This chapter discusses scenarios of potential future anthropogenic developments that could affect the concentrations of greenhouse gases, aerosols, and short-lived species and thereby the climate. Scenarios of this type have been published for more than a quarter of a century. The earliest work on this problem focused on fossil fuel and industrial CO2 emissions and employed timetrend extrapolations of fossil fuel resource utilization (Rotty 1977, 1978). This work was followed by the development of models that employed economic principles and considered energy markets explicitly in the development of scenarios that explored emissions and concentrations of CO2 in the absence of policies to limit anthropogenic emissions. See, for example, Nordhaus (1979), JASON (1979), Niehaus and Williams (1979), Häfele (1981), Nordhaus and Yohe (1983), and Edmonds and Reilly (1983, 1985). This literature evolved with other climate sciences and was extended to include land use change emissions of CO2, non-CO2 greenhouse gases, aerosols, and short-lived species, as well as a wide range of potential climate policy regimes. The range of global CO2 emissions reported in the literature by Nakicenovic et al. (2000) is shown in Figure 3.1. This broad literature is summarized in a variety of reviews and assessments, including Leggett et al. (1992), Edmonds et al. (1994), Weyant et al. (1996), Special Report on Emissions Scenarios (SRES) (Nakicenovic et al. 2000), Fisher et al. (2007), van Vuuren et al. (2008), and van Vuuren and Riahi (2011).