And this was half-bad, half-good news for early humans living in Africa.
Mild disaster
“Sulfate aerosols in the stratosphere are more effectively transported to the hemisphere that is experiencing winter,” Park says. For our ancestors, this meant the Toba’s sulfate veil drifted northwards and thinned over their heads slightly sooner than it would if the eruption happened in the northern summer. On the other hand, the Northern Hemisphere has far more land, which cools down faster and further than the ocean, so a northern-winter eruption must have cooled the globe a bit more than the same eruption in July. But overall, compared against the broader climate change trajectory, the Toba eruption was a rather mild disaster.
“The eruption did cool and dry the region, just not to such an extent as to significantly threaten the survival of humans,” Park argues. Across the 450-year window his team analyzed, eastern equatorial Africa was already sliding from a warmer, wetter regime into a cooler, drier one, tracking a natural cooling trend recorded in Greenland ice. Against that background, Toba’s impact looks tiny. “Our study shows that the magnitude of cooling and drying after Toba was much smaller, and within the natural range of variation in the last glacial–interglacial cycles,” Park says. “Humans had already experienced and survived that.”
The limitation in Park’s study is that Chala is just one lake. “It records the regional climate signals in eastern Africa, but it doesn’t show the impact on the whole globe,” Park explains. “There should be more studies using a similar approach, at more sites where the Toba ash is found.”
The team also hopes similar approaches will be used to learn more about other huge volcanic eruptions like Los Chocoyos in Guatemala and the Oruanui eruption in New Zealand 26,000 years ago. “They were not as big as Toba, but they were still super-eruptions. It would be interesting and exciting to see what happened,” Park says.
Science Advances, 2o26. DOI: 10.1126/sciadv.adv6851