From one international summit to the next, I can't help but notice a steady and discernible trend: National politicians are finding it ever harder to make their voices heard.

That's partly because there are more of them at the table: Once it was the G7, now it's the G20. Add the NGOs and the regional organizations, such as the EU and ASEAN, as well as all the emerging markets raising their voices. Don't forget the world's largest businesses that are rightly calling for a place at the negotiating table, too.

It's no different here in Cancun at the COP16 global climate change talks --  except in one regard: Cities. We are now joined at climate summits by numerous mayors and metropolitan leaders, and in fact, the C40 group of mega-cities met in Cancun two weeks before the summit to claim a seat at the table and demonstrate their willingness to commit to emissions reductions, no matter what. I was speaking last weekend at the World Climate Summit alongside the Mayor of Technology and Environment from Copenhagen and a city councilor from Vancouver, both cities that are leaders in the Siemens Green City Index.
 
In 1800, 3 percent of the world's population was living in cities. We're well past the 50 percent mark now, and since last year's climate talks in Copenhagen, approximately 44 million people have been added to the global total. Well over 300 million people will move from rural areas to cities in China by 2025, the equivalent of the entire population of the U.S. migrating in less than 15 years.

Some people would blame urbanization for climate change. Technically that's true: Cities are centers of industry and consumption, and produce more than 60 percent of the world's GHG' emissions, and probably up to 90 percent by the end of the century. Others see cities as mere victims. With 600 million people now living in urban areas at risk of rising sea levels, it is true that Mumbai, Jakarta, New York and others are increasingly vulnerable.

Cities, however, are also the place where so much of the solution lies. That's why mayors are making their voices heard at the Cancun climate summit. They are committed to finding sustainable solutions and are keen to criticize national governments for not helping them to succeed by continuing, for example, to subsidize fossil fuels.

Beyond their vulnerability to environmental changes, cities have a key motivation: Their attractiveness is at the heart of their competitiveness. They need to draw talent and investment in a world where capital and labor are footloose. And for many emerging markets, cities carry the responsibility for the national economy, such as Sao Paolo, which accounts for 40 percent of Brazil's GDP.