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 influence on Earth's climate. Many blame the cataclysmic eruption of the Indonesian volcano Tambora in 1815 for the ensuing "year without a summer" of 1816—when parts of the northeastern 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 David 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 filter out temperature changes caused by the cyclic weather phenomenon known as the El Niño-Southern Oscillation, which warms the sea surface in the equatorial Pacific and thereby warms the atmosphere. Such warming can mask the cooling brought about by an eruption, but it can also mimic volcanic cooling if 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 smaller drop than expected in the average temperature in the hemisphere (Northern or Southern) of the eruption—only half a degree centigrade or less—with 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 the unmelted snow would continue to reflect sunlight away from the surface, amplifying the cooling. The cool air over the region could, in turn, affect the jet stream. The jet stream tends to flow at the boundary between cool northern air and warm southern air, drawing its power from the sharp temperature contrast and the consequent difference in pressure. An unusual cooling in the region could cause the stream to wander farther south than normal, allowing more polar air to come in behind it and deepen the region's cold snap. Through such a series of feedbacks a small temperature drop could be blown up into a year without a summer.