The largest volcanic eruption in the last 2.6 million years may have had a smaller impact than previously believed. Evidence from a lakebed in East Africa indicates the Mount Toba supereruption 74,000 years ago caused only brief, mild cooling rather than the decade-long “volcanic winter” some scientists feared.
The lake that kept time
Geoscientist Jinheum Park and his colleagues at Johannes Gutenberg University in Mainz studied Lake Chala, a crater lake on the Kenya-Tanzania border, to resolve a long-standing debate. The lake’s conditions make it one of the few places where annual sediment layers, called varves, preserve climate records with exceptional clarity.
Unlike most lakebeds, where sediments mix over time, Chala’s deep, oxygen-poor waters remain undisturbed. Each year, a light layer of diatom silica forms during the dry season, followed by a darker layer of clay and soil during the wet months. This precision allowed the team to track the Toba eruption’s impact year by year.
The eruption’s ash appeared in the sediment as microscopic glass shards. Above it, two distinct layers marked the immediate aftermath: a darker layer followed by an unusually thick pale layer, suggesting a prolonged diatom bloom as cooler temperatures enhanced the lake’s mixing. Within three years, the layers returned to normal.
A chill, not a freeze
The researchers estimated the eruption cooled East Africa by about 0.5 degrees Celsius, significantly less than the 2–3 degrees some models had predicted. Park noted the change was nearly a quarter of the threshold needed to disrupt the lake’s nutrient cycles. If the cooling had been more severe, the varves would have become irregular. Instead, they remained consistent.
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This result supports a growing understanding of how massive eruptions affect climate. While sulfur dioxide from volcanoes can reflect sunlight and lower temperatures, the effect has limits. Park explained that larger sulfate aerosols settle faster because they are heavier. The most powerful eruptions may not have the climatic impact once assumed.
The study provides insight into how early humans endured natural disasters. Over the 450-year period analyzed, East Africa was already transitioning from a warmer, wetter climate to a cooler, drier one, a shift also recorded in Greenland ice cores. Against this backdrop, Toba’s impact was minor. Park stated that humans had already survived similar conditions.
However, the work has limitations. Chala’s record reflects only regional conditions, not global ones. Park emphasized the need for similar studies at other sites where Toba’s ash is found.
The findings do not eliminate the possibility that Toba caused hardship in other areas. Some regions may have faced harsher conditions, and indirect effects—such as disrupted migration or food shortages—remain uncertain. For early humans in East Africa, though, the disaster seems to have been nothing more than a temporary cold snap.
Improved forecast accuracy for natural disasters could help scientists better predict the consequences of future eruptions.