Mt. St. Helens shows little climate effect

There now seems little doubt that the Mt. St. Helens eruptions will have some impact on climate. But it will be too small to measure. That's the conclusion Alan Robock of the University of Maryland has drawn from computer simulations of the interaction between the eruptions' stratospheric veil and Earth's weather.

At its maximum, Mt. St. Helens' influence is expected to depress net January's average temperature in the north polar regionby only 0.1 degree C. That's too small to be picked out of the normal year-to-year variations which are generally some 25 times larger. Smaller cooling is expected for several years thereafter.

This finding reinforces climatologists' expectations of a minimal impact.The volcano is too far north and its major eruption, which blew out sideways, threw too little material into the stratosphere to have any significant climatic influence.

At the same time, the small cooling effect that Robock does predict is in line with theories that major volcanic eruptions with more substantial aerosol veils can change climate. Larger dust particles, which might have a warming effect, would fall out in under a year. But particles of acids, especially sulfuric acid, which volcanos eject copiously, form aerosol layers that diffuse around the Earth and persist. They would be expected to have a cooling influence, as Robock has shown. And for most big eruptions that would be expected to be substantially larger than Mt. St. Helens' cooling.

To be "substantial," such cooling would have to depress average global temperature only a degree or two for say a decade, an effect well within the range expected for major volcanic aerosol veils. Stephen C. Porter of the University of Washington has shown that a 1 degree C. average temperature drop could start glaciers advancing.

Meteorologists argue over whether or not such an effect actually has influenced Earth's climate significantly. Nevertheless, there does seem to be evidence for some kind of volcano-climate link. Porter notes that, while glacial activity over the past century is not synchronized between Northern and Southern Hemispheres, the sequence of glacier variations does match the frequency of volcanic eruptions in successive latitude belts.

It also pairs up with records of acidity and dust in ice cores taken from high northern and southern latitudes. These presumably reflect volcanic activity. For example, C. U. Hammer, H. B. Clausen, and W. Dansgaard of the University of Copenhagen have compared such data from Greenland ice cores with temperature indicators. They find that,f or the past 10,000 years, clustered volcanic eruptions are correlated with a considerable cooling effect.

Thus, as Porter says, volcanic eruptions may "be a major factor in modulating climate on the decadal scale." But don't blame your local weather on Mt. St. Helens.