The United Nations has advice for city planners around the world: Move people closer together.
Designing more compact, higher-density cities is key to improving the well-being of the world's burgeoning urban population, said Under-Secretary General Joan Cloas, the executive director of UN Habitat and a former mayor of Barcelona.
He said too many cities are characterised by urban sprawl that make it harder for people to get around and get access to basic services, especially in vast slums where the poor live far away from their jobs, medical services and food stores.
Growing suburbs, meanwhile, discourage the use of public transportation, biking and walking, which in turns contributes to pollution through reliance on cars. And mega-cities are encroaching on farm land and environmentally sensitive areas.
Clos said the world's average urban population density is "extremely low" at an estimated 2,000-3,000 inhabitants per square kilometre. For comparison, he said Manhattan has a population density of 56,000 inhabitants per square kilometre.
In the world's highest-density city, Hong Kong, he said the figure is 96,000.
More than 50 per cent of the world's population lives in urban areas, a number that is expected to reach 65 per cent within 40 years, Clos said.
"It's a huge transformation in the life experience of a lot of humans. And this requires, political attention, economic attention, social attention," he said at a news conference ahead of World Habitat Day on October 7.
Clos acknowledged the difficulties involved in trying to build high-density city centres and offered no easy solutions.
In New York and Hong Kong, high-density comes with some of the world's highest living costs. But Clos said those are extreme examples. In Europe, he said, the average urban population density is more like 15,000 inhabitants per square kilometre. He said offering affordable housing involves difficult political decisions, perhaps replacing subsidise gasoline with more subsidised housing.
Clos said high-density need not be the same as overcrowding.
"What cannot be sustained is spontaneous urbanisation. When we have spontaneous urbanisation instead of well-designed, high-density cities, we have overcrowding," he said.
"And that is what is happening in the favelas, the slums and other places.
Thomas Elmqvist, a Stockholm University professor, said there is opportunity for planning well-designed cities. Sixty per cent of the world's land projected to become urban by 2030 has yet to be built, according to a new study titled Cities and Biodiversity Outlook, which involved more than 200 scientists.
Elmqvist, the scientific editor of the study, said 25 per cent of the world's protected areas are now within 17 kilometres (11 miles) of urban areas. He said in 10 years, it will be 15 kilometres (9 miles).
Clos is the director of UN-Habitat. He spoke yesterday ahead of World Habitat Day on October 7.
