Last week, Time magazine asked, “Why Are Southerners So Fat?

There’s no simple answer, of course. Poverty, culture and climate all play a role in the South’s high obesity rates. But one factor that’s increasingly blamed by everyone from medical journals to the CDC is how Southern cities are built.

“The South doesn’t have many bus stops,” Time writes. “Public transportation is paltry, and for most people, the best way to get around is by car. … States like Mississippi and Tennessee also have a surprising lack of sidewalks, discouraging even the most eager pedestrians. Many roads are narrower than those in the North — where streets have wider shoulders to accommodate winter snow — and people who want to bike or jog find themselves uncomfortably close to traffic.”

All of which speaks to the fact that cities matter — to our health, as well as to the health of the planet. When we think of the environment in this country, we generally conjure up images in our mind of cuddly wildlife and pristine wilderness — the kind of things that we go on vacation to see, not what’s around us every day. But how we build our cities can play a very important role in preserving and protecting the environment.

“When it comes to global warming,” Time says, “green acres aren’t all that green — life in the crowded city is actually much more climate-friendly.”

There’s a tendency in America to believe that everyone wants to live on two-acre lots in the suburbs, but city living has made a comeback in recent years, in part because cities are working to improve quality of life and sprawl is turning out to be not-so-sustainable or desirable to many people.

Well-designed transportation systems, mixed-use development, progressive planning, energy and water conservation, recycling programs, open space preservation — all of these factors can help make a city more friendly to the environment and more livable for its residents.

A new website known as Smarter Cities, which launched earlier this month, aims to highlight the potential of cities to help reshape the environment responsibly. The site grew out of the Smarter Cities Project, formerly part of National Geographic’s Green Guide and now affiliated with the Natural Resources Defense Council.

Smarter Cities ranks communities across the country with a population of 50,000 or more on criteria of sustainability and livability. The data is collected and crunched with the help of a researcher from the Yale School of Forestry and Environmental Studies.

The result: “One of the nation’s most comprehensive and robust databases of U.S. urban progress toward sustainability,” according to the Smarter Cities site.

So how green is your city? It will probably come as no surprise that among the nation’s largest metropolises, perennial greenies such as Seattle, San Francisco and Portland topped the list (although you might be surprised at some of the other names in the top 15). Madison, Wis., is the top medium-sized city, while Bellingham, Wash., gets the small city nod.

Smarter Cities is far from the only attempt to identify the nation’s greenest burgs, and not everyone is going to agree. The criteria used, how they’re weighted, studying cities vs. metro areas, etc., can all make a difference. So while the rankings can be fun, it’s more important to look at what they’re based on and get a sense of what your city is doing right — and where it needs improvement.

Is your city on the right path?