Intra‐Annual Climate Anomalies in Northwestern North America Following the 1783–1784 CE Laki Eruption

The 1783–1784 CE Laki eruption in Iceland was one of the largest, terms mass SO2 emitted, high-latitude eruptions last millennium, but seasonal and regional climate response heterogeneous space time. Although did not begin until early June, tree-ring maximum latewood density (MXD) reconstructions from Alaska suggest that entire 1783 summer extraordinarily cold. We use high-resolution quantitative wood anatomy, model simulations, proxy systems modeling to resolve intra-annual effects on temperatures over northwestern North America. measured anatomical characteristics white spruce (Picea glauca) trees two northern sites. Earlywood cell ring are normal, while wall thickness is significantly anomalously reduced compared non-eruption years. Combined with complementary evidence experiments modeling, these features indicate an abrupt premature cessation thickening due a rapid temperature decrease toward end growing season. Reconstructions using conventional annual resolution MXD likely over-estimate total season cooling this year, width fails capture late-summer volcanic signal. Our study has implications only for interpretation climatic impacts America, more broadly demonstrates importance timing internal variability when comparing simulations. It further value developing cellular-scale measurements paleoclimatology.