Quantifying current greenhouse emissions and anticipating their future evolution is an important analytical input to policymaking. However, accurate emissions monitoring and reporting are not simple, and processes are still being improved globally. Predicting the future yet more challenging; models that do so are heavily affected by assumptions to create scenarios of economic, social and policy change over time. As a result there is a wide array of visions of the future, the difference between which are important to understand. This study focuses on carbon dioxide (CO2) emissions from fossil fuels. Key drivers of these emissions are activity, economics, energy intensity and carbon intensity. As reducing GDP or population is not a likely aim of climate policy, the primary means of affecting emissions is to change the last two of the four factors: reducing the amount of energy needed per GDP, and decarbonising the fuel mix.