Although most of the planet's man-made greenhouse gas contributions come from cities, that statistic is deceptive – the majority of the Earth's population live in urban settings. Per capita, the emissions of city dwellers is much lower than those living in more rural settings.

According to a recent study by the Brookings Institute focused on 2005 statistics, U.S. residents in the 100 largest metro areas were each responsible for 2.47 tons of CO2 emissions per year, compared to the national average of 2.87 tons.

The 14% reduction per person in cities is significant, and is largely due to population density and transportation choices. Those two factors go hand-in-hand: more compact urban settings make mass transit systems more effective and practicable. Walking and cycling between destinations become more viable alternatives as well, as commute distances shrink.

Researchers found that simple economics and weather patterns played a factor in cities' pollution levels. In California, where electricity and gas prices are higher, consumption and emissions decreased. Other variables include the source of electricity. Cities in Ohio, Kentucky, and Indiana – where coal is king – were heavy polluters.

Although cities may be the greener living alternative, the report also revealed an alarming trend: between 2000 and 2005, CO2 emissions in cities from transportation, electricity use, and home heating rose 7.5%. As with the rest of the nation, vehicle miles traveled increased in cities as well, though at a slower rate.

Much has changed since 2005 – predominantly in the past six months. Gas prices are way up and a possible economic recession looms. Mass transit ridership is soaring while car-driving miles continue to wane. Foreclosures are hitting the least sustainable, sprawling neighborhoods the hardest and car manufacturers and dealerships are finally shying away from SUVs.

When the Brookings Institute tallies up the data for 2008, I suspect city residents will continue to emit fewer emissions. Here's hoping we'll see the first real decline in annual pollution levels as well.

Source: Associated Press.

Photo via flickr by Creativity+ Timothy K Hamilton.

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