When Mount Pinatubo erupted, scientists noticed the rate at which carbon dioxide (CO2) filled the atmosphere slowed down for the next two years. Also during 1992 and 1993, ash and other particles from the volcano created a haze around the planet and slightly reduced the sunlight reaching Earth's surface and made the sun's radiation less direct and more diffuse.

Many scientists previously thought the reduction in sunlight lowered the Earth's temperature and slowed plant and soil respiration, a process where plants and soil emit CO2. But this new research shows that when faced with diffuse sunlight, plants actually become more efficient, drawing more carbon dioxide out of the air.

"There is evidence indicating that the drop in the atmospheric CO2 growth rate was probably too big to be explained by a reduction in respiration alone," said the study's lead author, Lianhong Gu, a researcher at the University of California Berkeley's Department of Environmental Science, Policy and Management.

Gu added that the respiration rates of plants and soil are sensitive to temperature changes. But "in order to explain the drop in atmospheric growth rate of CO2, we would need an average drop in global temperatures of about 3.6 degrees Fahrenheit (2� C), but the temperatures only dropped by about one degree (0.9) Fahrenheit (0.5�C) globally."

Plants take in carbon dioxide during photosynthesis in the day, and release it during respiration at night. But they don't necessarily photosynthesize and respire at the same rates. Since decreased plant and soil respiration could not explain the drop in carbon dioxide entering the atmosphere in 1992 and 1993, Gu and his colleagues deduced that enhanced photosynthesis by plants must be involved.

After Mount Pinatubo erupted, while overall solar radiation was reduced by less than five percent, data showed a reduction of direct radiation by as much as 30 percent. So, instead of direct light, the sun's rays were reaching leaves after colliding with particles in the air.

"Diffuse radiation has advantages for plants," Gu said. That's because when plants receive too much direct light, they become saturated by radiation and their ability to photosynthesize levels off.

In the layers of leaves from top to bottom, called the plant canopy, only a small percentage of the leaves at the top actually get hit by direct light. In the presence of diffuse light, plants photosynthesize more efficiently and can draw more than twice as much carbon from the air than when radiated by direct light.

Gu and his colleagues tested the CO2 uptake in various plant ecosystems around the world-including Aspen forests, mixed deciduous forests, Scots pine forests, tallgrass prairies, and a winter wheat field-based on the amount of solar radiation striking the leaves.

From these analyses, they generated parameters necessary for evaluating impacts of the Pinatubo eruption. On clear days following the eruption, they found that in all of the ecosystems, photosynthesis increased under the diffuse light.

While large volcanic eruptions are rare, this research has big implications for more regular phenomena such as the effects of aerosols and clouds on an ecosystem's ability to pull carbon from the atmosphere. Aerosols, or microscopic particles like soot or black carbon in the air, occur naturally but have also been increasing due to human activities since the industrial revolution.

Gu's research indicates that the maximum uptake of carbon dioxide by plant ecosystems occurs when cloud cover is about 50 percent.

The research was presented at a poster session of the American Geophysical Union (AGU) Fall Meeting in San Francisco, Calif. on December 14, 2001. A paper will be published soon in the Journal of Geophysical Research.

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