Boulder, Colo. — In the last month or so, sunsets have begun painting the sky with a brighter and more colorful palette than usual.
This is a purely aesthetic side-effect of the April 4 eruption of the Mexican volcano, El Chichon. The largest volcanic eruption in the world in 70 years, it is having a significant atmospheric impact: Scientists expect the gas and ash which were injected into Earth's stratosphere as a result to have subtle but significant effects on the world's weather patterns for several years to come.
While certain that the 10 million tons of volcanic ash and sulfuric acid, which are slowly enveloping the entire globe more than 12 miles overhead, will have measurable effects on the Earth's climate, experts say it is impossible to specify exactly what these will be.
''Anyone who claims to be able to predict the effect on weather patterns is a charlatan,'' says Steven Schneider of the National Center for Atmospheric Research (NCAR).
Still, scientists do believe the odds have shifted slightly toward an earlier and colder winter and a milder summer next year. They wouldn't be surprised to see autumn's first frost come a week or two early in some parts of the country, so gardeners and farmers take note.
The problem in predicting the volcano's effect is the atmosphere's complexity and a lack of recent experience with really large eruptions. While the blowout at Mount St. Helens was big as eruptions go, it was unimportant as far as weather effects went. That is because its magma (molten rock beneath the surface) did not contain much sulfur, and it is the amount of sulfur that a volcano manages to blast into the upper levels of the atmosphere which determine the magnitude of its weather effects.
The generally accepted theory is that a large volcanic eruption will cause a decrease in the average temperature at the planet's surface. The sulfuric acid droplets that it releases act as a stratospheric veil that reduces the amount of sunlight reaching the ground. In the tropics, scientists have measured a 5 percent drop in the amount of sunlight reaching the surface due to the El Chichon cloud. However, the upper layers of the atmosphere warm up and, according to current computer models, this warming more than compensates for the surface cooling. This has led a few scientists to argue that the overall climatic effect of an eruption could be one of warming, rather than cooling.
This minority view does not square with some of the anecdotal evidence on the subject, however. The most colorful case of past volcanic-climatic effects is undoubtedly the 1815 eruption of the Tambora volcano in Indonesia. Owen B. Toon, a researcher with the National Aeronautics and Space Administration (NASA) in Mountain View, Calif., enjoys spinning out the tale of Tambora and embroidering it with some scientific speculation.
In Europe, the summer following this giant eruption was cold and rainy, he recounts. As a result of the bad weather, Mary Shelley and some friends who were vacationing in Switzerland decided to hold a literary competition. It was then that the authoress got the idea for her famous novel, ''Frankenstein.'' Meanwhile, in New England, 1816 became known as the ''Year Without Summer'' because temperatures averaged 5 degrees F. (2.5 degrees C.) below normal and the Northeastern US was hit with a series of storms which brought unseasonably cold weather, Dr. Toon explains.
This spring's Mexican volcano was not nearly the size of the Tambora eruption , however. ''We're pretty sure that this will cause a cooling in the northern hemisphere of one half to 1 degree F. (1.9 to 1.25 degrees C.),'' says Toon, who is monitoring the volcanic cloud as part of an ongoing NASA program. This is small enough so that any weather effects probably will not be apparent to the average person, he cautions.
''The evolution of a year's weather is haphazard, as if dealt from a deck of cards,'' adds National Oceanic and Atmospheric Administration scientist Murray Mitchell. ''The volcanic cloud has reshuffled the deck, but its effects are very subtle and trying to predict them is a chancy thing at best.''
Dr. Mitchell speculates that the eruption might mean a milder than average hurricane season in the Gulf of Mexico. Hurricanes are whipped up by the heat in the tropical oceans. Because the volcanic cloud is currently concentrated in the equatorial latitudes, it is reducing slightly the rate at which tropical waters are warming and so may rob them of some of their usual steam, ''all things being equal, which they never are,'' says Mitchell.
Despite these uncertainties, the nation's climatologists are looking forward eagerly to studying the cloud and trying to piece together the picture of its weather effects. ''This will give us an important check on our theories,'' asserts NCAR's Schneider.
In particular, researchers will use this as a chance to calibrate the elaborate computer models of Earth's atmosphere which they use to estimate the long-term effects of the growing amount of carbon dioxide in the atmosphere.
''The really different thing about this volcano is that we are getting all these measurements of it,'' says Schneider. ''However, it will take two or three more before we will be able to really sort out volcanic effects on the climate.''