Half of the world’s population now lives in cities. By 2050, it is estimated that this ratio will grow to over 75%. Put another way, 3.5 billion people live in cities now. Global population is project to reach 9 billion by 2050. That means that cities will need to the housing and infrastructure to absorb upwards of 2.5 billion people within the next 30 years. It is a staggering thought, and one of the great challenges of our time.

An excellent new film on this topic is Urbanized, a documentary by Gary Hustwit. Tackling such a broad subject of urban design in 90 minutes is an ambitious endeavor, but Hustwit largely succeeds.

Viewers are transported around the world, from the slums of Mumbai to the deserted streets of Detroit, where solutions of all scales are presented. From bus rapid transit and bikeways in Bogota to a guerilla art project in New Orleans, the key message from Hustwit is that people and the social fabric are what make cities great, and some of the best solutions to urban sustainability come from deep community involvement.

The film features interviews with some of the heavy hitters of urban planning and architecture, such as Rem Koolhaas and Sir Norman Foster. Many of the cities highlighted are familiar – for example the bike networks of Copenhagen or the High Line in New York. But the film also presents unfamiliar urban solutions, such as novel concepts for subsidized housing in Santiago, Chile and participatory community design of walkways in the townships on the edge of Cape Town.

With cities responsible for upwards of 80% of global greenhouse gas emissions, they have a major role to play in climate change solutions. Luckily, urban residents generally have a smaller carbon footprint than suburban and rural residents. By implementing greater building and energy efficiency improvements, cities could become a model for solving climate change. And guess what, all of that urban fabric will have to be planned and designed.