Around 74,000 years ago, Mount Toba in Sumatra erupted on a scale unmatched by any other eruption in the past two million years. For decades, the eruption was believed to have triggered a global “volcanic winter” that lasted up to six years. The world became so cold and dark that the event was often described as having nearly wiped out the entire population of early humans.
But a study published in the July 2026 issue of Science Advances presents a much less dramatic picture. The researchers found that Toba’s climate impact in East Africa lasted less than two years and was relatively mild.
A Lake That Records Time Each Year
A research team led by Jinheum Park of the University of Cambridge’s Department of Geography, together with colleagues from Ghent University, the British Geological Survey, the University of Nottingham, and Lancaster University, looked for answers not in Sumatra, but at Lake Chala.
The crater lake lies on the eastern slope of Mount Kilimanjaro, straddling the border between Kenya and Tanzania.
The lakebed contains sediment deposits that have accumulated continuously for around 250,000 years. Samples were collected through DeepCHALLA, an international scientific drilling project under the International Continental Scientific Drilling Program (ICDP).
In November 2016, the project team recovered a 215-meter-long sediment core from the lakebed. The core spans a period covering much of the time that modern humans (Homo sapiens) have lived in East Africa.
What makes Lake Chala’s sediments particularly valuable is their alternating dark and light layers. Together, they form annual pairs, somewhat like tree rings preserved at the bottom of a lake.
Researchers call these layers varves. Each year leaves behind one layer, meaning the sediment can be read almost like a calendar when counted carefully.
Park’s team analyzed the microstratigraphy of these layers using micro-X-ray fluorescence scanning. They also used inductively coupled plasma mass spectrometry to determine mineral composition, measured stable carbon and nitrogen isotopes in organic matter, and analyzed diatom fossil assemblages at a resolution of two to three years.
Evidence from the Unbroken Layers
The findings indicate that the Toba eruption occurred during the Northern Hemisphere winter. Afterward, East Africa experienced only mild cooling accompanied by a sharp increase in dryness. Even then, the disruption lasted less than two years.
It was not an isolated event that appeared out of nowhere. The brief disturbance occurred on top of a cooling and drying trend that had already been underway for decades. That trend was part of the early transition toward the last ice age.
The researchers’ reasoning is relatively straightforward. Each year, the lake deposits one varve layer. If a major climate disruption had occurred, such as the 2–3°C drop in temperature envisioned by the long-standing “volcanic winter” scenario, the formation of these annual layers should have been disrupted.
The process might have stopped temporarily, or the layers might have become irregular.
But that is not what the Lake Chala record shows. The sediment layers remain orderly and continuous, with no gap around the time of the Toba eruption.
For the researchers, that continuity is itself strong evidence. If the cooling had been as severe as previously imagined, its effects should have left a visible mark in the sediment record. They did not.
Not Global Evidence, but a Window into East Africa
Park himself cautions against taking the findings too far. Lake Chala records what happened over East Africa, not the global impact of the Toba eruption.
Other parts of the world may have experienced much more severe cooling, particularly regions closer to Sumatra. A single lake on the Kenya-Tanzania border cannot answer that question.
That is why the team hopes this high-resolution approach, which can read sediment records year by year, can be applied to other sites that also received Toba ash. Doing so could help build a more complete picture of the eruption’s climate impact.
The researchers also want to apply the same method to other massive eruptions beyond Toba, including Los Chocoyos in Guatemala and Oruanui in New Zealand. Both are supervolcano eruptions that have also been suspected of causing major global climate disruptions.
That would allow researchers to make a broader comparison: whether the “not as severe as once thought” pattern was unique to East Africa or reflected a more widespread pattern.
