Scenario analysis is a method that tries to describe logical and internally consistent sequences of events to explore how the future might, could or should evolve from the past and present. The future is inherently uncertain. Through scenario analysis different alternative futures can be explored and thus uncertainties addressed. As such, scenario analysis is also a tool to deal explicitly with different assumptions about the future. Several definitions of scenarios can be found in the literature. In the definition of UNEP (2002), the uncertainty aspect is explicitly incorporated. "Scenarios are descriptions of journeys to possible futures. They reflect different assumptions about how current trends will unfold, how critical uncertainties will play out and what new factors will come into play". Another definition is the following: A scenario is a description of the present state of a social and or natural system (or a part of it), of possible and desirable future states of that system along with sequences of events that could lead from the present state to these future states (e.g. Jansen Schoonhoven and Roschar, 1992). Other definitions also include the purposes of the use of scenarios. Van Notten (2002) defines scenarios as "descriptions of possible futures that reflect perspectives on past, present, and upcoming developments in order to anticipate the future".

Different types of scenarios exist. Alcamo (2001) discerns baseline vs. policy scenarios, exploratory vs. anticipatory scenarios and qualitative vs. quantitative scenarios.

  • Baseline scenarios (or reference-, benchmark- or non-intervention scenarios) present the future state of society and environment in which no (additional) environmental policies do exist or have a discernable influence on society or the environment. Policy scenarios (or pollution control-, mitigation- or intervention scenarios) depict the future effects of environmental protection policies.
  • Exploratory scenarios (or descriptive scenarios) start in the present and explore possible trends into the future. Anticipatory scenarios (or prescriptive or normative scenarios) start with a prescribed vision of the future and then work backwards in time to visualise how this future could emerge.
  • Qualitative scenarios describe possible futures in the form of narrative texts or so-called "story-lines". Quantitative scenarios provide tables and figures incorporating numerical data often generated by sophisticated models.
  • Finally scenarios can be surprise-free or trend scenarios, which extend foreseen developments, on the one hand or including surprises and exploring the extremes (e.g. best case / worst case) on the other hand.