Global smog: newest greenhouse project.

Global smog: Newest greenhouse projection

Most forecasts of "greenhouse" climate changes focus on higher average land-surface temperatures and sea-level rises -- changes that could blight crops and inundate coastal communities. But farmers and beachfront dwellers aren't the only individuals likely to suffer directly from greenhouse effects, scientists reported last week. New studies suggest a greenhouse warming could greatly exacerbate air pollution -- especially smog-ozone levels -- throughout the world.

At NASA's Goddard Institute for Space Studies in New York City, David Rind and his co-workers have begun tweaking the institute's computer model of climate to explore weather-related changes that might result from a doubling in atmospheric levels of carbon dioxide, the primary greenhouse gas. In general, the model anticipates a growing sluggishness in weather systems, Rind told a Washington, D.C., conference on climate and air quality sponsored by the Environmental Protection Agency and American Gas Association.

As climate warms, temperatures climb faster at high latitudes (nearer the poles) than at low latitudes (nearer the equator), Rind says. Because latitudinal temperature gradients drive the circulation of air masses around the globe, diminishing the gradient can be expected to reduce the energy driving weather systems.

His model indicates a carbon dioxide doubling would in general slow surface winds, reduce winter and spring storms outside the tropics, reduce the intensity of storms that do occur and slow the eastward transport of pressure systems and air masses across the globe's mid-latitudes. A shift to fewer and weaker storms, combined with sluggish movement of air masses, suggests dirty air masses could be left hovering over industrial centers longer than they are today, he says.

Moreover, "with a warmer climate, there's more evaporation of moisture into the air," Rind says. His model predicts that a doubling of carbon dioxide could increase humidity "on the order of 30 to 40 percent." At the same time, there's likely to be more vertical exchange of air -- a convective mixing of high- and low-altitude air masses. "Our model also seems to imply that there will be a greater transport of ozone from the stratosphere [upper atmosphere] into the troposphere [lower atmosphere extending down to land]." The obvious implication, Rind says, is that urban smog-ozone levels may be enhanced by ozone generated in the stratosphere.

Greenhouse changes will also affect the chemistry of pollutants in the air at Earth's surface. An urban-smog model being developed at Systems Applications Inc. in San Rafael, Calif., indicates "air will became more reactive in the future," reports C. Shepherd Burton, the company's senior vice president. In a warmer environment, reactions between sunlight and air pollutants will occur more rapidly. This suggests that smog and other reactive by-products of such processes will form closer to their sources -- primarily in the centers of cities and industrial areas, where population densities tend to be highest. As a result, Burton says, "we would expect that attaining the federal ozone standard could become more difficult."

However, his computer model shows, this smog equation could change with a dramatic reduction in ozone precursors such as hydrocarbons and nitrogen oxides. The atmosphere's increased reactivity could alter the ozone precursors before they have time to generate much ozone. In that case, if one were to consider smog-ozone only, "if might look like climate change didn't end up hurting us at all," Burton says. However, he notes, the reaction products generated in place of ozone are oxidants -- such as peroxides and nitrogen compounds -- "whose health effects may be as bad as ozone's."

Greenhouse smog increases probably will not be restricted to urban areas, according to new computer analyses by Anne M. Thompson, a chemist at NASA's Goddard Space Flight Center in Greenbelt, Md. Using current global emission rates for methane and carbon monoxide, two important greenhouse gases, she calculates that by the year 2030, total atmospheric ozone levels in the low and middle latitudes could increase 10 percent globally, with levels highest at Earth's surface. Her data show that smog-ozone in distant suburbs and outlying areas could increase 20 to 25 percent. Even at sea, she says, ozone levels might increase 5 to 10 percent.

The ground-level ozone increases predicted by these studies justify expanding federal funding for air-quality monitoring and ozone mitigation, says John Topping, president of the Washington, D.C.-based Climate Institute. Moreover, he argues, when Clean Air Act amendments are drafted this year, they should permit the Environmental Protection Agency administrator to evaluate air-pollution-control strategies in light of their possible impact on larger environmental problems, such as greenhouse warming and stratospheric-ozone depletion.

Topping recalls being hampered by such restraints when he was staff director of EPA's office of air and radiation. He says there was a time, for instance, when regulations forced him to approve state requests to substitute chlorofluorocarbons (CFCs) for volatile and chemically reactive solvents, such as perchloroethylene because the reactive solvents directly contributed to urban air pollution and CFCs would not. Such requests struck him as ironic, he says, because at the time he was also initiating the CFC risk assessments that ultimately led to the Montreal Protocol, a treaty aimed at banning most CFC use.