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.

For more information about FLUXNET, see:

For more information on this story, see:

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 D