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LARGE
VOLCANIC ERUPTIONS AFFECT THE "GREENER GREENHOUSE"
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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.
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2 | | | "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.
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3 | | | 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. Back
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