That is the view of Mark Watts, director of multinational design firm Arup and a leading expert on sustainable planning.

Mr Watts told the MIPIM property conference in Cannes that cities faced unprecedented challenges - with the population soaring, and human beings now consuming more natural resources than the earth was able to replace.

But he said there were enough examples of smart, green thinking throughout the world to make him optimistic about the future.

Mr Watts outlined a model 'future city' investing heavily in cycling, electric buses, energy-generating buildings and urban farms.

And he said that ecologically sound policies adopted throughout the world had often been adopted for economic, rather than environmental, reasons.

Mr Watts said Copenhagen - where 35pc of all trips to work are by bike - had not only reduced spending on more expensive transport infrastructure, but was saving one dollar in health costs for every kilometre cycled.

And he said an app being used in San Fransisco, alerting drivers to empty parking bays throughout the city, had cut congestion by 10 per cent.

Mr Watts said the most successful 40 cities in the world had all seen the biggest increases on spending on cycling.

He added: "Most cities in the world which are judged to be the most liveable and most efficient are the ones that have sustainable transport systems.

"I think that we are going to see the return of the old-fashioned bicycle in the most successful cities in the world, moving forward.

"There are huge savings through going down this route."

Mr Watts said 'future cities' were going to have to 'run on data' - with a much better awareness of who was consuming what resources, and when.

And he said diminishing resources, natural disasters and uncertainty over energy supplies meant cities would have to be more 'self-reliant'.

He told delegates: "In the future we will see much more of a move back to cities growing much of their own produce, while also creating flood plains and green spaces for people to enjoy."

Mr Watts said national governments had often failed to work together on ecological issues - but city leaders had been much more forward-thinking, showing 'a huge amount of collaboration'.

He said: "The whole attitude is about seeing what works elsewhere, copying it and claiming it as your own.

"There are enough examples of really good ways of getting to be a sustainable future city throughout the world that if those can be replicated in every city, then there is hope for the future."