Volcanoes

Long after the lava has cooled, the effects of a major volcanic eruption may linger on. In the atmosphere a veil of fine dust and sulfuric acid droplets can spread around the globe and persist for years. Researchers have generally thought that this veil can block enough sunlight to have a chilling United States and southeastern Canada were hit by snowstorms in June and frosts in August.

The volcano-climate connection seems plausible, but, say scientists Clifford Mass and Davit Portman, it is not as strong as previously believed. Mass and Portman analyzed global temperature data for the years before and after nine volcanic eruptions, from Krakatau in 1883 to El Chichón in 1982. In the process they tried to the volcano happens to erupt just as an El Niño-induced warm period is beginning to fade.

Once El Niño effects had been subtracted from the data, the actual effects of the eruptions came through more clearly. Contrary to what earlier studies had suggested, Mass and Portman found that minor eruptions have no discernible effect on temperature. And major, dust-spitting explosions, such as Krakatau or El Chichón, cause a half a degree centigrade or less-a correspondingly smaller drop in the opposite hemisphere.

Other researchers, however, have argued that even a small temperature drop could result in a significant regional fluctuation in climate if its effects were amplified by climatic feedback loops. For example, a small temperature drop in the northeastern U.S. and southeastern Canada in early spring might delay the melting of snow, and of feedbacks a small temperature drop could be blown up into a year without a summer.