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March
8, 2007 Like a piece of
chalk dissolving
in vinegar, marine life with hard shells is in danger of being
dissolved by
increasing acidity in the oceans. Ocean acidity is
rising as
seawater absorbs more carbon dioxide released into the atmosphere from
power
plants and automobiles. The higher acidity threatens marine life,
including
corals and shellfish, which may become extinct later this century from
the
chemical effects of carbon dioxide, even if the planet warms less than
expected. A new study by "Before our study,
there was
speculation in the academic community that climate change would have a
big
impact on ocean acidity," Jain said. "We found no such impact." In previous studies,
increasing
levels of carbon dioxide in the atmosphere led to a reduction in ocean
pH and
carbonate ions, both of which damage marine ecosystems. What had not
been
studied before was how climate change, in concert with higher
concentrations of
carbon dioxide, would affect ocean chemistry and biology. To investigate
changes in ocean
chemistry that could result from higher temperatures and carbon-dioxide
concentrations, the researchers used an Earth-system model called the
Integrated Science Assessment Model. Developed by Jain and his graduate
students, the model includes complex physical and chemical interactions
among
carbon-dioxide emissions, climate change, and carbon-dioxide uptake by
oceans
and terrestrial ecosystems. The ocean-surface pH
has been
reduced by about 0.1 during the past two centuries. Using ISAM, the
researchers
found ocean pH would decline a total of 0.31 by the end of this
century, if
carbon-dioxide emissions continue on a trajectory to ultimately
stabilize at
1,000 parts per million. During the last 200
years, the
concentration of atmospheric carbon dioxide increased from about 275
parts per
million to about 380 parts per million. Unchecked, it could surpass 550
parts
per million by mid-century. "As the
concentration of
carbon dioxide increases, ocean water will become more acidic; which is
bad
news for marine life," Cao said. "Fortunately, the effects of climate
change will not further increase this acidity." There are a number
of effects and
feedback mechanisms built into the ocean-climate system, Jain said.
"Warmer water, for example, directly reduces the ocean pH due to
temperature effect on the reaction rate in the carbonate system. At the
same
time, warmer water also absorbs less carbon dioxide, which makes the
ocean less
acidic. These two climate effects balance each other, which results in
negligible net climate effect on ocean pH." The addition of
carbon dioxide
into the oceans also affects the carbonate mineral system by decreasing
the
availability of carbonate ions. Calcium carbonate is used in forming
shells.
With less carbonate ions available, the growth of corals and shellfish
could be
significantly reduced. "In our study, the
increase
in ocean acidity and decrease in carbonate ions occurred regardless of
the
degree of temperature change associated with global warming," Jain
said.
"This indicates that future changes in ocean acidity caused by
atmospheric
carbon-dioxide concentrations are largely independent of climate
change." That’s
good news. The
researchers’ findings, however, call into question a number
of engineering
schemes proposed as mitigation strategies for global warming, such as
lofting
reflective balloons into the stratosphere or erecting huge parasols 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. "Even if we could
engineer
our way out of the climate problem, we will be stuck with the ocean
acidification
problem," Caldeira said. "Coral reefs will go the way of the dodo
unless we quickly cut carbon-dioxide emissions." Over the next few
decades, we may
make the oceans more acidic than they have been for tens of millions of
years,
Caldeira said. And that’s bad news.
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