This lecture will explain how we assign a present monetary value to the future climate impacts of today’s greenhouse gas emissions, emphasising the importance of scenarios; equity and welfare; the notion of discounting future harms and benefits; bottom-up versus top-down, and model-based versus empirical, approaches to quantifying global economic impact; and dealing with uncertainty and risk. We’ll discuss the challenge of monetising irreversible impacts such as species extinction, including the (special?) case in which the species in question is Homo Sapiens.