In 2001 a panel representing virtually all the world's governments and climate scientists announced that they had reached a consensus: the world was warming at a rate without precedent during at least the last ten millennia, and that warming was caused by the buildup of greenhouse gases from human activity. The consensus itself was at least a century in the making. The story of how scientists reached their conclusion - by way of unexpected twists and turns and in the face of formidable intellectual, financial and political obstacles - is told here. Spencer R. Weart explains the emerging science, introduces to us the major players, and shows us how the Earth's irreducibly complicated climate system was mirrored by the global scientific community that studied it. This book portrays scientists working on bits and pieces of a topic so complex that they could never achieve full certainty - yet so important to human survival that provisional answers were essential. Weart unsparingly depicts the conflicts and mistakes, and how they sometimes led to fruitful results. His book reminds us that scientists do not work in isolation but interact in crucial ways with the political system and with the general public. The book not only reveals the history of global warming but also analyzes the nature of modern scientific work as it confronts the most difficult questions about the Earth's future.