From John Shannon who writes about green energy, sustainable development and economics. Email him at john[AT]borderstan.com.

A massive shift in demographics has taken place in the past few decades as millions of people leave rural lands and move to ever-expanding cities. This phenomenon is not limited to any one continent, it’s happening everywhere.

Today, more than half the world’s population live in cities and close to 80 percent of global economic and environmental output is tied to cities — as opposed to rural or undeveloped areas. The migration of people towards cities looks set to continue for some time.

And not alone in this is New York City, where the Big Apple’s three-term Mayor Mike Bloomberg recently commented about the NYC experience.

“Recent college graduates are flocking to Brooklyn not merely because of employment opportunities, but because it is where some of the most exciting things in the world are happening — in music, art, design, food, shops, technology and green industry. Economists may not say it this way but the truth of the matter is: being cool counts. When people can find inspiration in a community that also offers great parks, safe streets and extensive mass transit, they vote with their feet.”

“New York has an impact on the rest of the world” — according to Bloomberg. “The environmental stuff, whether or not it reverses climate change, influences you and your kids today–the air you breathe, the water you drink, the economy you have, the opportunities you have. Today. That’s how you sell the environment. Don’t try to sell it for 40 years from now. Think ‘my air, my water.’ And cities are where it’s all going to happen,” according to Bloomberg.

Acting in their own self-interest, a growing list of cities are joining together to find and share new environmental and economic solutions via the C40 Cities Climate Leadership Group (C40), which “is a network of the world’s megacities committed to addressing climate change.”

Acting both locally and collaboratively, C40 Cities are having a meaningful global impact in reducing both greenhouse gas emissions and climate risks. Through a partnership with the Clinton Climate Initiative, C40 brings together a unique set of assets and creates a shared sense of purpose. C40 offers cities an effective forum where they can collaborate, share knowledge and drive meaningful, measurable and sustainable action on climate change (C40).

As cities now comprise more than half of the world’s total population, which is slightly more than 7-billion as of January 2013, it makes sense for responsible and visionary leaders to find solutions to common problems and to find better ways to enhance the enjoyment of citizens. From Vancouver, Canada, to Washington, DC, to Copenhagen, Denmark, mayors and other civic leaders are at work improving the present model for more than three billion city-dwellers worldwide.

• Explore an interactive list of the C40 Cities and their current initiatives. (C40)
• Explore our latest research further to learn more about our work. (C40)

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