New research aims to create accurate account of a city’s carbon burden

With more than half the world’s population now living in cities, it is becoming crucial to determine how curbing greenhouse gas emissions from cities can help mitigate global warming. Densely populated areas are capable of impressive energy efficiencies per person, but cities on the whole are major contributors to the human-propelled greenhouse gases that cause global climate change.

Many cities are developing plans and policies to cut their carbon emissions. But their efforts have been hampered by the lack of a reliably accurate, standardized methodology for inventorying their total emissions, as well as reliable data on energy use.

New research led by Christopher Kennedy, a professor of civil engineering at the University of Toronto, takes steps toward establishing that methodology by examining why and how emissions differ between cities. In a recent ES&T paper (2009, DOI 10.1021/es900213p), Kennedy and his colleagues argue that a combination of geophysical factors, including local climate and technical factors, play a major role in assessing a particular city’s total emissions.

The paper investigates greenhouse gas emissions across seven components central to the “urban metabolism”: electricity, heating and industrial fuels, industrial processes, ground transportation, aviation, marine activity, and waste management. The ten urban areas Kennedy and his team have examined—ranging in population from 432,000 to 9,519,000—are: Los Angeles County, Greater Toronto, Canton of Geneva, Greater Prague, Cape Town, Denver City and County, New York City, Greater London, Barcelona, and Bangkok. Both emissions occurring within a city and the upstream life-cycle emissions for fuels used within the cities were calculated.

“This paper is certainly an advancement in knowledge in the area” of assessing greenhouse gas emissions from cities, says Peter Marcotullio, a specialist in urban environments and development at Hunter College of the City University of New York. “The big takeaway messages that are really interesting for me are that it’s not the absolute amount of energy that’s consumed that describes heterogeneity between cities, but what they call the fuel mix,” he says. “In terms of greenhouse gas emissions and the effect on the planet, it’s really the type of fuel you’re using. I think that’s significant, although intuitive.”

Another significant finding, says Marcotullio, is that a city’s relative level of prosperity plays a much smaller role than the city’s climate in total greenhouse gas emissions. “There is a major theory out there that uses income, or GDP, to distinguish between the environmental burdens of cities,” he says. By comparing the number of “heating degree days”—days on which the temperature falls at or below 18.0 °C—across different cities, the researchers found more similarities in energy use than the average income of its residents.

“That this study says [income] just isn’t that important is interesting, and potentially very controversial,” Marcotullio says.

Lily Parshall, a Ph.D. candidate in sustainable development at Columbia University, says that a major challenge for assessing emissions in cities is the lack of comparable data. The biggest contribution of this study “is trying to develop consistent comparisons across a number of cities in different parts of the world, using the best available data,” she says.

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