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Engineered strategies to mitigate global warming could influence biosphere
James
E. Kloeppel, Physical Sciences Editor
(217) 244-1073; kloeppel@illinois.edu
12/12/01
SAN FRANCISCO — Blocking
the sun may not be such a cool way of counteracting climate change,
scientists at the University of Illinois say. Potential effects upon
the biosphere could be important to agriculture and forest production,
and also could create secondary feedback mechanisms that may further
change the climate.
A number of engineering schemes have been proposed as mitigation strategies
for global warming, such as lofting reflective balloons into the stratosphere
or erecting a huge parasol in orbit. By blocking some of the sunlight,
these devices would create a cooling effect to offset the warming caused
by increasing levels of greenhouse gases.
But, even if such feats become practical, there are concerns about how
the biosphere would respond to a reduction in solar radiation. To compensate
for the climate effects of doubling the amount of carbon dioxide in
the atmosphere, for example, the amount of sunlight striking Earth would
need to be decreased by nearly 1.8 percent.
"The biosphere plays a very important role in determining how much
carbon dioxide is in the atmosphere," said Donald Wuebbles, a professor
and head of atmospheric sciences at the UI. "Through photosynthesis,
carbon dioxide is removed from the atmosphere and stored in plants.
Decreasing the solar constant by 1.8 percent could impact the amount
of biomass produced, and therefore affect how much uptake and storage
of carbon dioxide occurs."
To study such effects upon the biosphere, Wuebbles and his colleagues
– John Foley, a professor of atmospheric and oceanic sciences at
the University of Wisconsin at Madison, and UI graduate student Vaishali
Naik – used a dynamic global ecosystem model to simulate the response
of vegetation to engineered climate conditions.
First, the researchers doubled the amount of carbon dioxide in the atmosphere.
Then they decreased the solar constant by 1.8 percent and compared the
results.
The researchers found a definite influence on the biosphere. There was
a decrease in net production of biomass in tropical forests and in boreal
forests located in higher northern latitudes. However, there was a slight
increase in biomass production in the mid-latitudes.
"We think this mixed message is coming as a result of feedback
mechanisms such as changes in water stress," Wuebbles said. "Without
those feedbacks, we would have seen a decrease in net primary production
everywhere."
Photosynthesis depends not only on atmospheric carbon dioxide and incident
solar radiation, but also on an ample water supply, Wuebbles said. "In
water-stressed regions, such as deserts, biomass is heavily dependent
on the availability of water, as well as sunlight. If you reduce the
solar radiation, there will be less evaporation from the soil, leaving
more water for plants to consume. With less water stress, the plants
can grow better, creating more biomass."
While previous studies have indicated that elevated levels of carbon
dioxide in the atmosphere would stimulate photosynthesis, resulting
in increased primary production of vegetation, the situation is not
quite that simple, Wuebbles said. "For example, blocking some sunlight
would decrease plant growth, but that would also decrease uptake, which
would give a positive feedback on the amount of carbon dioxide in the
atmosphere, which could lead to more climate change."
More work needs to be done to understand the impacts on the biosphere
from an engineered response to climate change, Wuebbles said. "In
particular, we need to take into account these various feedback mechanisms
that may affect the amount of carbon dioxide in the atmosphere."
Wuebbles will present the team’s preliminary findings at the American
Geophysical Union meeting in San Francisco on Wednesday, Dec. 12.
