A vast volcanic eruption in Indonesia about 74,000 years ago blasted hundreds of cubic miles of ash and lava into the atmosphere in probably the largest eruption in human history.
The eruption of the Toba volcano probably chilled the planet – but early humans were shielded from the worst effects, a new study has concluded.
Researchers have previously not understood why humans weathered the crisis from the largest volcanic eruption in the past 2 million years.
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Lead author Benjamin Black said: "We were able to use a large number of climate model simulations to resolve what seemed like a paradox.
"We know this eruption happened and that past climate modelling has suggested the climate consequences could have been severe, but archaeological and paleoclimate records from Africa don't show such a dramatic response.
"Our results suggest that we might not have been looking in the right place to see the climate response. Africa and India are relatively sheltered, whereas North America, Europe and Asia bear the brunt of the cooling.
"One intriguing aspect of this is that Neanderthals and Denisovans were living in Europe and Asia at this time, so our paper suggests evaluating the effects of the Toba eruption on those populations could merit future investigation."
The researchers analysed 42 global climate model simulations in which they changed various factors such as sulphur emissions, time of year of the eruption, and background climate state.
This allowed the team to make a probabilistic assessment of the range of climate disruptions the Toba eruption may have caused.
Black said: "By using a probabilistic approach, we aim at understanding the likelihood that some regions were less impacted by Toba, considering the wide range of estimates of its size and timing, in addition to our lack of knowledge of the underlying climate state.
The results suggest there was likely significant regional variation in climate impacts. The simulations predict cooling in the Northern Hemisphere of at least 4°C, with regional cooling as high as 10°C depending on the model parameters.
In contrast, even under the most severe eruption conditions, cooling in the Southern Hemisphere (ncluding regions populated by early humans) was unlikely to exceed 4°C.
The results explain independent archaeological evidence suggesting the Toba eruption had modest effects on the development of hominid species in Africa.
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