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