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 , 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鈥檚 eruption produced vast quantities of sulphur dioxide, a gas that behaves in the opposite way to a greenhouse gas 鈥� it cools Earth by increasing the atmosphere鈥檚 ability to reflect the sun鈥檚 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鈥檚 people and plants. But this idea has proved controversial, and the latest evidence makes it look even less likely.
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Chemical clues
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鈥檚 team was able to estimate that Toba鈥檚 eruption caused a cooling of about 1.5聽掳C over a period of 20 to 30 years. 鈥淭here doesn鈥檛 seem to be any significant impact on the climate in east Africa,鈥� Lane says. 鈥淚 would infer the global climate effects were insignificant. There shouldn鈥檛 have been any effect on humans.鈥�
Geophysicist of Rutgers University in New Brunswick, New Jersey, isn鈥檛 convinced.
Robock studies volcanoes鈥� effect on climate, and agrees Toba probably didn鈥檛 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鈥檚 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 (, doi.org/cx2fj5). 鈥淚n our model, all the trees died and there was a huge impact on vegetation.鈥�
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.
of the University of Cambridge, who has also modelled the effects of supervolcanoes on climate, agrees. According to Graf鈥檚 model, the vast sulphur emissions created sulphate particles that grew large and so reflected less light, reducing the cooling effect (, doi.org/cpk3fm).
If Graf and Lane are right, a supervolcano eruption might not cool the global climate much. But that doesn鈥檛 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鈥� survival would be where this cloud headed. The Indonesian 鈥渉obbits鈥� (Homo floresiensis) only survived Toba because the ash cloud travelled west 鈥� 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鈥檚 Eyjafjallaj枚kull in 2010 was enough to disrupt air transport for weeks. 鈥淚f Yellowstone went up, US industry would be very strongly harmed,鈥� says Graf.
Journal reference: , DOI: 10.1073/pnas.1301474110
