Within three weeks of the Mt. Pinatubo eruption, the largest volcanic blast of the century, a band of sulfur aerosol had encircled the globe. By early 1992, the volcanic gases and aerosols had diffused through the stratosphere, veiling the earth. During that time, global carbon dioxide levels fell more sharply than any other decline on record. Some scientists suggested that global cooling caused ecosystem respiration to drop, lowering the amount of carbon dioxide emitted into the atmosphere. But Lianhong Gu of Oak Ridge National Laboratory, lead author of the Science report, didn't think that could be the only explanation.
Gu knew that crop scientists had discovered that plants grow best in diffuse light. When sunlight is too intense, some leaves fall into shadow, unable to photosynthesize, while others bask in the direct beams but will reach a photosynthetic saturation point. Moderate cloud cover and aerosols block direct beams, but allow light to bounce back and forth off water vapor and other molecules, creating a "softer" light that reaches leaves that would otherwise be shaded. As a result, the plants photosynthesize more, using up carbon dioxide in the process. Gu and his collaborators determined that the same principles apply to forest canopies. The Harvard Forest data show that carbon dioxide levels dropped for two years following the eruption at Mt. Pinatubofindings that the scientists suggest represent a worldwide phenomenon given that the eruption had a global atmospheric effect. "Up until now we hadn't linked aerosols and clouds with carbon studies," Gu says. "In order to understand atmospheric carbon dioxide concentrations, which affect climate, we have to look at how aerosols and clouds affect the global carbon cycle."