This chapter describes past trends in greenhouse gas emissions, and future projections. It presents the potential consequences of climate change when no action is taken, and discusses the associated risks and uncertainties. The chapter assesses four different scenarios for stabilising greenhouse gas concentrations, and examines how differences in the stabilisation target, peaking year and level of overshooting of the target affect the costs of action. These scenarios all assume that mitigation policies are cost-effective, i.e. that all carbon emission sources can be priced globally, to provide a benchmark for more realistic scenarios examined in later chapters.