BioEd Online

Immense volcanic eruptions are not necessarily triggers for catastrophic global cooling, say researchers in China and the United States. Super-eruptions are predicted to occur roughly once every 50,000 years, so these findings are relatively reassuring.

It has long been known that giant volcanic eruptions leave an imprint on the climate. When Mount Pinatubo in the Philippines blew in 1991, for example, it lowered average global temperatures by about 0.25 °C for a few years, as the dust it spewed into the atmosphere shielded the surface from the Sun. Super-eruptions, which are much bigger, are widely expected to be able to trigger an effect similar to a 'nuclear winter', killing off life world-wide.

But Meng-Yang Lee of the Academia Sinica in Taipei, Taiwan, and colleagues now find that one super-eruption in Indonesia coincided with the opposite climate switch: from an ice age to a warmer 'interglacial' period¹. This doesn't rule out the possibility that dust from the eruption blocked out sunlight and cooled the Earth, but it does show that the effect wasn't sufficient to induce long-term climate cooling and glaciation.

When Toba in Indonesia exploded some 74,000 years ago, it released about 2,800 cubic kilometres of magma - enough to fill a quarter of all the world's lakes. In 1992, geologists Michael Rampino and Stephen Self proposed that dust from this event may have killed off vegetation worldwide and induced a drastic decline in the population of humans2 - an effect much the same as when a giant meteorite slams into the planet. The climate was already shifting to ice-age conditions, but Rampino and Self proposed that the Toba super-eruption probably sped this up.

Lee's team looked at a volcanic deposit from an earlier super-eruption at Toba. Sediments from the bottom of the South China Sea and the Indian Ocean contain layers of ash that rained down from the sky after that ancient volcanic eruption, which they deduce happened about 788,000 years ago.

Although it seems to have been slightly smaller than the eruption studied by Rampino and Self, producing a mere 800-1,000 cubic kilometres of magma, it was still immense by the standards of most volcanic eruptions. Yet it didn't produce a switch to a cold climate. On the contrary, this early super-eruption coincides with a period of global warming.

Lee and colleagues conclude that these huge volcanic outbursts might not be as environmentally devastating as we thought.