Loretta J. Mickley, a research associate at Harvard University's Division of Engineering and Applied Sciences, will report on these findings Saturday, Feb. 19, at the annual meeting of the American Association for the Advancement of Science in Washington, D.C. Her work is based on modeling of the impact of increasing greenhouse gas concentrations on pollution events across the United States through 2050.
Using this model, Mickley and colleagues found that the frequency of cold fronts bringing cool, clear air out of Canada during summer months declined about 20 percent. These cold fronts, Mickley said, are responsible for breaking up hot, stagnant air that builds up regularly in summer, generating high levels of ground-level ozone pollution.
"The air just cooks," Mickley says. "The pollution accumulates, accumulates, accumulates, until a cold front comes in and the winds sweep it away."
Ozone is beneficial when found high in the atmosphere because it absorbs cancer-causing ultraviolet radiation. Near the ground, however, high concentrations are considered a pollutant, irritating sensitive tissues, particularly lung tissues.
"If this model is correct, global warming would cause an increase in difficult days for those affected by ozone pollution, such as people suffering with respiratory illnesses like asthma and those doing physical labor or exercising outdoors," Mickley says.
Mickley and her colleagues used a complex computer model developed by the Goddard Institute for Space Studies in New York, with further changes devised by her team at Harvard. It takes known elements such as the sun's luminosity, the earth's topography, the distribution of the oceans, the pull of gravity and the tilt of the earth's axis, and figures in variables provided by researchers.
Mickley gradually increased levels of greenhouse gases at rates projected by the Intergovernmental Panel on Climate Change, a group charged by the United Nations to study future climate variation. Her model looked at the effect the changing climate would have on the concentrations of two pollutants: black carbon particles -- essentially soot -- and carbon monoxide, which could also indicate ozone levels. When the model first indicated that future climate change would lead to higher pollution in the Northeast and Midwest, Mickley and her colleagues were a bit surprised.
"The answer lies in one of the basic forces that drive the Earth's weather: the temperature difference between the hot equator and the cold poles," Mickley says.
Between those extremes, the atmosphere acts as a heat distribution system, moving warmth from the equator toward the poles. Over mid-latitudes, low-pressure systems and accompanying cold fronts are one way for heat to be redistributed. These systems carry warm air poleward ahead of fronts and draw down cooler air behind fronts.
In the future, that process could slow down. As the globe warms, the poles are expected to warm more quickly than the equator, decreasing the temperature difference between the poles and the equator. The atmosphere would then have less heat to redistribute and would generate fewer low-pressure systems.
With fewer cold fronts sweeping south to break up hot stagnant air over cities, the air would sit in place, gathering pollutants. Mickley's model shows the length of these pollution episodes would increase significantly, even doubling in some locations.
Mickley's collaborators include Daniel J. Jacob and B. D. Field at Harvard and D. Rind of the Goddard Institute for Space Studies.
Their work was funded by a Science to Achieve Results (STAR) grant from the Environmental Protection Agency.
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