Around 73 thousand years ago a supervolcano in Sumatra erupted on a scale unprecedented in the last 2 million years. It left a 100 by 30 km elliptical caldera now occupied by Lake Toba, and explosively ejected 2800 of magma, about 800  km³ falling as ash as far afield as the Greenland ice cap. Although ice-core records show little if any sign of associated climate change in polar regions, the vast amount of ash and sulfate aerosols blasted into the stratosphere must have had some ‘global winter’ effect. Large areas of South Asia were blanketed by thick beds of ash. Human migration from Africa into Eurasia was probably underway at the time, indeed stone tools are found directly beneath and above the Toba ash in southern India and Malaysia. Some palaeoanthropologists have seen the stresses imposed by the Toba eruption as possible means of reducing the entire human population to a mere few thousand: a genetic ‘bottleneck’ that could have led to rapid evolution among surviving generations that may have shaped changes in human behaviour and culture.

There is a widening range of views on the climate changes that may have followed Toba. It has even been suggested that global mean surface temperature fell by as much as 10°C (Robock, A. et al. 2009. Did the Toba volcanic eruption of ∼74 ka B.P. produce widespread glaciation? Journal of Geophysical Research: Atmospheres, v. 114, DOI: 10.1029/2008JD011652), although not so far as to produce a worldwide glacial surge but sufficient to devastate vegetation. This bleak look back to a critical point in human affairs resulted from modeling of the effects of a global reflective cloud of ash and sulfate. A later modeling study factored in particle and aerosol sizes (Timmreck, C. et al. 2010. Aerosol size confines climate response to volcanic super-eruptions. Geophysical Research Letters, v. 37, doi:10.1029/2010GL045464) to give a less dramatic, but still severe maximum global cooling due to Toba of ~3.5°C.

The focus has now shifted from modelling to a more direct look at the environmental effects of the Toba super-eruption, preserved in sediments beneath Lake Malawi in southern Africa (Lane, C.S. et al. 2013. Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka. Proceedings of the National Academy of Science, v. 110, doi/10.1073/pnas.1301474110). The sediments contain a thin ash layer that is very different from those produced by East African Rift volcanism but chemically and texturally similar to the Toba ash from the Indian Ocean and India. The sediments, diatom fossils and chemical biomarkers immediately above the ash show little sign of a significant temperature fall. At most it records a 1.5°C fall, and the authors conclude little chance of a human genetic bottleneck among Africans living at the time.

There is clearly a conflict between results of modeling and real-world climatic data, which is interesting in its own right. But the Malawi findings do not rule out ‘bottlenecks’ resulting from severe stress in South Asia where the ash itself would have severely affected game and vegetation for long enough to face migrating human bands with the prospect of starvation. Obviously, some survived to move on and to leave their tools behind on top of the Toba Ash.

Related articles

• Supervolcano eruptions may not be so deadly after all (newscientist.com)
• Our Past (and Future) With Supervolcanoes (theratchet.ca)
• Volcano catastrophe idea ‘dismissed’ (bbc.co.uk)