In 1991, the eruption of Mount Pinatubo in the Philippines spewed sulfate aerosols across the globe, reducing direct sunlight but increasing the amount of reflected light everywhere. This change boosted plants' ability to grab carbon dioxide from the atmosphere and turn it into energy, according to a report in the 28 March issue of Science. The study is a step toward understanding how volcanic eruptions and shifting cloud cover can affect a key greenhouse gas and influence climate change.
The 20th century's steady climb in atmospheric carbon dioxide levels slowed markedly after Mount Pinatubo erupted. Most researchers assumed this was because the volcano's debris blocked sunlight and reduced temperatures, which in turn reduced the amount of carbon dioxide produced by breathing plants and animals. But environmental biophysicist Lianhong Gu of Oak Ridge National Laboratory in Tennessee and colleagues suspected the decline in carbon dioxide was due to more efficient photosynthesis. Crop scientists have long known that plants use diffuse radiation--familiar to photographers as "soft lighting"--more efficiently than direct sunlight. That's because diffuse light creates fewer shadows, so it reaches more leaf area, which more than compensates for its reduced intensity.
To test their hypothesis, the researchers studied the exchange of carbon dioxide between the atmosphere and Harvard Forest in eastern Massachusetts. Using a tower perched above the forest canopy, they took measurements of gas exchange in 1992, the year in which Mount Pinatubo had its biggest impact, and compared it with measurements from 1995 to 1997, when the dust from Pinatubo had settled. After using a computational model to compensate for temperature and time of year, they found that photosynthesis on cloudless days was 23% higher in 1992 than in the later years.
The findings should prompt climate scientists to take a much closer look at the impact of diffuse radiation, something they've tended to neglect in the past, Gu says. They may also help researchers understand the effects of cloud cover, which also creates diffuse light and represents the biggest source of uncertainty in climate models, he says.
Environmental biophysicist John Norman of the University of Wisconsin, Madison, says the authors make a solid argument for an increase in photosynthesis. And they were lucky to have started taking the necessary measurements just when Pinatubo erupted, he says. "They got the classic case here to make this point."