> So at least for carbon footprints, those don't necessarily seem to be linked to density.

Every study I have seen shows the opposite, that carbon footprints are strongly linked to density and urban-suburban divisions. In 2012, for example, climate researchers at Berkeley put together a map for the entire United States showing average household carbon footprint by zip code [0]. They found that suburban emissions are markedly higher than emissions in their respective central cities almost everywhere, confirming a result that Edward Glaeser and other urban economists had already established [1].

The FAQ [2] for the Berkeley researchers' paper [3] summarizes the headline finding like this:

There is an inverted-U relationship between population density and HCF [household carbon footprint]; HCF increases ... from low to medium population densities, and decreases from medium to high population densities. The turning point is about 3,000 persons per square mile, which is very close to medium population density of all locations, and a little higher than the population density of larger suburbs (which have densities of 2,700 persons per square mile).

In other words, low- and high-density places (rural areas and cities) have low household carbon footprints and middle-density places (suburbs) have high footprints. Controlling for income, the authors found that the inverted U goes away and the relationship becomes more monotonic: population density is negatively correlated with carbon emissions across the spectrum of density levels. The upswing of the inverted U-curve that you see before controlling for income happens because, as American communities are actually arranged, middle-density areas (which are relatively dense suburbs) tend to be wealthy, and emissions are also strongly correlated with consumption; the consumption effect dominates the efficiency gains due to higher densities as you go from rural areas to suburbs, and it's not until you get to city-level densities (and poorer urban populations) that the efficiency gains due to density win out over consumption-driven emissions.

So part of the reason that suburbs have high emissions is that the people who live there are relatively rich, and the richer you are the more stuff you consume. But no matter how rich or poor you are, the study suggests, you will likely have a noticeably lower carbon footprint if you live in a city and pursue a lifestyle typical of your income class there rather than doing the likewise in a suburb.

The authors also found, by the way, that suburban sprawl around the central cities of large metropolises effectively offsets the carbon-efficiency of the dense cities themselves [4]; the larger the central city, the greater the sprawl surrounding it, and the number of inefficient suburban households tends roughly to balance out the carbon efficiency of households in the central city.

0. http://coolclimate.berkeley.edu/maps

1. See, e.g., http://www.city-journal.org/2009/19_1_green-cities.html

2. http://coolclimate.berkeley.edu/files/coolclimate/Jones-Kamm... [PDF]

3. http://pubs.acs.org/doi/abs/10.1021/es4034364 [PDF]

4. http://newscenter.berkeley.edu/2014/01/06/suburban-sprawl-ca...