Big Ideas: Dickson Despommier's Vertical Farming

[youtube]http://www.youtube.com/watch?v=1clRcxZS52sIf there weren't any pesky practical limitations, what world-changing device would you invent? In the second installment of Babelgum and GOOD's new Big Ideas competition, Columbia professor Dickson Despommier imagines filling New York's skyscrapers with farms.