The largest volcanic eruption in human history resulted in a few years of warm weather, according to an analysis of ancient sediments, and not a severe volcanic winter as some researchers had thought.
The Toba supervolcano, located on the Indonesian island of Sumatra, erupted 74,000 years ago, shooting thousands of cubic kilometres of volcanic material into the atmosphere.
鈥淭here is no doubt that the Toba super-eruption was colossal,鈥� says at Griffith University in Queensland, Australia. But the effect it had on the global climate is a matter of debate.
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Volcanic eruptions can have a cooling effect by releasing sulphur dioxide, which forms aerosol particles in the stratosphere that block sunlight. Prior research suggested the debris from Toba triggered a period of global cooling, perhaps lasting 1000 years, leading to multiple species extinctions and a sharp fall in the human population.
To find out more, at the Max Planck Institute of Geoanthropology in Germany and colleagues, including Petraglia, studied Jwalapuram, an archaeological site in southern India. The low-lying site has clearly defined layers of volcanic material, known as tephra, deposited by the Toba super-eruption.
鈥淭he amount of tephra was so massive that it completely buried the local vegetation at the site,鈥� says Jha.
Seasonal monsoon rains consolidated ash in the environment and concentrated it in low-lying areas like Jwalapuram. Later, a desiccated top layer called hardpan would form on the tephra when drier weather resumed. This cycle repeated year after year, forming layers like the pages of a book that recorded the aftermath of the eruption.
Freshly deposited tephra is highly susceptible to environmental factors like precipitation and temperature. To understand the layers of debris left by the eruption, Jha and his colleagues used various technologies, including X-ray and electron microscopy, to assess the rock鈥檚 mineral composition and what it can reveal about past climate.
The team found that after the eruption, Jwalapuram first saw one year of environmental cooling. The next five years after that were warmer, strongly contradicting the volcanic winter hypothesis.
鈥淲ithin six years of the eruption, ecosystems stabilised and returned to conditions that were favourable for hunting and gathering societies present in the region,鈥� says Petraglia.
Worldwide volcanic winters lasting many years have been documented after other major eruptions, including one in AD 536 linked to the decline of multiple civilisations. But this doesn鈥檛 seem to have followed from Toba, possibly because of the monsoon weather system.
鈥淲hile a few years of warming and drying would have posed challenges to local ecosystems, near-extinction events seem unlikely,鈥� says Jha.
In future work, Jha would like to expand his search to unearth further details about Toba and its effects around the globe.
at Columbia University in New York City hopes this work will help put an end to claims about extreme volcanic winters resulting from Toba, which he views as long discredited. 鈥淭he story offered by the initial hypotheses is simply too captivating for people to easily let go, but the breadth of contradictory evidence is getting too hard to ignore,鈥� he says.
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