(Photo used courtesy of PhotoA.nl @ flickr. Used under the Creative Commons lisence.)

I’d like to make a simple argument: that our world’s cities must play a vital role in the fight against climate change. Indeed, I think they already do.

But first, let’s put the progress of our world’s cities into perspective.

In 1900, only 160 million people, or one tenth of the world’s population, lived in urban areas. As of shortly after 2000, that number had grown twenty-fold to nearly half the world’s population, or 3.2 billion people.

According to UN projections, this process of urbanization is only going to accelerate. By 2025, the UN predicts 70 percent of the human population will be living in cities. By the end of this year, over half will live in urban areas for the first time in human history.

Similarly, in 1950, the only city to exceed 10 million people was New York City. By 2015, of the 44 cities with 5-10 million inhabitants, as many as 39 of them will be in developing countries – the areas most poorly equipped to handle the challenges presented by a changing climate.

In contrast, the wealthiest 25 percent of the world consume 80 percent of the world’s economic output. Of this 25 percent, more than 80 percent live in cities. And so it is that cities consume 75 per cent of the world's energy and produce 80 per cent of its greenhouse gas emissions -– most of these from the Northern Hemisphere.

This represents an enormous shift in the history of human civilization. For a majority of the 2 million year history of our species, we lived in caves, villages and suburbs. Now, for the first time, our environment will be a built, rather than an organic, environment.

What do we take from this?

Through the lens of increased urbanization, matched by increased concentrations of greenhouse gas concentrations, we see a world much as former President Bill Clinton described his vision of Rwanda, in his acceptance of the TED Prize. “We live in a world that everyone knows is interdependent but insufficient,” he said. Insufficient because it is “profoundly unequal.”

It would be easy to accept these as unavoidable conditions of urban development. Urban development has spread rapidly, population growth has continued apace and, many argue, affluence has bred consumption which has fueled degradation.

Furthermore, some will argue that global problems must be met with global solutions. They will also argue that the scope of multi-nationals and the concentrated power of federal governments mean that the emission targets of cities can be easily circumvented or easily overruled. Cities, according to this logic, are either besides or below the point.

But it this really true?

It is my belief that this position -– this conviction that cities are confined to the squalor of trash and not to the splendor of trees -- defies our better understanding of cities’ historic role. They are barometers of progress and, as such, reflect the vitality of their home country.

Consider the growing international interest in local climate networks.

In the US, for instance, there is the US Mayor’s Climate Protection Agreement. Created in 2005, more than 800 mayors have signed. Much the same is true in the UK. Since its creation in 2000, over 300 local authorities have signed the Nottingham Declaration. Ken Livingston, former Mayor of London, launched the Energy Strategy for London. Michael Bloomberg, Mayor of New York, has launched a similarly ambitious program.

Outside of these national networks, there are a serious of international networks as well. Included in this list is the ICLEI, as well as the CCP program, which now includes 675 local authorities. There is also C40 of global cities, representing a commitment from 40 of the world’s largest cities to tackle climate change. It is perhaps The Climate Alliance, which has more than 1,300 members in 17 European countries, that reflects the most international example of locally-based alliances.

Given the global consequence of localized environmental problems, these networks provide a multi-dimensional strategy that fills the strategic gaps left by one-dimensional regimes. At best, these networks provide effective depth and breadth – what the academics Michele Betsill and Harriet Bulkeley call “vertical tiers of government and horizontally organized form of governance.”

It comes down to a simple truth: more people are moving into ever-larger cities. For the first time, more people live in urban areas, than don’t. This presents two conflicting challenges.

One, to reduce the personal impact of urban environments on local citizens. And two, to reduce the diffuse impact of urban centers on our global environment.

This means working with environments traditionally disassociated with the natural world, and recalibrating our understanding of the locus of global influence away from the affluence of national governments to the main and often gritty streets of global cities. It is not that we need either local or national governments. We need both –- and we need both to be on their best behavior.