Erosion rates of permafrost coasts along the Beaufort Sea accelerated over the past 50 years synchronously with Arctic-wide declines in sea ice extent, suggesting a causal relationship between the two. A fetch-limited wave model driven by sea ice position and local wind data from northern Alaska indicates that the exposure of permafrost bluffs to seawater increased by a factor of 2.5 during 1979–2009. The duration of the open water season expanded from ∼45 days to ∼95 days. Open water expanded more rapidly toward the fall (∼0.92 day yr−1), when sea surface temperatures are cooler, than into the mid-summer (∼0.71 days yr−1).Time-lapse imagery demonstrates the relatively efficient erosive action of a single storm in August. Sea surface temperatures have already decreased significantly by fall, reducing the potential impact of thermal erosion due to fall season storm waves.