Air Pollution on Global Temperature

Researchers at 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.

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. As ozone decreases and methane increases, the net effect on the radiative budget of the Earth is small, because the contributions from these two greenhouse gases partially cancel each other out.

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

Global climate models are used by researchers to predict future conditions and to aid global policy matters such as the Kyoto Protocol. One example is the MIT Integrated Global System Model, which includes an economic development model; a two-dimensional land and ocean resolving interactive chemistry-climate model that divides the planet into 24 latitudinal bands; a terrestrial ecosystems model, and a natural emissions model.

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 Dr. Mayer. "It takes months just to run a global climate model without the chemistry."

Dr. Mayer and her colleagues, Chien Wang, Mort Webster, and Ronald G. Prinn, applied a method that allowed them to derive a computationally efficient urban air chemistry model from a state-of-the-art urban airshed model. They then coupled the "reduced form" urban air pollution model to the MIT integrated global climate model. This makes the global chemistry-climate model more powerful and comprehensive, because it is able to take urban air pollution into account in a way never before possible. "Our model takes about one day on a conventional workstation for a 100-year simulation. This is a powerful way to get interactions among air pollution, methane and other tropospheric gases," Dr. Mayer said.

Among the questions the more powerful model may help answer: How do air pollution and climate policies interact? What are the long-term effects of regional regulations regarding air pollution? What are the long-term effects or cost savings of targeting only greenhouse gases or more stringent Environmental Protection Agency air pollution regulations?

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 100-year 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.