New analysis of sediment from a Kenyan lake suggests the massive Toba eruption caused only minor cooling, challenging theories that it nearly wiped out humanity.

A study of sediment records from Lake Chala in East Africa indicates that the Mount Toba eruption, which occurred approximately 74,000 years ago, had a limited impact on the climate. Researchers found that the volcanic event resulted in roughly 0.5 degrees Celsius of cooling and lasted less than two years. These findings contradict previous hypotheses suggesting the eruption caused a catastrophic volcanic winter that threatened the survival of early human ancestors.

High-Resolution Climate Records

To analyze the eruption's impact, researchers examined sediment cores from Lake Chala, a crater lake on the border of Kenya and Tanzania. The lake's deep, oxygen-starved water preserves annual sediment layers known as varves, which act as a precise climate record. By identifying microscopic glass shards from the Toba eruption within these layers, the team was able to track climate changes with high resolution.

Evidence of Limited Cooling

The research team measured silicon-to-aluminum and manganese-to-iron ratios to estimate temperature and moisture changes. The data showed that the cooling effect was approximately 0.5 degrees Celsius, a magnitude significantly smaller than what would be required to disrupt the lake's annual sediment patterns. The study concludes that the eruption's impact was within the range of natural climate variation already being experienced by early humans at the time.

Timing the Eruption

The sediment analysis also allowed scientists to determine the season of the eruption. Because the ash was found near the end of a dark sediment lamina, researchers estimate the event occurred in January or February. This timing suggests the sulfate veil drifted northward, potentially thinning over Africa sooner than it would have had the eruption occurred during the northern summer.