Erosion of Icy Coastlines in the Face of Changing Sea Ice

Across the Arctic, over the satellite record, the open-water season has increased in duration, and sea-ice extent in all months is decreasing. This represents a regime shift for Arctic coasts, as sea ice provides a first-order control on the physical vulnerability of the coastline to erosion, inundation, and damage to settlements and infrastructure by ocean water. A pan-Arctic analysis of satellite-based sea-ice concentration along the coast indicates that the median length of the 2012 open-water season has expanded by between 1.5 and 3-fold relative to 1979. There is no simple connection between existing observations of coastal erosion and sea ice change; longer open water seasons permit greater erosion rates, but specific locations are heavily influenced by the details of geology, geomorphology and storm tracks. Along a central portion of Alaska’s Beaufort Sea coast, characterized by ice rich permafrost, erosion rates increased from 6.8 to 19 m/yr between 1955 and 2012 while the sea ice-free season increased from 45 to 100 days since 1979. I develop a numerical model of bluff retreat to assess the relative roles of the length of sea ice-free season, sea level, water temperature, nearshore wavefield, and permafrost temperature in controlling erosion rates in this setting. Erosion is highly episodic, as high water levels are a prerequisite for erosion. The certain increase in sea level, expansion of the open water season, and potential changes in storminess will likely increase future coastal erosion rates. Looking towards the future, with no efforts to control greenhouse gas emissions, Arctic sea ice will continue to decline. Using a climate model ensemble that captures the observed sea ice change and constrains internal variability, I examine the predicted open water season over the 21st century. I identify the year when each point starts and finishes its transition out of the locally defined pre-industrial open water regime. The majority of the Arctic nearshore area started to shift in 1990, will start to emerge from the noise of internal variability in 2040 and, will be covered by ice for only half of the year by 2070. This change will usher in a fundamentally different Arctic coastal system