ONE OF THE JOYS THAT I associate with New York City is looking down on rooftop gardens. Columbia University Professor Dickson Despommier takes rooftop gardens to a whole new level. In The Vertical Farm: The World Grows Up, he envisages city centres with entire buildings devoted to growing fruits and vegetables. The illustrations in his book should be required viewing for city planners because 80 per cent of the world's population will live in urban areas by 2050. Despommier argues, an area of land 20 per cent larger than Brazil will be required to feed all these city dwellers. Where is all that arable space? In his online writings and in his book, Despommier suggest that multi-storey buildings could be designed as giant hothouses used to grow food year-round on an industrial scale at the heart of cities. And if a skyscraper occupant doesn't want to get his hands dirty, the space could be let out as part of a multi-storey allotment scheme.