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June 5, 2002
LARGE VOLCANIC ERUPTIONS AFFECT THE "GREENER GREENHOUSE"
Last year, scientists discovered that the
northern hemisphere was becoming
increasingly greener due to increased
warming. Furthering that study,
researchers have discovered that
tremendous amounts of tiny pollutant
particles thrust into the atmosphere by
large volcanic eruptions slow plant
growth, but still enhance the ability of
lands to act as a sink for carbon, as
reported in the May 31st issue of
Science.
Last September, researchers used
satellite data and confirmed that plant
life has been growing more vigorously
since 1981 above 40 degrees north
latitude, which represents a line stretching from New York to Madrid to Beijing.
They also determined that the growing season has increased by several days,
especially in Eurasia. One suspected cause of this "greener greenhouse" they
cited was rising temperatures that are possibly linked to the buildup of
greenhouse gases in our atmosphere.
"When we looked at the data between
1992 and 1993, the warming period of
the last 20 years was interrupted
because of the tiny dust particles
(aerosols) shot into the atmosphere by
the eruption of Mount Pinatubo," said
Ranga Myneni, co-author of the study,
and professor at Boston University. The
increased amount of aerosols cast into
the upper atmosphere from the eruption
caused more sunlight to be reflected
back into space and cooled the Earth.
As a result of the Earth cooling, and
sunlight being reduced, plants produced
less new foliage, because they use
sunlight, carbon dioxide and water to
make sugars for growth of new stems, leaves and structures. Myneni said that the
computer model he and his colleagues used simulates photosynthesis, and
calculates the amount of plant sugars that are produced versus how much carbon
is given off.
During the 1980s and 1990s the land
was taking in more carbon than it was
putting back in the air, including the
years immediately following Mt.
Pinatubo's eruption. "That is because
although the plants produced less
sugars, the soil actually released less
carbon, through the soil "respiration"
process, which is much like a plant
giving off carbon dioxide (CO2) through
plant respiration," Myneni said. The end
result was that the plants did not
produce as much new foliage and take
in as much carbon.
The model also showed that soil
respiration, the amount of CO2 released
by the microbial life that lives in the soil
also decreased. The reduction was so
great that for the two-year cooler period
following Pinatubo, the land mass of the
northern hemisphere actually acted as a
"carbon sink," where more carbon was taken in than was given out.
The Pinatubo eruption acted as a kind of experiment by Mother Nature to test the
researchers. The aerosols thrown into the air by the volcano had a two-year
cooling effect, during which time the worldwide greening trend lessened. This
effect was exactly predicted in the computer simulation. Carbon dioxide is a main
greenhouse gas and is suspected of playing a role in rising global temperatures.
If the northern forests are greening, they may already be absorbing more carbon
-- a process that can impact global temperature changes.
The computer simulations, directed by Wolfgang Lucht of the Potsdam Institute
for Climate Impact Research, were produced using the Lund-Potsdam-Jena
Dynamic Global Vegetation Model. The model predicts several factors, including
the growth of green leaves and the overall exchange of carbon in the ecosystem.
The researchers used satellite data based estimates of green leaf area to
compare the model simulation of plant growth. "The agreement between the two,
and also the model simulation of net carbon exchanges with changes inferred
from atmospheric
CO2 concentration, lend credibility to this study," Myneni said.
Wolfgang Lucht is the lead author of the researchers' findings, "Climatic Control of
the High-Latitude Vegetation Greening Trend and Pinatubo Effect."
This work was made possible through funding by NASA Headquarters' Earth
Science Enterprise, a long-term research program dedicated to understanding
how human-induced and natural changes affect our global environment.
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Contacts:
Rob Gutro
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-4044)
Bob Zalisk
Boston University
(Phone: 617/353-7628)
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Greening in the North
This view of the top of the world shows
enhanced plant growth over the last 20 years,
from the north pole, stretching southward to the
30 degree North latitude line (the circle).
Full Caption
The Temperature Connection: Temperature Influences Greenery
Researchers using satellite data have
confirmed that plant life above 40 degrees
north latitude, from New York to Madrid to
Beijing, has been growing more vigorously
since 1981 due to rising temperatures and
buildup of greenhouse gases.
Full Caption
Eruption of Mount Pinatubo, Philippines, July 1991
When Mount Pinatubo exploded in June 1991,
it sent enormous clouds of volcanic ash and
acidic gases into the stratosphere to altitudes in
excess of 100,000 feet. The largest ash cloud,
from a June 15 eruption, was carried by upper
level winds to the west and circled the globe in
about three weeks.
After the eruption, ash and other particles from
the volcano created a haze around the planet
for the next two years and slightly reduced the
sunlight reaching Earth's surface and made the
sun's radiation less direct and more diffuse.
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 Earth after colliding with particles in
the air.
Full Caption
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