A scenario is a story or story outline. Thinking about the future normally involves creating alternative scenarios, or stories, about the possible evolution of drivers. Some such scenarios are exploratory and consider the possible unfolding of different futures around key uncertainties, such as the rate of some aspect of technological advance or the fragility of some element in the global environment. Other scenarios are normative and develop stories about preferred futures, such as a global transformation to sustainability.
Scenarios in a computer model typically are built from multiple interventions that collectively help build a coherent story about the future. Often, but somewhat imprecisely, the word scenario is used more loosely to refer to any intervention (such as the change of a fertility rate for a country or an alternative assumption about oil resources).
Scenarios or interventions with respect to what? When IFs or other computer simulations are "run", without making any changes to parameters or initial conditions specified as the default values, they generate a forecast that is typically called the base case (sometimes reference run). The IFs base case, always available when a model session is initiated, is itself a scenario. Sometimes the base case is incorrectly referred to as a trend extrapolation or a "business as usual" scenario. More accurately, however, the base case of IFs is a computation that involves the full dynamics of the model and therefore has very nonlinear behavior, often quite different from trends. It is a good starting point for scenario analysis for two reasons. First, it is built from initial conditions of all variables and on that has been given reasonable values from data or other analysis. These initial conditions and parameters make up the package of interventions that constitute the base case scenario. Second, the base case is periodically analyzed relative to the forecasts of many other projects across the range of issue areas covered by IFs and is to a degree "tuned" to reproduce the behavior of respected forecasters.
The Quick Scenario Tree allows users to create and save two different kinds of files: Scenario-Load-Files (.sce) and Run-Result-Files (.run). The Scenario-Load-Files files represent changes that were made to the scenario tree but that were not yet entirely run through IFs software. The Run-Result-Files represent files that were originally changes to the scenario tree that were eventually entirely run through IFs software. The running a Scenario-Load-Files file will make those changes permanent and therefore a Run-Result-File.
In addition to the base case, most versions of IFs will include a number of other previously-run scenarios (see Lesson 0 for additional important terminology), perhaps the set of scenarios for the National Intelligence Council’s (NIC) 2020 Project or those for the Global Environmental Outlook (GEO) of the United Nations Environmental Program. If you look, for instance, at the Packaged Display form, you will see a list of previously-run scenarios in the box at the bottom left of the screen. Because those have already been run, based on a set of interventions constituting their foundations, the user can immediately display their results.