Even the most utopian visions of a low-carbon world include cars, and even if they're fueled on electricity or ethanol, cars need places to park. But if parking structures have to exist, at least they can double as urban farms. In Vancouver, a company called Valcent Products is building a high-density "VertiCrop" farm on the roof of a parking garage.

The farm will be super high-tech, with motorized conveyors moving layers of plants into position for optimum sun, watering, and easy harvesting. Valcent says the farm will produce as much as as a 16-acre field in California. Plus, nobody likes parking on the roof of parking garages anyway, since it makes the inside of the car feel like Hades, so the farm is making use of otherwise wasted space.