"We are not saying that it wasn't difficult for humans after Toba," says Mike Petraglia at the
Hominin life appeared to continue in the same vein immediately after the eruption, with hundreds more stone tools in the layers immediately above the ash fall. The team uncovered a similar story 1000 kilometres further north of Jwalapuram, in the Middle Son river valley. "We see very little change in tool technology across the Toba ash. They may have had to relocate for a short period of time, but within a generation or so they were back where they were before, making the same kinds of stone tools," says Chris Clarkson, a stone-tool specialist from the University of Queensland in Brisbane, Australia, who worked at the digs in India.
Volcanic winter at wikipedia. Volcanic winter was predicted with the same climate models used for nuclear winter predictions
The new work portrays a somewhat different view of the eruption from the most popular current theory. Previous computer models of the eruption had suggested the event was truly cataclysmic - very nearly a doomsday for early humankind. With calculations based on the assumption that Toba belched out 100 times more aerosols than the 1991 eruption of mount Pinatubo in the Philippines, and scaling the environmental effects accordingly, the models suggested global temperatures dropped by about 10 °C following the blast. This supports the idea of a decade-long "volcanic winter" and widespread catastrophe
Yet this theory has drawn some criticism since it was first put forward 17 years ago, with scholars such as Hans Graf, an atmospheric scientist at the University of Cambridge, believing that the climate change following the explosion has been wildly overestimated.
For Graf, the crux of the argument concerns the precise cooling effect of the sulphur dioxide released by the explosion. During smaller eruptions, like that of mount Pinatubo, most of the released sulphur dioxide reacts with hydroxide (OH) ions from water molecules in the atmosphere to form particles of sulphate - a highly reflective substance that bounces sunlight back into space before it can warm the Earth.
Previous estimates had placed Toba's sulphur dioxide production at 100 times that of mount Pinatubo's output. Graf thinks this figure is misguided: recent chemical analyses of Toba's fossilised magma suggests it should be roughly half that. "We think Toba was more of a giant in ash production, not sulphur," says Graf.
What's more, he says, the atmospheric effect of a super-eruption is incomparable to a smaller, typical eruption. Whereas most of the sulphur dioxide from mount Pinatubo would have been rapidly converted to sulphate particles, there simply would not have been enough OH ions in the atmosphere available to react with all the sulphur dioxide released by Toba, delaying the formation of these reflective particles. Even those particles that had formed would have probably clumped together and settled to the ground rather than stay in the atmosphere.
Taking all of this into account, Graf and his colleagues suggest a new estimate of global cooling of just 2.5 °C, which lasted for just a few years. According to this model, the effects were also highly regional. In places like
This new view is highly contentious. Alan Robock from Rutgers University in New Brunswick, New Jersey, who came up with the original simulations, stands by his original predictions. "Our model showed that extra water would be lofted into the stratosphere because of warming at the top of the troposphere [the lowest layer in the atmosphere], so water would not be a limiting factor," he says. "We simulate a decade or two of very cold, dry, dark conditions, which would have been difficult for humans to adapt to."
Yet recent archaeological and geological work in
To build further evidence, Ditchfield analysed the ratio of different carbon isotopes - which are each absorbed at different rates by different plants - in ancient plant remains in the Jwalapuram region of southern India and the Middle Son river valley in central northern India, both of which are around 2000 kilometres from Toba. He saw only a slight increase in the carbon-13 isotope after the Toba eruption, which suggests there was just a small increase in grassland environments at this time. "Woodlands weren't obliterated by Toba. We see a diverse range of habitats persisting after the eruption, which would have provided a diverse range of game and hunting opportunities," he says.
Nevertheless, hominin species living at the time of the eruption would undoubtedly have faced tough conditions. The blanket of ash, for example, would have been quickly washed into the freshwater supplies: Ditchfield found deposits up to 3 metres deep on the valley floors where rivers would once have flowed. And there is no doubt that in the years immediately following the eruption the early humans would have had to adjust to colder temperatures, probably having to economise significantly as food resources dwindled.