A couple of months ago, I criticized the notion that suburban sprawl actually improves air quality by reducing density. However, I missed one pro-sprawl argument: the claim, based on an Australian study [pdf], that apartments (especially high-rise apartments) have higher per-capita greenhouse gas (GHG) emissions than houses. Does this study mean that sprawl pollutes less than compact development? Probably not, for several reasons.

First, walkable urban development need not require high-rise apartment buildings. In fact, low-rise urban areas can be quite dense. For example, Brooklyn's Park Slope neighborhood is dominated by rowhouses and other low-rise buildings- so much so that only 15 percent of its housing structure have five or more units. Yet Park Slope has just over 64,000 people per square mile (roughly twice the Brooklyn-wide average), and the overwhelming majority of its residents do not drive to work.

Second, the Australian study may be of limited uses because of its miniscule sample size. It studied only 17 high-rise buildings, 12 mid-rise buildings and 10 low-rise buildings. As a result, the authors wrote that their study’s results "are not statistically representative of… multi-unit residential buildings" even in Sydney, Australia where the study was conducted, let alone in the United States.

Third, evidence from Australia may not be tremendously relevant to the United States, because personal vehicles account for only 10 percent of Australian GHG emissions, as opposed to 25 percent of U.S. GHG emissions. As a result, development patterns that reduce driving are likely to reduce overall per-household emissions to a greater extent in the United States than in Australia.

Fourth, because the study focuses on per-capita emissions its findings are only valid as long as household sizes do not change—that is, as long as apartments are more likely than houses to be dominated by single people. But if development becomes more compact and multifamily dwellings become more popular with families, per-capita emissions for apartments will decline, because each new resident does not significantly increase the overall emissions for a unit. For example, if a two-bedroom apartment is occupied by a family of three instead of by one person, their apartment building is unlikely to need additional elevators or hallway lighting.