In October, the IPCC, the UN body that reports on climate change, issued a stark warning. It said that if the world wanted to avoid catastrophic environmental damage, we needed to reduce carbon emissions by almost half in the next 12 years. So what needs to be done now if that target is to be reached? What's the position in China, the world's largest emitter of carbon dioxide? And how important is the stated intention of President Trump to withdraw the United States from the Paris Agreement on greenhouse gas emissions? We hear from, among others, Professor Lord Nicholas Stern, the author of the seminal Stern Review in 2006.