Reading between the lines of tree rings to reconstruct exactly what happened in Alaska the year that the Laki volcano erupted half a world away in Iceland can help fine-tune future climate predictions. In June 1783, Laki spewed more sulfur into the atmosphere than any other Northern Hemisphere eruption in the last 1,000 years. The Inuit in North America tell stories about the year that summer never arrived. Benjamin Franklin, who was in France at the time, noted the “fog” that descended over much of Europe in the aftermath, and correctly reasoned that it led to an unusually cold winter on...