Climate change presents the greatest challenge ever faced by our domestic and international institutions, and a great deal of the difficulty lies in the science of the issue. Because human influence on global climate differs in important ways from other environmental threats these peculiarities set the context for discussion of what can be done to reduce greenhouse gas emissions and to adapt to change that cannot be avoided. Following a brief summary of current understanding of how Earth’s climate works, five ways are presented by which the science of climate impinges on attempts to construct a policy response.