It seems that we simply can't resist the allure of the world's cities. Most are already bursting at the seams, yet they keep on getting bigger. It's reckoned that by 2050 more than 70 percent of us will live in urban areas. How can we refashion our cities to make them smarter, more liveable and more green? Justin Rowlatt unpicks the issues with guests: Ed Bryan at IBM, Martin Powell at the engineering and technology company Siemens and Maria van der Hoeven, Executive Director of the International Energy Agency.