Climate change is damaging people’s lives today. Even if world leaders agree the strictest possible curbs on greenhouse gas (GHG) emissions, the prospects are very bleak for hundreds of millions of people, most of them among the world’s poorest. This paper puts the dramatic stories of some of those people alongside the latest science on the impacts of climate change on humans. Together they explain why climate change is fundamentally a development crisis. The world must act immediately and decisively to address this, the greatest peril to humanity this century.