There are two principal reasons for this. First, the decisions made by mayors over the next 5 to 10 years—about the pattern of their cities' development—will lock us into a high or a low carbon path. It's a question of simple math: half of the world already lives in cities and that number is rising, while 70 percent of greenhouse gas emissions are the product of urban consumption.

Seeing the change

But cities can get off that high carbon pathway if they take some basic measures: using urban planning to cut sprawl, enabling citizens to shift from private vehicle use to public transit, cycling and walking, investing in more energy efficient buildings and reducing waste. 

Research shows that big city mayors around the world tend to control many of the levers necessary to move in this direction. For example, London's mayor used road pricing to reduce emissions and addressed the congestion crisis facing the city; in Copenhagen, one of the wealthiest cities in the world, the city’s success in promoting cycling means 40 percent of its population cycles to work, not for environmental reasons but because it makes economic sense and promotes healthy living.

United we stand?

The second reason that cities are so pivotal to getting the world onto a low carbon pathway is because city leaders have shown a far greater propensity to work together to tackle climate change than their counterparts in national government. That's one reason the Pope called a gathering of leading world mayors this summer.

"Action by city leaders, business and other non-state actors will be crucial to closing this emissions gap."

The inter-governmental climate negotiations have been stymied for two decades, but in 10 years of working together, cities in the C40 have doubled the number of climate actions taken. For example, through its waste program ‘One Bin Fits All’, Houston residents can place all their trash, recyclable or otherwise, in the same bin. The city estimates the scheme could help divert 55-75 percent of its waste from landfill and now many other cities are studying Houston and thinking of copying the idea.

New York City’s “One City Built to Last” energy efficiency program aspires to cut carbon emissions by roughly 3.4 million metric tons equivalent annually by 2025. This initiative will also create an estimated 3,500 construction-related jobs and generate $8.5 billion in total cost savings for New Yorkers.

The next step

When the world's national government leaders gather in Paris in December, we all hope and expect that they will agree upon a new climate treaty. A deal of some kind now looks more likely and this will be a cause for celebration, considering how difficult the process has been.

No one expects that nation states will make significant enough commitments to get even close to keeping global temperature rises below 2 degrees, the level that scientists say would enable us to stay within an acceptable margin of risk of avoiding runaway climate change. Action by city leaders, business and other non-state actors will therefore be crucial to closing this emissions gap. Meeting with Pope Francis this summer, C40 city mayors made clear they stand ready to step up to the task.