Professor Julian Cribb, author of The Coming Famine, said the global food crisis was a forewarning of what could be expected as civilisation ran low on water, arable land and nutrients, and experienced soaring energy costs.

Professor Cribb said the urban farmers of the future - who would primarily grow vegetables - would play a much larger role in the global diet.

"We need new skills in designing this diet and developing the intensive vegetable culture needed to support it," he said.

"This intensive urban vegie culture is an entirely new industry and will need a new professional - the urban farmer who can grow food on the roofs and sides of buildings, in intensive biocultures and by other novel methods to feed the megacities of 30 million-plus inhabitants.

"If we don't, by 2050 we will have more than three-quarters of the human population - almost 8 billion people - living in places where they are totally without the means or the knowledge of how to feed themselves.

"Our giant cities will be gigantic death traps, at the mercy of even quite minor glitches in regional or global food supplies."

The City of Sydney council has commissioned a report to look at ways to encourage the greening of rooftops.

Deputy Lord Mayor Marcelle Hoff said a group of environmental experts was working on a report - due before Christmas - on retrofitting green roofs to multi-storey apartment buildings.

The council was adding six community gardens and employing a dedicated staff member to co-ordinate them.

The council owns two community gardens and supports another eight.

"Green roofs would create more open space and enhance bio-diversity," Ms Hoff said.

"They will also reduce energy consumption by insulating buildings, reduce stormwater run-off, reduce greenhouse gases and could be practical, too, by growing fruit and vegetables."

Green Roofs Australia executive Jim Osborne, a landscape architect, said councils and governments needed to provide incentives such as greater planning and monetary support for rooftop gardens.

The benefits of these gardens had been established overseas but more scientific research examining Australian conditions was needed.