Recently, Bob Silvestri, a proponent of auto-oriented, low-density development, argued that auto-orientation is more energy efficient than person-orientation and, therefore, superior. Yet his data, while implying that New York City or Paris are terrible polluters, does not support his thesis that Marin is the pinnacle of environmental quality. That's not to say his data doesn't have problems (it does), but let's take the assumption that he's measuring the right things and that the studies he cites are unimpeachable.
Houses, density, and greenhouse gases
Silvestri cites a (rather flawed) study (PDF) of greenhouse gas emissions per unit for a number of housing types, from high-rises to detached homes. Single-family detached homes were scored second best for all emissions in the Australian suburbs studied, with only town houses scoring better.
Simultaneously, Silvestri makes the quite important point that open space is a carbon sink. It's undeniable that the more open space we preserve as a region, the better off we’ll be from a sequestration standpoint. The EPA says open space takes in 2.5 metric tonnes of CO2 per acre per year (MTCO2/year), agricultural or recreational land takes in 1.5 MTCO2/year, suburban land takes in 1 MTCO2/year, and urbanized land takes in 0.2 MTCO2/year. Town homes, which lie somewhere between urbanized and suburban land, still leave plenty of open space in the back yard (often 50 percent). We need to estimate, but let's put that as 0.7 MTCO2/year. I will assume these numbers take into account commercial development patterns as well.
Silvestri measures San Francisco's net emissions against Marin's net emissions, but that's not the way to evaluate optimal conditions. It unfairly punishes San Francisco for having small political boundaries and rewards Marin for having expansive boundaries. Rather, we need to establish a baseline of nature and determine how different methods of development will change the carbon status of the same land area.
Two towns
So, we have 640 acres (1 square mile) of virginal open space producing a net negative 1,600 MTCO2/year. We'll people that with 100 households in a traditional suburban setting of about 4 homes per acre, which again will include commercial development. Using the average household size in the US, that means 259 people on 25 acres.
According Silvestri's Australian data for per-unit emissions, people living in suburban areas emit about 2.5 MTCO2/year apiece. With 259 people, our little town emits 647.5 MTCO2/year. Subtracting our sequester, which is now 615 acres of virgin land and 25 units of suburban, our square mile goes from a net negative 1,600 MTCO2/year to a net negative 915 MTCO2/year. Not too shabby.
Next door, another 100 families has set up shop on another square mile of land, but, inspired by Europe, these guys want a village of town houses at a relatively loose 25 units per acre. Rather than 259 people on 25 acres, this village will only use up 4. Since town home denizens pollute less than suburbanites, they're only emitting 518 MTCO2/year. Since they're living on less than a sixth the land area, there's more virgin open space to absorb their footprint. All told, the village goes to a net negative 1,075 MTCO2/year.
This other village, of course, will reap the other benefits of compact development. They will need to maintain fewer fire stations, fewer roads, fewer pipes, etc. Changes to travel patterns will mean less driving over the baseline and more walking, bicycling, and more transit users. That means they won't have to maintain large parking lots or such wide streets (which means more environmentally friendly stormwater management), and the citizens won't need to go to the gym to stay healthy.
As a bonus, with the money saved (and it would be substantial), they could electrify the whole transit system, rendering moot Silvestri's argument that transit as too carbon-intensive. Then again, a townhome-style city is ideal for cycling and walking, so there wouldn't be as pressing a need for transit anyway.
Far from supporting single-family housing, Silvestri should be supporting the kind of densities town homes provide, which can go as high as 60 units per acre. They are far more financially and space-efficient and less carbon-intensive than single-family homes. That's in his data, clear as the day.
I don't know why Silvestri would try to twist the data into saying something it doesn't, but the study itself does the same thing. The author, Wendell Cox, has done some good research on cities but has come to some odd conclusions: that suburbia as we know it is the result of free-market choices (it isn't, and is instead the result of $450 billion in annual federal subsidies) and that Seattle's suburbs are growing much faster than the city proper (they're not). I've found it's best to approach anything associated with Cox or his firm, Demographia, with a healthy dose of skepticism.
I hope Silvestri will join me and other urbanists in support of the kind of infill development that he has championed in the past. It offers a much better path to lower greenhouse gases than the Santa Clara-style sprawl his ideas advocate in the farmland and open spaces of Napa, Solano, and Sonoma.