Advocates of “geoengineering“–proposed approaches to cool the planet to offset the effects of greenhouse gases–point to major volcanic eruptions as evidence that the techniques could work. Very large eruptions, like the one at Mount Pinatube in 1991, actually did cool the planet by injecting sulfates into the upper reaches of the atmosphere, where they circulated for over a year, shading the earth. Such advocates have been hoping that another major eruption would come along that could be studied for clues on how best go about artificially cooling the planet.

The eruptions of Iceland’s Eyjafjallajökull volcano, while impressive and large enough to shut down air traffic in Europe, don’t fit the bill. Here’s what Alan Robock, a professor of environmental studies at Rutgers University, who has studied the impact of volcanoes on climate, told me:

So far the volcano has only put out less than 0.004 Mt of SO2, compared to the 20 Mt that the 1991 Mt. Pinatubo eruption injected into the stratosphere. Furthermore, the Icelandic emission was only into the lower atmosphere, where the lifetime is on the order of one week, as opposed to a couple years in the stratosphere. So too little, and staying in the atmosphere for too short a time. So far.

To make an impact globally, the volcano would have to erupt much more violently (or perhaps trigger nearby volcanoes to erupt violently) so that the larger amounts of sulfates would reach the stratosphere and stay in place, he says. To follow the size of eruptions at the volcano, click here.