CITIES will save humans - we just need to lose the family car, have less kids, and move into apartments to achieve a sustainable population.

The founder of international talkfest TED Chris Anderson told a Sydney audience last night the world's population was set to rise to 10 billion within the next 70 years, and only cities would stem a population growing too fast to feed and resource itself.

"I think urbanisation is going to save humanity,’ he said.

"One million people a week, every week, are moving into cities. That will go on for the next 70 years. There will be seven billion, out of nine or 10 billion people living in cities.

"The global population is going to stabilise at 10 billion. In a village, an extra kid is an asset, to help work on the farm, but in a crowded city not so much."

He said education, and women being exposed to medical services, would see family sizes reduce from six or seven people, to just two or three.

"We can get to a sustainable number of people," he said.

With two thirds of the population living in cities, the forest could regrow itself where settlements once were, he said.

Mr Anderson cited New York City as one of the most sustainable on earth - where residents produce half the amount of carbon emissions as Sydneysiders.

"They walk to work, they settle for smaller living spaces," he said.

While nations grew more frustrated at their federal governments floundering on major decisions, Mr Anderson said: "At a city level, things can get done."

He said the future would depend on the kind of city that developing nations chose to live in.

"If they pick suburbia, big houses, lives build around driving a car, we're screwed," he said.

"That is a lifestyle we have chosen in the west.

"Vertical living has so many of the answers."

He said it was important architects realised that people did not want to live in a rat trap, or in a box jammed together.