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Blast zone. Toba's massive eruption 74,000 years ago left behind this crater, now a lake, in Indonesia (inset).

NASA; (inset) Thinkstock

Giant Eruption Cut Down to Size

More than 2000 times as massive as the blast that ripped open Mount St. Helens in 1980, the Indonesian "super-volcano" Toba ejected millions of metric tons of volcanic ash, sulfur, and other debris into the atmosphere 74,000 years ago. The eruption darkened the skies, cooled the globe by 10˚C for half a decade, and redirected the course of human evolution. At least that's what some climatologists and archeologists have concluded. But a new model indicates that Toba's climate effects were milder and abated quickly, suggesting that humans may have made it through the incident relatively unscathed.

Scientists hotly debate the climate impacts of Toba's eruption. Although a handful of recent studies have suggested that the blast locked the world in a deep freeze, a 1992 estimate indicated a milder climate impact. Based on scaling up smaller eruptions like the 1991 eruption of Mount Pinatubo in the Philippines, volcanologist Stephen Self of The Open University in Milton Keynes, U.K., and paleobiologist Michael Rampino of New York University in New York City put the cooling at only 3˚ to 5˚C.

The impact on humans is also controversial. A volcanic winter could have decimated human populations, especially in the Toba region, contributing to a genetic "bottleneck," where only the survivors' genes got passed on, as some researchers have reported. But based on archeological sites in India, anthropologists led by Michael Petraglia of the University of Oxford in the United Kingdom have suggested that humans living close to the blast zone would have survived the eruption handily.

To address these uncertainties, researchers led by climate modeler Claudia Timmreck of the Max-Planck Institute for Meteorology in Hamburg, Germany, created a climate model that they believe more realistically mimics the impact of the Toba eruption. They focused especially on the way the blast's sulfate aerosol particles—formed in the stratosphere from erupted gaseous sulfur dioxide—cooled the atmosphere by reflecting the sun's rays. Based on ice core data, and to match previous models' assumptions, they assumed Toba pumped out about 850 million metric tons of sulfur into the atmosphere—100 times the amount released by Mount Pinatubo.

The simulation revealed that Toba's impact was not as extreme as some scientists believed. Temperatures dipped only 3˚ to 5˚C across the globe, for example. The model also showed that the high concentrations of sulfur particles were short-lived; they settled out of the stratosphere—where they can have the largest cooling effect—within 2 to 3 years, the team reports online this month in Geophysical Research Letters. Extreme temperature changes in Africa and India lasted only a year or two, with a temperature decrease of at most 10˚C in the first year after the eruption, followed by 5˚C the second year. Overall, Toba didn't wipe out flora and fauna, Timmreck says, but it would have made life harder for a few years.

Self says that Timmreck and colleagues have provided the first calculations using realistic aerosol chemistry for super-volcanoes. Still, he says, it's unclear just how much sulfur Toba released. If the team's model is off, the impact of the volcano could have been much different.

Anthropologist Stanley Ambrose of the University of Illinois, Urbana-Champaign, who originally proposed the evolutionary bottleneck for modern humans, says the team's model has some flaws. He notes, for example, that the researchers started with modern climate conditions instead of simulating conditions from 74,000 years ago. Ambrose also cites Greenland ice core data that suggest sulfur stuck around in the atmosphere longer than just a few years and that Earth had already entered a cold snap. Toba's eruption could have chilled an already cooling world: "In reality, it was the worst, most prolonged, unrelentingly cold [time]" that modern humans would have had to survive, he says.

Petraglia agrees that Toba probably made a bad situation worse, but he says the impacts may not have been so serious for humans. "Populations survived, but it could have been environmentally nasty for a few years." The next step, he says, is to blend the simulations with actual field observations about Toba's environmental impacts.