Urban growth and climate change.

• Author: Kahn, Matthew E.
• Position: Research Summaries

My recent research focuses on the implications of urban economic growth for greenhouse gas production. I also examine how the quality of life in different cities around the world may be affected by climate change.

Low Carbon Cities

Edward L. Glaeser and I ranked U.S cities with respect to their household greenhouse gas emissions. (1) Using micro data on household consumption of transportation, electricity, and home heating, we document significant differences across major cities. For example, if the average household chose to live in Houston versus San Francisco, it would produce roughly 16 more tons of carbon dioxide each year. San Francisco is ranked as a "greener" city because of its temperate climate, which means that households there use less electricity, the region relies more on natural gas rather than coal for power generation, and this lowers the electric utility's emissions factor. San Francisco is also more compact than Houston, with more of the metropolitan area's total employment located downtown. The suburbanization of employment has contributed to a rising carbon footprint. When people work in the suburbs, they are likely to live in the suburbs, and to live in a larger home and rely on a private vehicle for transportation. (2) Policies such as declining center city crime and improved urban public schools help to shrink a city's carbon foot

print because they encourage densification and living closer to the city center. (3)

Today, households who live in Daqing--China's "brownest city"--produce only one fifth of the emissions of households who live in San Diego, the U.S.'s "greenest city". Building on my U.S research, I recently ranked the household carbon footprint for 74 Chinese cities using high quality data from the 2006 Chinese Urban Household Survey. (4) In the case of China, the dirtiest cities are to the North where coal is used for winter heating. These results are significant because Chinese regional economic development policy is encouraging growth in the Northern region to deflect growth away from the mega cities along the eastern coast. If Northern China's cities continue to rely on coal for heat rather than substituting to natural gas, then this spatial trend could have significant aggregate carbon implications.

Durable Capital

Over time, new versions of products such as the Toyota Prius or Tesla electric car or Zero Net Energy homes will have much smaller carbon footprints than earlier makes of cars and homes. Such "green" products often represent a tiny share of the existing capital stock because cars and buildings are long lived capital. This means that it can take decades for average energy efficiency of homes or vehicles to improve.

I examine the consequences of durable capital for the greenhouse gas mitigation progress in one study with Lucas Davis. We collected detailed vehicle-level information on the scale and composition of used vehicles exported by the United States to Mexico under NAFTA. (5) As it turns out, NAFTA can be viewed as an early "cash for clunkers" program. While it provided cash for U.S households, this type of trade can have detrimental environmental consequences. Even though Mexican households drive fewer...