Half the world's population live in cities, and by 2050 the figure will increase to two-thirds, or about 6 billion people. The environmental impact is already extensive. As the global population expands, so too do pollution and pressure on resources.

"earthrise" travels to Colombia and Singapore to meet some of the people who are finding ways to meet growing demand while also making our cities more sustainable. Juliana Schatz heads to Bogota to see how Conceptos Plasticos builds homes out of discarded waste. Russell Beard travels there to meet some of the entrepreneurs who are helping the island city-state of Singapore earn its green title.