NSF PR 00-66

Press Release

NSF PR 00-66 - September 27, 2000

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Worsening Urban Air Pollution Won't Increase Global Temperature Over Next 100 Years

Researchers funded in part by the National Science Foundation (NSF) and affiliated with the Massachusetts Institute of Technology have found that although urban air pollution is expected to increase significantly in the coming century, it will not have a big effect on global temperature change.

While there may be temperature increases in certain regions, global mean surface temperature will not go up significantly because of urban air pollution, researchers at MIT's Joint Program on the Science and Policy of Global Change wrote in a paper to be published in the September 27 issue of the Journal of Geophysical Research--Atmospheres.

"It is very important to understand the interplay between air quality and climate, and recent advances in coupled chemistry climate models make that possible," says Anne-Marie Schmoltner, program director in NSF's division of atmospheric sciences. "Changes in air quality could affect global climate, and conversely, climate change could affect air quality. The results presented by the MIT group provide insight into these complex interactions."

Using a method that allows global coupled-chemistry climate models to take urban air pollution into account in a new way, MIT researchers found that compared to a reference run excluding urban air pollution, the average tropospheric ozone concentration decreases while high concentrations of ozone are projected in the urban areas. As a consequence of the change in the chemical composition of the troposphere, the lifetime of methane increases. This leads to higher ambient methane concentrations, even if emissions are unaltered.

"People thought things would go in this direction, but they couldn't quantify it before," said Monika Mayer, research scientist at MIT and lead author on the paper, "Linking local air pollution to global chemistry and climate."

While scientists agree that urban air pollution can alter concentrations of greenhouse gases such as ozone in the troposphere, they have left the complicated chemistry of urban air pollution out of global climate models. "Global-scale models that do not take into account urban areas' highly nonlinear atmospheric chemistry most likely overestimate tropospheric ozone production due to unreasonably high background nitric oxide concentrations," the authors write.

Yet, "high-resolution climate models don't have chemistry coupled to them," said Mayer. "It takes months just to run a global climate model without the chemistry."

Population projections show that in the next 100 years, the concentration of people in urban areas will increase dramatically. While 30-40 percent of air pollution currently comes from urban areas, as much as 70 percent may originate from cities in the future. The researchers carried out three simulations of 100year projections that factored in the effects of increased urban air pollution tied to population increases and economic development in these areas. They found that even with significant increases in air pollution, global mean temperature should not change much, although there may be more pronounced regional effects.

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