The risks of climate change are potentially immense. The benefits of taking action are also clear: we can see that economic development, reduced emissions, and creative adaptation go hand in hand. A committed and strong low-carbon transition could trigger a new wave of economic and technological transformation and investment, a new era of global and sustainable prosperity. Why, then, are we waiting? In this book, Nicholas Stern explains why, notwithstanding the great attractions of a new path, it has been so difficult to tackle climate change effectively. He makes a compelling case for climate action now and sets out the forms that action should take.Stern argues that the risks and costs of climate change are worse than estimated in the landmark Stern Review in 2006 -- and far worse than implied by standard economic models. He reminds us that we have a choice. We can rely on past technologies, methods, and institutions -- or we can embrace change, innovation, and international collaboration. The first might bring us some short-term growth but would lead eventually to chaos, conflict, and destruction. The second could bring about better lives for all and growth that is sustainable over the long term, and help win the battle against worldwide poverty. The science warns of the dangers of neglect; the economics and technology show what we can do and the great benefits that will follow; an examination of the ethics points strongly to a moral imperative for action. Why are we waiting?