When world leaders gather in Paris in December to negotiate a global agreement on climate change, they will have a powerful ally standing behind them and urging them onward: mayors. In fact, mayors have already helped set the stage for success in Paris, by establishing models of cooperation that provide a strong foundation for the negotiations.

Through international coalitions like the C40 Cities Climate Leadership Group and the Compact of Mayors, cities around the world have been committing to major reductions in carbon emissions, while adopting a common measurement system and public reporting process to ensure that they can be held accountable. In other words: Many cities are already doing what the United Nations is urging national governments to do.

Climate Change

Mayors have had good reason to take a leading role in this work. Urban centers are now home to half of the global population, and are responsible for an estimated 70 percent of annual greenhouse-gas emissions. Cities are not only a big part of the cause of climate change -- they will also bear the brunt of the effects. Ninety percent of cities are built along rivers and coastal plains, which are particularly vulnerable to sea-level rise and extreme weather, both of which are byproducts of climate change.

Necessity is the mother of invention, but mayors have other powerful incentives to take immediate action to curb emissions. Energy efficiency measures save taxpayers money and save innocent people from respiratory and other diseases. A 2014 study by the Clean Air Task Force found that five years ago, U.S. coal pollution was linked to the deaths of 13,000 people a year. Today, thanks in part to President Barack Obama's leadership in helping to move old, heavily polluting coal plants offline (work that Bloomberg Philanthropies has long encouraged), that number is down to 7,500.

Mayors also have economic incentives to act, because people want to live in cities with clean air. And where people want to live, businesses want to invest. The more cities can attract talented and hard-working people, the more their economies will grow.