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"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. |
