Radiocarbon ages and lichen-dated moraines from 17 glaciers in coastal and near- coastal British Columbia and Alaska document a widespread glacier advance during the first millennium A.D. Glaciers at several sites began advancing ca. A.D. 200–300 based on radiocarbon-dated overridden forests. The advance is centered on A.D. 400–700, when glaciers along an ∼2000 km transect of the Pacific North American cordillera overrode forests, impounded lakes, and deposited moraines. The synchroneity of this glacier advance and inferred cooling over a large area suggest a regional climate forcing and, together with other proxy evidence for late Holocene environmental change during the Medieval Warm Period and Little Ice Age, provide support for millennial-scale climate variability in the North Pacific region.

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