On Start the Week Sue MacGregor asks what happens when the world's population reaches ten billion. The computer scientist, Stephen Emmott argues that time is running out for humanity unless we radically change our behaviour, but the geographer Danny Dorling believes that we should be preparing for the inevitable population decline. Jill Rutter explores the impact of differing scientific advice on politics, and the complexity of evidence-based policy. And with India's population set to exceed that of China, the Nobel prize-winning economist Amartya Sen lambasts political inaction in raising standards for the poorest in society. Producer: Katy Hickman.