Less Sunlight for Alaska as a Result of Mexican Volcanic Eruption

There has been a good deal of coverage in the media lately about the earth gradually warming up-because of the "greenhouse effect" caused by increasing amounts of carbon dioxide in the atmosphere.

Paradoxically, the past year has actually been a period of decreased direct radiation from the sun, caused by material in the air. On April 4, 1982, the Mexican volcano E1 Chichon exploded with a force that has been compared with the eruption of Krakatoa in 1883. That explosion destroyed an island in Indonesia and was heard 3000 miles away. The material ejected into the atmosphere traveled around the globe, and resulted in spectacular sunrises and sunsets all over the world for three years.

The eruption of E1 Chichon is also having worldwide effects. Measurements made by the Geophysical Institute at Fairbanks reveal that between November 15, 1982 and May 31, 1983, the amount of direct sunlight reaching the ground on clear days was reduced by almost 25%. The reason that temperatures did not drop correspondingly is that a greater proportion of radiation was being diffused in the upper atmosphere and reached the ground by less direct routes. The increased amount of scattered radiation was caused by tiny particles of debris from the E1 Chichon eruption. While most Americans are more aware of the Mt. St. Helens eruption a year earlier, the El Chichon explosion had many more far-reaching effects because it propelled material well into the stratosphere, while the Mt. St. Helens eruption resulted in material transport barely beyond the troposphere (that region of the earth's atmosphere where most weather agents, such as clouds, operate).

It is interesting that the volcanic cloud was confined to latitudes less than 35 degrees north during the summer of 1982 and only began to spread to higher latitudes in the fall. When it did, however, it did so very rapidly. Even though the first observations of anomalous radiation changes from the sun were not made in Fairbanks until more than 7 months after the eruption, similar effects were noted about the same time in Oregon--20 degrees to the south. This indicates that the northward spreading occurred very slowly between the tropical and mid-latitudes, but very rapidly after that.