LARGE
VOLCANIC ERUPTIONS HELP PLANTS ABSORB MORE CARBON DIOXIDE FROM THE ATMOSPHERE
New
NASA-funded research shows that when the atmosphere gets hazy, like it did after
the eruption of Mt. Pinatubo in the Philippines in June 1991, plants photosynthesize
more efficiently, thereby absorbing more carbon dioxide from the atmosphere. 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 will be 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. Aside
from NASA, the study was also funded by the National Oceanic and Atmospheric Administration
(NOAA), the Department of Energy, and other organizations, through the FLUXNET
program. Editor's
Note: AGU Title, Time and Location "Roles of Volcanic Eruptions, Aerosols
and Clouds in Global Carbon Cycle" Friday, December 14, 2001, 8:30 AM,
Moscone Center Hall Back
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