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NSF PR 01-32 - April 19, 2001
Climate Researchers Warn of Impact from Poorly
Designed Air Pollution Efforts
Climate researchers are warning that efforts to reduce
air pollution could, if not well designed, make global
warming worse. Limiting emissions of man-made nitrogen
oxides, a strategy to control ozone in the lower atmosphere,
would result in increased methane abundance and lead
to additional greenhouse warming, they say.
Nitrogen oxides, commonly abbreviated NOx, are shorthand
for the combination of nitric oxide and nitrogen dioxide
(NO plus NO2) that are produced by aircraft and automobile
emissions, in biomass burning, and by some industrial
processes, as well as by such natural events as lightning.
"Traditionally, atmospheric chemistry research has
focused on processes in the natural and polluted atmosphere,
while climate modeling has investigated the impact
of greenhouse gases," says Anne-Marie Schmoltner,
director of the National Science Foundation (NSF)'s
atmospheric chemistry program, which funded the research
along with NASA. "However, it is important to recognize
the interplay between the chemistry and the distribution
of greenhouse gases. Increasingly complex models such
as the one employed in this study are now able to
look at these important interactions."
The research was conducted by Oliver Wild and Hajime
Akimoto of the Frontier Research System for Global
Change in Yokohama, Japan, and Michael J. Prather
of the University of California, Irvine. It will appear
in the May 1 issue of the journal, Geophysical
Research Letters, published by the American Geophysical
Union.
The reason not to concentrate only on reducing nitrogen
oxide emissions, they say, is that there is a marked
difference in the short- and long-term effects of
doing so. Increased nitrogen oxide emissions do indeed
lead, as is commonly expected, to short-term warming
from increased short-lived ozone in the troposphere,
the lower part of Earth's atmosphere.
Over the following decade, however, these nitrogen
oxide emissions lead to reductions in methane and
even ozone, and thus to a net cooling. Overall, the
net impact is a slight cooling for a wide range of
locations of nitrogen oxide emissions, and thus reductions
in these emissions, such as from pollution control
measures, will eventually add to global warming.
The scientists note, however, that when emissions of
carbon monoxide (CO), which usually result from the
same processes that produce nitrogen oxides, are added
to the equation, the net result is back to global
warming. Therefore, they say, efforts to address issues
of urban air quality and global warming must involve
combined emission controls and not just the "quick
fix" of reducing local air pollution by controlling
emissions of nitrogen oxides.
It has been difficult for scientists to quantify the
greenhouse effect of short-lived pollutants, such
as nitrogen oxides and carbon monoxide, which do not
themselves have a significant impact on climate. But
these gases control the major greenhouse gases --
methane, ozone, and the hydrofluorocarbons - through
tropospheric chemistry. This work adds further evidence
to the role of such urban pollutants as indirect greenhouse
gases, which was also reported in the recent assessment
report of the Intergovernmental Panel on Climate Change.
Wild and his colleagues have developed a new method
of quantifying the effect of these short-term chemical
interactions. It expands on their previously published
research that described a tropospheric Chemical Transport
Model (CTM) developed at the University of California,
Irvine. This model determines the impact of short-lived
regional emissions on the long-term global climate
effect of the methane-carbon monoxide-ozone combination.
By calculating separately the short-term regional
effects of those gases and the long-term global trends
of greenhouse gases in general, the authors are able
to determine their combined impact on climate change.
Using the Irvine CTM model, Wild and his colleagues
conclude that man-made surface emissions of nitrogen
oxides, taken alone, consistently cause cooling through
their impact on ozone and methane. The amount of cooling
varies greatly, depending on the region in which the
emissions occur. The model shows, however, that combined
industrial emissions of nitrogen oxides and carbon
monoxide always yield a positive result, that is,
increased warming. Therefore, they conclude, "decisions
to control global atmospheric ozone and hence greenhouse
warming by cutting nitrogen oxides emissions alone
would produce the opposite effect when the long-term,
global changes to both methane and ozone are considered."
The authors urge that further research be conducted
on specific regional impacts of man-made emissions,
which may require the development of regional models
to compare with the CTM.
The study was funded in part by NSF's atmospheric chemistry
program and NASA's atmospheric chemistry modeling
and analysis program.
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