Planes: larger role in global warming?

Nitrogen oxides ([NO.sub.x]) constitute a family of combustion gases that can foster ozone, both in urban smog and in the rarefied atmosphere high above Earth's surface. Through their production of ozone -- a greenhouse gas when trapped within the upper troposphere -- they may also contribute to global warming. A controversial analysis now suggests that the [NO.sub.x] emitted by cruising aircraft pose a small but growing greenhouse threat.

Since the 1970s, chemists have recognized that high-flying aircraft should pose a more potent warming threat, per gram of [NO.sub.x] emitted, than cars and other ground-level sources. Why? Adding [NO.sub.x] to regions with low ambient levels of this pollutant, such as the upper reaches of the troposphere (8 or more kilometers above Earth's surface), drives far more ozone production than would an equal addition into a nitrogen-oxide-rich environment, such as downtown Los Angeles, explains Colin Johnson of the Atomic Energy Authority's Harwell Laboratory in Didcot, England. Moreover, he says, "the greenhouse warming per molecule of ozone is greater [at higher levels] in the atmosphere."

But until recently, no one had quantified both of these factors in connection with aircraft, Johnson says. "Because we had written a new model of the atmosphere, we thought it would be an ideal opportunity to analyze the question," he told SCIENCE NEWS.

In the Jan. 2 NATURE, Johnson and his colleagues conclude that aircraft may contribute roughly as much to global warming as surface [NO.sub.x], even though they produce only about 3 percent of combustion-generated [No.sub.x].

Together, all sources of nitrogen oxides will contribute only about 3.5 percent as much to global warming as will carbon dioxide over the next century, they estimate. However, if air traffic maintains its present rate of growth, "we've got to keep a careful eye on [NO.sub.x] emissions]," Johnson warns.

Others remain skeptical. Michael J. Prather, an atmospheric scientist at NASA's Goddard Institute for Space Studies in New York City, questions the team's reliance on a two-dimensional (latitude and altitude) model of the global atmosphere. Such models, he says, are "inadequate" to predict the dispersion of aircraft contrails and pollutant plumes, since they make no provision for convective mixing of short-lived gases such as [NO.sub.x].

Johnson agrees that an evaluation with three-dimensional models is needed. Indeed, he says, "that's the next phase of our study."