Super-Eruptions, Climate and Human Survival
The explosive eruption of Mt. Pinatubo in 1991 injected enough sulfur-containing compounds into the stratosphere to substantially reduce the amount of sunlight reaching Earth's surface. In response to the increased reflectivity of the planet, the surface temperature cooled by about 0.3°C during 1992, with temperatures returning to their normal levels by 1994. But what happens when a much, much large eruption occurs?
Roughly 74,000 years ago, a "super-eruption" took place in Indonesia, the largest know eruption in the past 100,000 years. The Toba eruption was enormous, throwing out roughly 1000 times as much rock as the 1980 eruption of Mt. St. Helens (Fig. 1). Dust trapped in polar ice cores shows that ejected material spread around the globe, indicating that the eruption injected substantial material into the stratosphere, where it can strongly affect climate. How much and for how long the Toba eruption actually affected climate and life on the Earth's surface has been the subject of intense debate.
Recently, we used state-of-the-art climate models to examine this question. Our study included climate models developed by the National Center for Atmospheric Research (NCAR) in Boulder, Col., and by the NASA Goddard Institute for Space Studies (GISS) in New York City. These are the same models used for climate projections of the near future in studies of global warming. In this case, we simulated the response to an enormous volcanic eruption to test how various processes might affect the climate response. Depending on the assumed size of the eruption and the processes included in the models, the maximum global mean cooling was 8-17°C. This is an enormous change, roughly 10-20 times the size of the warming since pre-industrial times and about the same magnitude as the transition to an ice age. Among the most interesting findings was that in response to the reduced sunlight able to penetrate the think blanket of ash and particles in the atmosphere, broadleaf evergreen trees and tropical deciduous trees virtually disappeared for several years. However, the Earth's climate returned to near-normal conditions within a decade in most simulations.
Cooling of the Earth lasted longer in GISS model simulations including interactive chemistry in the atmosphere, however. This played an important role as the injection of volcanic material was so large in the Toba eruption that some chemical processes saturated, leading to a longer presence of sulfur-containing particles in the stratosphere. This extended the time of extreme planetary cooling so that in these simulations the Earth remained at least 10°C colder than normal for a full decade (Fig. 2).
An ice sheet did not begin to form in any of the simulations as the climate change did not persist for a long enough period. Hence the results do not support the theory that the super-eruption might have triggered an ice-age. However, a "volcanic winter" occurring suddenly and lasting a decade or two could still have devastating consequences on life at the surface, with abrupt massive decreases in food production and potential extinctions of some species. Indeed, there is some evidence for such extinctions and for the presence of a "genetic bottleneck" in human population coincident with the eruption. Our results thus suggest that the sudden and severe climate response to the Toba super-eruption may have wiped out a substantial portion of the world's human population at that time.
Robock, A., C. M. Ammann, L. Oman, D. Shindell, S. Levis, and G. Stenchikov 2009: Did the Toba volcanic eruption of ~74k BP produce widespread glaciation? J. Geophys. Res., 114, D10107, doi:10.1029/2008JD011652.