The authors of this study, published in The Journal of Urban Planning and Development, quantified the emissions from building materials and construction, home heating and power demands, and transportation energy, in both urban suburban neighborhoods in the Toronto metro area.  And they found that downtown residents use radically less energy, and consequently emit about two-thirds less climate-warming CO2 than their suburban counterparts.  Take a look:Norman Study Graph

First, they found that the biggest difference between city and suburban living is in transportation emissions. Compared with center-city dwellers, suburbanites used 3.7 times more transportation energy. Low density development requires more and longer car trips compared to higher density areas. Even transit is more carbon-intensive in suburbs.

Second, the greenhouse gas impacts of building materials—all the wood, steel, concrete, glass, and the like—are tiny, compared with the energy used in resident’s daily lives.  In fact, the “embodied energy” in the building materials  isn’t much more than a rounding error. It’s still important to reduce the environmental impacts of building materials and construction, but  when weighing the long-term climate impacts of new construction, the most important consideration is how we live once we’re in the buildings. That means the energy that goes into construction may be less important than the location of the building.

To, me, the most interesting thing about the study is the relative importance of transportation to GHG emissions. Whether the emissions are counted per square foot of living space or per person, low density development generates more emissions and requires more energy in large part because of residents dependence on car trips and longer, diesel-burning transit trips. The study suggests that the kind of transit trips are important with high density dwellers relying on shorter electric powered street cars or subways rather than longer bus trips.

Norman Study Figure 6

From the study:

It is notable that the overall trend between densities has not been fully reversed by changing the functional unit, which suggests a high level of overall energy use and GHG emissions intensiveness for low density development. This is largely due to the significantly higher level of automotive transportation emission and energy use association with low density development compared with high density development.

So while the study has its limits—it compares just two neighborhoods in a single city– it points, as other studies do, to the evidence that sprawl and car dependence are closely linked, and are responsible for a disproportionate share of GHG emissions.