Scientists from the Frontier Research System for Global Change in Yokohama, Japan and the University of California, Irvine, announced the results of research on NOx, primarily produced by aircraft and car emissions, on 16 April. The reason not to concentrate only on reducing emissions of NOx, they say, is that there is a marked difference in the short and long-term effects of doing so, with increased NOx emissions leading to short-term warming from increased short-lived ozone in the troposphere, the lower part of Earth’s atmosphere. However, over the following decade, these emissions help produce reductions in methane and even ozone, and thus a net cooling.

Overall, the net impact is a slight cooling for a wide range of locations of NOx emissions, the researchers say, and therefore reductions in these emissions, as pursued by many governments, will eventually add to global warming. The scientists note, however, that when emissions of carbon monoxide (CO), usually resulting 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 NOx and CO, which do not themselves have a significant impact on climate, but which control the major greenhouse gases, methane, ozone, and hydrofluorocarbons, through tropospheric chemistry. However, the researchers have developed a new method of quantifying the effect of short-term chemical interactions, expanding on previously published research that describing a tropospheric Chemical Transport Model (CTM), which 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 these 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 previous CTM model, the researchers conclude that manmade surface NOx emissions, taken alone, consistently cause cooling through their impact on ozone and methane, but that the amount of cooling varies greatly, depending on the region in which the emissions occur. However, combined industrial emissions of NOx and CO always result in increased warming. They conclude, therefore, that “decisions to control global atmospheric ozone and hence greenhouse warming by cutting NOx emissions alone would produce the opposite effect when the long-term, global changes to both methane and ozone are considered.”

The research will appear in the 1 May issue of the journal, Geophysical Research Letters, published by the American Geophysical Union.

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