Supervolcano eruptions may not be so deadly after all

It was the biggest bang in human history. Around 75,000 years ago, the Toba supervolcano exploded on the Indonesian island of Sumatra, blasting enormous volumes of gas and ash into the air. Yet a new analysis suggests it had little impact on the climate, or on humans. So could such vast eruptions be survivable?

Supervolcanoes are capable of releasing more than 1000 cubic kilometres of material in one eruption, enough to cover an entire continent with ash. There are only six supervolcanoes on Earth, the most famous being Yellowstone. Toba was the last to erupt.

Toba’s eruption produced vast quantities of sulphur dioxide, a gas that behaves in the opposite way to a greenhouse gas – it cools Earth by increasing the atmosphere’s ability to reflect the sun’s rays back into space. Archaeologists and volcanologists have suggested that this triggered a 1000-year volcanic winter that wiped out a significant proportion of the planet’s people and plants. But this idea has proved controversial, and the latest evidence makes it look even less likely.

Chemical clues

Christine Lane of the University of Oxford and her colleagues were looking for clues to past climate change in the sediments at the bottom of Lake Malawi when they came across a layer of ash from the Toba eruption. The team was able to relate the depth of each sediment layer with the climate at the time when it was laid down by looking at chemical traces left by microbes, which adapt their structure to the climate.

By studying these chemical signatures, Lane’s team was able to estimate that Toba’s eruption caused a cooling of about 1.5 °C over a period of 20 to 30 years. “There doesn’t seem to be any significant impact on the climate in east Africa,” Lane says. “I would infer the global climate effects were insignificant. There shouldn’t have been any effect on humans.”

Geophysicist Alan Robock of Rutgers University in New Brunswick, New Jersey, isn’t convinced.

Robock studies volcanoes’ effect on climate, and agrees Toba probably didn’t cause a 1000-year chill. But the short-term cooling would still have been devastating, he says, and might have been too brief to show up in Lane’s temperature record.

Robock has developed his own model predicting the impact of climate changes on plant life. He reckons that short-term cooling could have had a devastating impact on plants worldwide (Journal of Geophysical Research: Atmospheres, doi.org/cx2fj5). “In our model, all the trees died and there was a huge impact on vegetation.”

Muted cooling

Lane agrees that there might have been a short-term period of severe cooling, but because more dramatic dips have occurred without affecting humans, the shift was unexceptional.

Hans-F Graf of the University of Cambridge, who has also modelled the effects of supervolcanoes on climate, agrees. According to Graf’s model, the vast sulphur emissions created sulphate particles that grew large and so reflected less light, reducing the cooling effect (Geophysical Research Letters, doi.org/cpk3fm).

If Graf and Lane are right, a supervolcano eruption might not cool the global climate much. But that doesn’t make it harmless. Anyone within 100 kilometres of the blast would almost surely be killed by a vast quilt of hot ash and gas smothering the landscape. The key point affecting species’ survival would be where this cloud headed. The Indonesian “hobbits” (Homo floresiensis) only survived Toba because the ash cloud travelled west – away from their home island of Flores.

And even if the climate did not change much, there would be global consequences if a supervolcano erupted today. After all, the minor eruption of Iceland’s Eyjafjallajökull in 2010 was enough to disrupt air transport for weeks. “If Yellowstone went up, US industry would be very strongly harmed,” says Graf.

Journal reference: PNAS, DOI: 10.1073/pnas.1301474110