Volcanic Blast's Devastation Confirmed by Pollen

It takes a heck of a disaster to wipe the trees off of India. But 73,000 years ago, the titanic eruption of Toba in Indonesia did exactly that, according to a new study, brushing the region clean almost overnight as it kicked the planet into a cold snap that would persist for almost 2,000 years.

Toba's eruption may be the most important volcanic event in human history, whittling our ancestors in Africa down to a population of perhaps just 30,000 survivors. But scientists debate whether the catastrophe was as bad as all that, with some arguing that there was only a small blip in climate at the time.

Now a new study led by Martin Williams of the University of Adelaide in Australia proves the environment was thrown into chaos, at least in India.

Pollen and soil samples collected from the Bay of Bengal and across central India depict a moist tree-covered land devastated by the eruption. Nourishing monsoon rains were shut off and a cold, dry spell gave rise to a grassy savanna.

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The new work, published recently in the journal Paleogeography, Paleoclimatology, Paleoecology is the first direct evidence of an ecosystem decimated by the eruption. Previous studies have shown that Toba's ash extends through much of India and the Indian Ocean, and that sulfur from the eruption accumulated in large quantities on the Greenland ice sheet.

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For co-author Stanley Ambrose of the University of Illinois at Urbana-Champaign, the study represents the latest in a long line of evidence that argues Toba was indeed a massively destructive event.

"Climate models estimate that temperatures dropped 16 degrees Celsius (29 degrees Fahrenheit) for 50 years in Africa," he said. "Today the average temperature in Nairobi is 25 degrees Celsius (77 Fahrenheit). Just imagine what that would be like."

But Michael Petraglia of the University of Oxford thinks the researchers have overstated their case.

"Two of their three samples in India come from the same river valley," he said. "What they're sampling is a local situation, and they're making a huge leap of interpretation that all of southern Asia was like this."

Petraglia added that his own research suggests early humans were already in India at the time, and survived the eruptions.

"I'm not denying the Toba super-eruption had ecological effects; I think it did. But the interpretations they're making about human evolution and genetics are tenuous and weak. The linkages between the eruption and genetics have not been robustly proven yet," he said.