This post originally appeared on Orphan Road.

Wired has a fascinating article this month on Dongtan, a “green city” of 500,000 residents (roughly the size of Seattle) being planned for an area near Shanghai. The London-based urban design team working on the project made some interesting discoveries in their efforts to build an environmentally-friendly city:

The team found research on energy consumption in cities around the world, plotted on a curve according to population density. Up to about 50 residents per acre, roughly equivalent to Stockholm or Copenhagen, per capita energy use falls fast. People walk and bike more, public transit makes economic sense, and there are ways to make heating and cooling more efficient. But then the curve flattens out. Pack in 120 people per acre, like Singapore, or 300 people, like Hong Kong, and the energy savings are negligible.

For reference*, based on numbers I grabbed from Wikipedia and some quick calculations:

Portland, OR = 6.5 residents per acre
Seattle = 11
Vancouver, BC = 21
Copenhagen = 24
Stockholm = 41
New York City = 42

Wikipedia has limited data on Stockholm, so take those numbers with a grain of salt. In either case, this stuff is important to keep him mind when we have heated conversations about density. Not all densities are created equal, and it seems clear that adding a few more residents per acre could cut Seattle’s per-capita energy use dramatically without turning us into Manhattan.

*Wikipedia separates the land within the city limits from the water. I used the land-only numbers, since you can’t build on water. I don’t know if that’s what the Dongtan guys did or not. I suspect they, too, used land-only, since NYC’s density drops to 27 res. per acre if you count all the water.