The volcanic ash th at Mount St. Helens spewed into the atmosphere could confound efforts to detect climatic effects of human pollution. The volcanic debris now circling earth could block out some sunlight, causing a slight cooling of the earth's surface that would mask the expected warming effects of increasing carbon dioxide in the atmosphere and delay its detection for more than a decade, according to Kirby Hanson, director of the NOAA Geophysical Monitoring for Climatic Change program.
Measurements, some going back to 1958, have shown a 5% increase in atmospheric carbon dioxide over the past two decades. Furthermore, the annual rate of increase is growing, and it is predicted that within 50 years the concentration of carbon dioxide in the atmosphere could be double what it was before the Industrial Revolution. The temperature effect of that doubling, Hanson says, would be a net warming near the earth's surface of about 2° or 3°—averaged over the northern hemisphere.