In 1600 eruption of Huaynaputina in Peru put a large amount of sulfur into the atmosphere. Sulfur reacts with water in the air to form droplets of sulfuric acid, which cool the planet by reducing the amount of sunlight reaching the Earth's surface.
The droplets soon fall back to Earth, so the cooling effects lasted only a year or so, but the global impact on human society was much greater, according to a new study of contemporary records by geologists at UC Davis.
o one had looked at the agricultural and social impacts, said Ken Verosub, professor of geology at UC Davis. "We knew it was a big eruption, we knew it was a cold year, and that's all we knew."