Toba "cut down to size"

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Science last week carried a news article by Naomi Lubick, describing a new model for the climatic effects of the Toba volcanic eruption, around 74,000 years ago.

The simulation revealed that Toba's impact was not as extreme as some scientists believed. Temperatures dipped only 3 degrees to 5 degrees C across the globe, for example. The model also showed that the high concentrations of sulfur particles were short-lived; they settled out of the stratospherewhere they can have the largest cooling effectwithin 2 to 3 years, the team reports online this month in Geophysical Research Letters. Extreme temperature changes in Africa and India lasted only a year or two, with a temperature decrease of at most 10 degrees C in the first year after the eruption, followed by 5 degrees C the second year. Overall, Toba didn't wipe out flora and fauna, Timmreck says, but it would have made life harder for a few years.

The issue comes down to the assumptions they have to make when they scale up the measured effects of recent volcanic eruptions such as Mt. Pinatubo, Philippines. The new model is argued to be consistent with ice core data about atmospheric sulfate concentrations after the eruption.

I think these climate models continue to shift too much to really interpret the importance for ancient human populations. A global reduction in temperature and biosphere productivity is not going to be happy times for most Pleistocene hunter-gatherers. But the kind of extreme, prolonged population contraction seems like it must require a rather more severe event, seriously forcing global climates out of their usual range of variation.

I’ve been a very consistent Toba skeptic, because a global catastrophic event in the Late Pleistocene really is not helpful in explaining the present pattern of human genetic diversity. But with a little clever science, it might become possible to look for more temporary effects, or those limited to a few regions of the world. What’s necessary is to bring the expectations into the same range of realistic alternatives.

In that view, a more precise climate model that may show a shorter and smaller range of climate effects may be very useful.