The resulting cooling on Earth's surface can last for months or years.
Not all eruptions have that effect, however. For instance, the continuing eruption of Indonesia's Merapi this fall has killed dozens, but this latest episode is probably not big enough by itself to effect large-scale weather changes, scientists believe.
As for rainfall, in the simplest models, lowered temperatures decrease evaporation of water from the surface to the air. Less water vapor translates to less rain.
But matters are greatly complicated by atmospheric circulation patterns, cyclic changes in temperatures over the oceans, and the shapes of land masses.
Until now, most climate models incorporating changes in the sun and the atmosphere have predicted that volcanic explosions would disrupt the monsoon by bringing less rain to southeast Asia--but the researchers found the opposite.
They studied several eruptions, including one in 1258 from an unknown tropical site, thought to be the largest of the last millennium; the 1600-1601 eruption of Peru's Huaynaputina; Tambora in 1815; the 1883 explosion of Indonesia's Krakatau; Mexico's El Chichn in 1982; and Pinatubo.
Tree rings showed that huge swaths of southern China, Mongolia and surrounding areas consistently dried up in the year or two following big events, while mainland southeast Asia received increased rain.
The researchers believe there are many possible factors involved.
"The data only recently became available to test the models," said Rosanne D'Arrigo, one of the paper's co-authors. "There's a lot of work to be done to understand how all these different forces interact."
In some episodes pinpointed by the study, it appears that strong cycles of the El Nio-Southern Oscillation, which drives temperatures over the Pacific and Indian Oceans and
|Contact: Cheryl Dybas|
National Science Foundation