HARVARD GAZETTE ARCHIVES

Loretta J. Mickley and colleagues found that the frequency of cold fronts bringing cool, clear air out of Canada during the summer months declined by about 20 percent. (Staff photo Kris Snibbe/Harvard News Office)

Loretta J. Mickley, a research associate at the Division of Engineering and Applied Sciences; Daniel J. Jacob, the Vasco McCoy Family Professor of Atmospheric Chemistry and Environmental Engineering; B. D. Field of the Division of Engineering and Applied Sciences; and D. Rind of the Goddard Institute for Space Studies in New York examined the impact of increasing greenhouse gas concentrations on pollution events in the United States through 2050.

Mickley and colleagues found that the frequency of cold fronts bringing cool, clear air out of Canada during the summer months declined by about 20 percent. These cold fronts, Mickley said, are responsible for breaking up the hot, stagnant air that builds up regularly in summer, generating high levels of ground level ozone pollution.

"The air just cooks," Mickley said. "The pollution accumulates, accumulates, accumulates, and finally a cold front comes in, the winds sweep it away."

Ozone is a highly reactive compound that is beneficial when found high in the atmosphere because it absorbs cancer-causing ultraviolet radiation. Near the ground, however, high concentrations of ozone are considered a pollutant, damaging vegetation and irritating sensitive tissues, particularly lung tissues.

If the model is correct, Mickley said, there would be 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 and colleagues' results were reported in the journal Geophysical Research Letters and will be presented at an upcoming conference of the American Association for the Advancement of Science.

Mickley did her crystal ball gazing through a complex computer model developed by the Goddard Institute for Space Studies, with further changes devised by the team at Harvard. It takes known elements such as the number of hours of sunshine per day and the tilt of the earth's axis, and figures in variables provided by researchers.

In this case, Mickley gradually increased the levels of greenhouse gases at the rates projected by the Intergovernmental Panel on Climate Change, a group charged by the United Nations to study future climate variation. The result was roughly a two degree Celsius increase in global temperature.

Mickley's study looked at effects across the entire United States, but she found the most significant change in these Northeastern and Midwestern pollution incidents.

"The question we had was whether any of those processes [that drive pollution] would change as we undergo global climate change," Mickley said.

Mickley's model looked at the effect warmer temperatures would have on the concentrations of two pollutants: black carbon particles - essentially soot - and carbon monoxide, which Mickley said could also indicate ozone levels. Emissions of these pollutants were kept constant over time. When the model first indicated that the result of increasing global temperatures would be higher pollution in the Northeast and Midwest, Mickley said she and her colleagues were a bit surprised and had to search the model for the reason.

The answer, Mickley said, lies in one of the basic forces that drive the Earth's weather: the temperature difference between the hot equator and the cold poles.

Between those two extremes, the Earth's atmosphere acts as a heat distribution system, moving warmth from the equator toward the poles. Over the mid-latitudes, low-pressure systems and the cold fronts that accompany them are one way for that heat to be redistributed. These systems carry warm air poleward ahead of the front and draw down cooler air behind the front.

In the future, that process could slow down. As the globe warms, the poles are expected to warm up more quickly than the equator because the melting of snow and ice would decrease the solar energy that is reflected back to space and increase the solar energy absorbed by the Earth. With warmer poles, Mickley said, there would be less difference between the temperature at the poles and at the equator. That would mean the atmosphere would have less heat to redistribute and so would generate fewer low-pressure systems to do that work.

Another way the atmosphere redistributes energy is through transport of tropical water vapor to the mid-latitudes, where the vapor condenses and releases heat. That process would increase in importance in a warmer world, ultimately leading to still fewer low-pressure systems.

With fewer cold fronts sweeping south to break up the hot stagnant air over the cities, the air would sit in place, gathering pollutants. The model, Mickley said, shows the length of these pollution episodes would increase significantly, even doubling in some locations.

"We're getting these high pressure stagnation events, but the cyclones were being a little tardy in coming," Mickley said. "The cold fronts were a little late in the future climate and pollution builds up."

Mickley said that the increased pollution was largely due to a longer life for the episodes of stagnant air, not because of more frequent occurrences.

"We see an intensity change in the atmosphere," Mickley said. "It becomes more sluggish, with fewer of these low pressure systems coming through."

alvin_powell@harvard.edu

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