Exploding into an ice age.

Scientists have considered it a coincidence that the largest explosive volcanic eruption of the past 2 million years occurred near the beginning of the last ice age. But a new computer study suggests this mammoth eruption about 73,500 years ago may have accelerated Earth's cooling.

The eruption of Toba on the Indonesian island of Sumatra lofted more than 1 billion tons of volcanic ash and sulfur gases 27 to 37 kilometers into the atmosphere, say Michael R. Rampino of New York University and Stephen Self of the University of Hawaii in Honolulu. This debris could have created a cloud that reduced the sunlight striking Earth's surface. To estimate the impact of the cloud on global climate, the researchers extrapolated from the known effects of smaller, more recent eruptions such as the 1815 Tambora eruption. Rampino and Self calculate that Toba's cloud could have caused average global temperatures to drop by 3[degrees]C to 5[degrees]C for two to three years.

Supposedly, airborne volcanics dissipate too quickly to start an ice age. But Rampino argues that eruptions may speed global cooling if glaciation is under way. Toba's "volcanic winter" could have induced additional cooling and "the extra 'kick' that caused the climate system to switch from warm to cold states," the researchers propose in the Sept. 3 NATURE.

Rampino and Self also suggest that Toba may exemplify a more general "feedback" relationship between volcanic activity and ice ages: The ice age then under way may have set off Toba's eruption by lowering sea levels, relieving pressure on the volcano. The ensuing eruption, in turn, hastened the cooling. They plan more modeling studies of volcanic debris in the atmosphere during global cooling, Rampino says.

V. Ramaswany of Princeton University, in a commentary accompanying the report, urges a search of paleoclimatic records for any link between huge eruptions and past ice ages.