The two great challenges of the 21st century are the battle against world poverty and managing climate change. Unless we succeed on one, we cannot succeed on the other. Success requires the world, developed and developing countries, to find the path to low-carbon growth. We understand, not perfectly but broadly: the necessary scale of action required to reduce the risks, and the costs of that action; we can see many of the technologies that can deliver low-carbon growth and can encourage more; and we can describe the economic instruments that will give the necessary incentives. What is now required is political will and a global deal.