluff materials provide substantial mechanical strength, but limited resistance to thermal erosion. Furthermore, the fine-grained materials disaggregated from the bluffs can be easily transported away in suspension, which limits any strong negative feedback on erosion rates. The observation that coastal erosion is driven primarily by thermal processes in this coastal zone implicates a direct relationship between climatic warming and landscape change in this setting.