Editor's Note: This post, originally published for Huffington Post, was co-authored by Stephen M. Ross, Chairman and Founder, Related Companies.

The world's cities are growing by 1.4 million people every week. This remarkable demographic shift is creating densely populated communities that can be centers of innovation and economic opportunity. These cities can also be healthier for people, with cleaner air, and more resilient to climate change, but only if urban leaders with real influence have the vision to meet the challenges of the 21st century.

New York, which will host the UN Climate Summit on September 23, has been a leading example of a city with a sustainable development strategy, PlaNYC -- initiated by former Mayor Michael Bloomberg -- and a dedicated office to make the city more resilient to extreme weather and other climate impacts. Over the past decade, New York's economy has surged, while its CO2 emissions have dropped 19 percent below 2005 levels, putting it ahead of schedule to achieve its emissions reduction target of 30 percent by 2030.

But not all cities are as forward-looking as New York. Many are expanding at their fringes, sprawling out instead of growing up, building on land that is less expensive, but with a greater environmental impact. The result can be built-in inefficiency: more traffic congestion and air pollution, and expensive, unsustainable water and power systems. The urban infrastructure built today could last for a century, locking in a dangerous, high-carbon pathway.

The challenge is great. Cities are at the forefront of the climate change: 23 of the 25 largest U.S. cities are near coastlines, where rising sea levels, storm surges, and extreme weather pose major risks. We all saw how Superstorm Sandy exposed the vulnerability of even a great city like New York.

Cities also use about 70 percent of global energy and produce 70 percent of energy-related greenhouse gases. And many cities are growing with unstructured development which brings massive economic, social, and environmental costs. In the United States, urban sprawl costs an estimated $400 billion a year for public services like water and waste, greater capital investment requirements for roads and other infrastructure, and increased congestion, accidents and pollution.

If well-designed, though, cities can become centers of innovation and technological advancement. Encouraging examples of thriving economies can be seen around the world, including in Stockholm, Portland (Oregon), and Singapore.

A just-released report, Better Growth, Better Climate: The New Climate Economy, by the Global Commission on the Economy and Climate (WRI is one of eight research partners) found that the world can save $3 trillion in the next 15 years by building cities that are more compact, efficient, and connected.

Clearly, any solution to climate change needs to have cities at its heart. The UN Climate Summit will have a major focus on cities, with hundreds of national and city-level leaders expected to make commitments that will accelerate the transition to a low-carbon future. One such example is an announcement around a new Compact of Mayors, an initiative that will drive deeper reductions in urban emissions.

The first step to reducing emissions is measuring them. Underlying the Compact is an effort by WRI, the C40 Cities Climate Leadership Group, and ICLEI to create a new common standard to measure city-level emissions. At least 60 cities around the globe have already used this system, identifying the main drivers of their emissions and opportunities to reduce them. This is just one example of the innovations we need to design and build cities in a new way.

It's time to shift the paradigm on urban development. But this is only truly possible through strong leadership and real commitments to make the change a reality. Let's hope leaders seize the opportunities today to shape more sustainable, prosperous cities of the future.

The two authors have recently partnered to create the WRI Ross Center for Sustainable Cities, an initiative that will galvanize action on sustainable urban development and improve the lives of people around the world.