Air pollution control efforts will add to global warming if carbon monoxide is not curbed along with nitrogen oxides
provided by University of California, Irvine
limate 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.
Nitrogen oxides are shorthand for the combination of nitric oxide and nitrogen dioxide that is 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 Michael J. Prather, professor of Earth system science at UC Irvine, and Oliver Wild and Hajime Akimoto of the Frontier Research System for Global Change in Yokohama, Japan. It appeared in the May 1 issue of Geophysical Research Letters.
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 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, 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 themselves do not 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 noted in the recent assessment report of the Intergovernmental Panel on Climate Change.
Prather 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 UC 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, Prather 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.