Columbia University professor Dr. Dickson Despommier envisions feeding urban centers of the future with 30-story-tall indoor farms -- he estimates that 150 could feed New York City. Capable of producing produce year round, these farms could potentially address issues ranging from global warming to population growth. The farm concept runs off of solar power, wind power, and inedible plant waste. Irrigation is supplied with treated sewage water and evaporation from the plants produces moisture suitable for producing bottled water. Despommier believes that these towers could allow existing farm land to be reforested to combat CO2.
Its all very pie-in-the-sky and sounds like something that might come out of an engineering design contest, but somehow it sounds achievable.