Research Briefs

The Toba mega-eruption, global cooling, and human evolution

A satellite image.

Mt. Toba, located on the island of Sumatra in Indonesia, produced the largest volcanic eruption of the last two million years. The caldera, shown here, measures 30 by 100 kilometers (18 by 60 miles) and is 1,700 meters (5,577 feet) tall. The caldera probably formed in stages, with large eruptions occurring about 840,000, 700,000, and 75,000 years ago. (Image courtesy NASA and the U.S./Japan ASTER Science Team.)

NCAR researchers are studying whether the eruption of Indonesia’s Mt. Toba supervolcano about 70,000–75,000 years ago may have cooled Earth enough to initiate an ice age and potentially alter the course of human evolution.

Genetic evidence suggests that all modern humans evolved from a few thousand individuals of the species homo sapiens just several tens of thousands of years ago, whereas fossil evidence suggests that the roots of the homo sapiens lineage are much broader and older. If only a small number of humans survived a catastrophic event such as the Toba eruption, both scenarios could have occurred, with the eruption reducing human population enough to create a genetic bottleneck in our evolution.

Volcanic eruptions are known to produce global cooling when their masses of sulfur dioxide and other gases reach the stratosphere, where they efficiently reflect the Sun’s rays into space, thereby cooling Earth. Energy released during the Toba eruption was about three thousand times greater than during the 1980 eruption of Mt. St. Helens, with similarly larger amounts of sulfur.

Working with a team of scientists, Caspar Ammann and Sam Levis used NCAR’s Community Climate System Model (CCSM) with a coupled dynamic vegetation model to simulate the effects of the Toba eruption. Colleagues at NASA and in the university community used a different model to look specifically at stratospheric chemistry following Toba.

Results from the combined study, published in the Journal of Geophysical Research, show that the Toba eruption would likely not have initiated an ice age. However, it could have been strong enough to trigger a volcanic winter at least a decade long, affecting plant and animal life seriously enough to contribute to a genetic bottleneck in human evolution.

“While our results show that indeed the eruption could have produced great stress on humans and their environment, the effect would have been quite concentrated in the few very dark, cold, and dry years immediately following the eruption,” Caspar says.