Mount Toba was the biggest volcanic eruption in the last 2.6 million years, and some have suggested it nearly wiped out humanity. Around 74,000 years ago, a caldera on what is now Sumatra emptied thousands of cubic kilometers of magma in about two weeks, roughly a thousand times more than Mount Pinatubo spewed out in 1991. “People thought it might have caused massive cooling of the planet, and hence threatened the survival of our ancestors,” says Jinheum Park, a geoscientist at Johannes Gutenberg University in Mainz, Germany.

Park and his colleagues went looking for the records of that catastrophe in the mud taken from the bottom of a small crater lake on the Kenya-Tanzania border. They found instead that the effects of the Mount Toba eruption lasted under two years and amounted to perhaps half a degree of cooling.

Muddy calendars

Volcanic eruptions inject sulfur dioxide into the stratosphere, where it becomes a haze of tiny droplets that reflect sunlight back into space. Bigger eruptions eject more sulfur dioxide and, in principle, cause more cooling. But this trend stops working when the eruption exceeds a certain magnitude. “Bigger sulfate aerosols settle quickly, because they are heavier,” Park explains. This quick settling makes them less effective in scattering incoming solar radiation.

Estimates of Toba’s sulfur output varied so widely that computer models had the eruption appearing as anything between a near-extinction-level event to a mild nuisance for our early ancestors, depending on the number we punched in. To settle this argument, scientists tried to look at geological evidence: Toba’s ash preserved in seafloor or lake cores. But this also proved tricky.

Underwater mud cores usually don’t work that well when it comes to dating and constraining abrupt, violent events like volcanic eruptions. When sediment settles at the lake or ocean bottom, materials from different years get a bit mixed. Even if we cut a mud core taken from the bottom of the sea into very thin slices and attempt to resolve climate changes at smaller intervals, our slices would reflect the mixed influence of yearslong climate signals. With most such records, we can track how past climates evolved decade by decade. A volcanic winter, even a severe one, would last one to three years.