Image of New York City courtesy of Flickr user Paul Beattie under a Creative Commons license.

Cities present a kind of environmental paradox. The construction of all our roads, housing and office towers requires a lot of resources and energy. Collectively, we produce a ton of trash. And we generate a lot of pollution while we're at it (not to mention of the light and noise varieties, which are equally obnoxious to nature).

On the other hand, two families living in an apartment building likely consume less energy than two families in detached houses outside the city. And the more people who pack into cities, the more land we can conserve outside of them.

The most obvious illustration of the environmental benefit of cities is shown in the graph below (from a new OECD report that I am having too much fun with this week). It tracks carbon emissions per capita from ground transportation in several metropolitan areas around the world. The densest places have the lowest emissions; the least dense have the highest.

OECD. Area in the population density calculation excludes green space. Unit for emissions (tCO2e) are tonnes CO2 equivalent. Reference years are 2005-2011, depending on the city.

Many of the compact metros above with the lowest emissions also have better public transit. And all that density makes it possible for people to consume less energy — by sharing transit or traveling shorter distances — getting from home to work to the grocery store and back again.

This chart leads to a fairly obvious takeaway if you're into urban planning, but it's an important one for anyone who doesn't view cities through the lens of housing patterns and transportation networks:

This suggests that large cities are not high polluters per se, but rather that their impact on the climate and environment depends on urban form and the way they are organised.