If the concept of a sustainable city sounds like a paradox, that’s because it is, according to physicist Geoffrey West.

Ironically, because of their urban “metabolism,” cities require only 85 per cent of the resources necessary to double in size, and they’re more energy efficient than rural communities.

A UN-HABITAT report State of the World's Cities 2010/2011 found that cities’ density and economies of scale provide more benefits to the environment than rural living— and could even reverse the impact of climate change by reducing per capita emissions.

This seems counterintuitive to the agrarian revolutionary who believes a return to rural communities is a solution to climate change. Urban living may increase our problematic reliance on destructive factory farming, but increased worldwide migration to cities is helping curb population growth because there is no need for large families for labour.

“When villagers migrate to the city, their family size drops, on average, by at least one child per family, often below the steady population rate of 2.1 children,” writes Doug Saunders in his recent book, Arrival City. “Without massive rural-to-urban migration, the world’s population would be growing at a far faster pace.”

As more people move to cities to find work, even though their family size shrinks, consumption rears its head. More people means a clamour for more food, energy, and products. According to West, the only way to keep the unsustainable urban machine going is the innovation produced by cities that constantly finds new resources to exploit.

The upshot is if we want to live in a sustainable world, we’ll need bigger cities, and more of them. As a physicist who applied his training to the study of urban environments, West believes we need more megalopolises.

But as we found in the fifth annual Corporate Knights Sustainable Cities ranking, not all cities are created equal.

We studied 28 indicators of sustainability in five categories—ecological integrity, economic security, infrastructure and built environment, governance and empowerment, and social well-being. Seventeen Canadian cities were surveyed, giving us a picture of the country’s urban sustainability.