National and sub-national governments must set ambitious GHG emission reduction goals and assess different policies to achieve them. Scenarios allow planning emissions over time -as well as other variables such as employment or GDP- taking «assumptions» into account and answering questions such as: What happens if emissions grow uncontrollably? What if we set a goal aligned with science? How many emissions could be reduced by a carbon tax of USD 10 per tCO2eq? How many emissions could be reduced by a 30% increase in the use of renewable energy?