Did an Ancient Volcano Freeze Earth?

Remnant. Toba today comprises a caldera lake, a newly arising cone (central island), and a pip-squeak of a volcanic progeny named Pusukbukit (left).


One fine day about 74,000 years ago, a giant volcano on Sumatra blew its top. The volcano, named Toba, may have ejected 1000 times more rock and other material than Mount St. Helens in Washington state did in 1980. In the process, it cooled the climate by at least 10°C, causing a global famine. But could the aftermath have been even worse? A new study puts to rest questions about whether Toba plunged Earth into a 1000-year deep freeze and whether an equivalent event today could jump-start a new, millennia-long ice age.

Giant volcanic eruptions such as Toba briefly cause the opposite of global warming. Although eruptions do emit greenhouse gases such as carbon dioxide, volcanoes also spew sulfur dioxide. Combined with water vapor, sulfur dioxide forms sulfate aerosols, which can spread around the globe, blocking solar radiation and chilling the air before becoming acid rain and snow.

Paleoclimate evidence suggests that the Toba eruption, which occurred during the last ice age, emitted lots of sulfur dioxide--vastly more than Mount St. Helens did. The eruption also seems to have coincided with the start of a 1000-year period of even colder temperatures. Some scientists have suggested that Toba caused the deep freeze and that perhaps such an event happening today could bring on a new ice age. But models developed by NASA and the National Center for Atmospheric Research in Boulder, Colorado, argue otherwise.

Researchers led by climatologist Alan Robock of Rutgers University in New Brunswick, New Jersey, ran scenarios that featured eruptions producing up to several times more sulfur dioxide than Toba. The result, published 27 May in the Journal of Geophysical Research—Atmospheres, was a cooler climate that lasted only a few decades. So the 1000-year cold spell was probably part of the natural cycle that has produced more than a dozen ice ages over the past couple of million years.

"The results virtually eliminate mega volcanic eruptions as one of the key drivers of global-scale glaciation," says climatologist Ellen Mosley-Thompson of Ohio State University in Columbus, who was not involved in the study. So, paleoclimatologists should focus on more likely climate coolers, she says, such as changes in ocean circulation or cyclical variations in Earth's orbit around the sun.

Still, if Toba erupted today like it did in the past, the results would be catastrophic. Although the volcano isn't expected to blow its top for thousands of years, Robock and colleagues estimate a megaeruption could lower global temperatures by as much as 17°C for several years, followed by a recovery to normal conditions that could take decades. That would hit the human population with the double whammy of dramatically reduced agricultural production and widespread loss of vegetation, leading to widespread food shortages and starvation.

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