Humanity’s ongoing and accelerating disruption of the planetary climate system is a unique and existential threat to human civilization. In this chapter the author provides an overview of the basic scientific knowledge from which all discussion of response should proceed: Earth is warming, driven by human activity (principally the emission of greenhouse gases into the atmosphere), which is disrupting the entire planetary climate system, with extreme risks for humans. He also examines what the science has to say about the nature and scale of meaningful response. Specifically, he outlines the notion of a carbon budget. Finally, he examines: what science has to say about the nature of this challenge as a so-called “superwicked” problem; what natural systems have to teach us about navigating such a challenge; the role of communities; and the distinction between robust and optimized solution paths.