Apr. 19, 2001 WASHINGTON - Climate researchers are warning that efforts to reduce air pollution could, if not well designed, make global warming worse. Limiting emissions of manmade nitrogen oxides, a strategy to control ozone in the lower atmosphere, would result in increased methane abundance and lead to additional greenhouse warming.
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.
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 manmade 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 manmade emissions, which may require the development of regional models to compare with the CTM.
The study was funded in part by the National Science Foundation's Atmospheric Chemistry program and NASA's Atmospheric Chemistry Modeling and Analysis Program.
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