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.