World cities, home to most people, to add 2.5 billion more by 2050: U.N.
High-rise office buildings are seen during sunset in Tokyo December 10, 2013. REUTERS/Toru Hanai
A general view of the Shanghai's financial district of Pudong, with the Shanghai Tower (R), which is undergoing construction and scheduled to finish by the end of 2014, in Shanghai July 31, 2013. REUTERS/ Carlos Barria