A Daunting Emissions Quest for U.S. Cities More than 1,000 American cities have voluntarily committed themselves to ambitious targets for reducing carbon dioxide emissions. A recent case study focusing on Allegheny County, Pa., home to Pittsburgh, highlights how hard it will be for some to meet those goals, however. At first glance, one would think that the county is on track. According to a study published this month in the journal Environmental Science and Technology, total carbon dioxide emissions in Allegheny County declined by an average of 1 percent a year from 1970 to 2000. And the Pittsburgh region's current carbon reduction goals are, conveniently, 1 percent per year through 2023. The difficulty here is that over the same three decades, Allegheny County lost one-quarter of its population and the bulk of its energy-intensive steel industry; that's what accounts for the overall decline in fossil fuel emissions. Per-capita emissions were actually unchanged. If continued economic contraction and depopulation were a policy prescription for the Pittsburgh area - and it's not - the targets set through 2023 might seem more achievable. More broadly, researchers at Carnegie Mellon University analyzed trends for energy use and fossil fuel emissions from 1900 to 2000 in Allegheny County. The team then paired these trends with historical changes in the local economy to shed light on the feasibility of the Pittsburgh area's present climate commitments. Hard study of the numbers gives one "a sobering pause," said H. Scott Matthews, a professor of civil and environmental engineering who co-authored the study with Rachel Hoesly, a doctoral student. This isn't going to be as easy as it looks. "We want the results to help people understand the magnitude of change that's required," Dr. Matthew said. He said he recognized the rallying effect of sweeping emissions reduction proclamations but that he was concerned that they often are no more than lip service. Pittsburgh and some 1,000 other cities have signed the Conference of Mayors" Climate Protection Agreement promoting strong and voluntary commitment to cuts in municipal greenhouse gas emissions. On average, participating cities have agreed to reductions of 1 to 2 percent per year. Across Europe, almost 4,000 cities have signed a similar agreement. "The effort required to make significant change is truly astounding," said Matthew Mehalik, a program manager at the nonprofit group Sustainable Pittsburgh, which coordinates local business and government sustainability initiatives. Dr. Mehalik, who has collaborated with Dr. Matthews, said he agreed with the paper's basic conclusion. "The grass-roots organic process alone is not going to cut it," he said of emissions reductions.