Forget Curbing Suburban Sprawl

Building denser cities would do little to reduce CO₂ emissions, a new NAS report concludes.

Urban sprawl has rightly been blamed for contributing to increasing fuel consumption in the United States, since many commuters have little choice but to drive to work. But policies designed to make cities more compact will do little to reduce gas consumption by 2050, in time to prevent the worst effects of climate change, according to a new report from the National Academy of Sciences (NAS).

City sprawl: An aerial view of downtown Chicago. Credit: Wikipedia

Urban planners hoping to help mitigate CO₂ emissions by increasing housing density would do better to focus on fuel-efficiency improvements to vehicles, investments in renewable energy, and cap and trade legislation now being voted on in Congress, according to the study, released Tuesday. It concludes that increasing population density in metropolitan areas would yield insignificant CO₂ reductions.

Even if 75 percent of all new and replacement housing in America were built at twice the density of current new developments, and those living in the newly constructed housing drove 25 percent less as a result, CO₂ emissions from personal travel would decline nationwide by only 8 to 11 percent by 2050, according to the study. If just 25 percent of housing units were developed at such densities and residents drove only 12 percent less as a result, CO₂ emissions would be reduced by less than 2 percent by 2050.

Policy changes aimed at reducing emissions through city planning have to come at the local or state level. An exception to the nationwide trend of sprawling suburban homes is Portland, OR, where residents drive 17 percent fewer miles per day than the national average because of boundaries set on urban growth and a light rail system that both got their start in the mid-1970s. The state of California followed Portland's lead in 2008 by passing land-use policies with a goal of curbing urban sprawl, reducing automobile travel, and as a result, cutting statewide greenhouse-gas emissions by 3 percent by 2020.

One of the study's authors doubts whether major increases in housing density are even possible. "I think the 75 percent figures are completely unrealistic," says Anthony Downs of the Brookings Institution. "Twenty-five percent is much closer to realistic and that may even be high. Nationally we've had no increase in housing density in the last 30 years; I don't see that reversing."

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Downs points out that Portland is an exception to the national rule. "Portland is only one out of 350 metropolitan centers in the country that has strong transportation and housing policies directed at increasing population density. It's not exactly a groundswell movement," he says.

Changing local zoning rules to increase population density across the country would face a lot of opposition from homeowners without yielding significant emissions reductions, Downs adds. "It's an enormous amount of effort to achieve a tiny amount of outcome," he says. "If your principle goal is to reduce fuel emissions, I don't think future growth density is the way to do it."