Scenarios can have qualitative or quantitative aspects or both. Qualitative (descriptions) and quantitative (modelling) aspects of scenarios can also be complementary. Commonly, the approach has been a story and simulation approach that combines a descriptive story line with numerical modelling.
Qualitative aspects of scenarios are typically a family of story lines. Story lines are particularly useful for aspects of the scenario for which something is not quantifiable or there is insufficient data to quantify it with the required accuracy. For instance, story lines allow social, cultural, and institutional factors to be addressed explicitly, even if current knowledge does not allow these uncertain factors to be treated in a quantitative way.
The quantitative part of the scenario commonly involves scientific modelling used to examine different assumptions. We might examine the consequence of alternative actions (policy scenarios) or we might investigate the consequences of a single course of action under different key assumed uncertainties relating to the social-ecological system (baseline scenarios).
The quantitative aspect of the scenarios is valuable for its ability to check the internal consistency of the story line, to enrich the story and provide geographic detail. This is because it is possible for models to have a level of complexity that is greater than our mental models.
Developments of the qualitative and quantitative aspects of scenarios usually happen concurrently and are iteratively refined in response to each other. Development using participatory methods is growing especially to meet the challenges of complex issues.
Development of the qualitative storylines typically involve stakeholder engagement to negotiate plausible futures that are coherent and internally consistent. This process could involve a formal or informal dialogue (workshops, interviews, surveys etc.) involving both experts and stakeholders and ideally incorporating a relevant range of different worldviews. The process may involve both analysis and imagination (e.g. even though ideal worlds do not exist, we can imagine them and work towards that as a goal). Quantitative scenarios are computed using models. When available and given adequate resources, a number of models are used.
In general, the following iterative development process is used:
- identifying the main driving forces affecting the state of a system
- determining the current state
- identifying the critical uncertainties
- making assumption about how uncertainties will evolve
- identifying options for mitigation and
- analysing the implications
Scenario exercises can be exploratory or normative. Exploratory scenarios contrast one or more baseline scenarios (or reference, benchmark, or non-intervention scenarios) with one or more policy scenarios (or mitigation, or intervention scenarios) which consider the role of deliberate human actions and choices in shaping the future. Note, often the baseline scenario is incorrectly referred to or perceived as a ‘business as usual’ scenario which simply extrapolates trends, however; not all futures might be derivable as simple extrapolations of past trends.
Baseline scenarios reflect uncertainty (for example of driving forces or other parameters), in that there can be more than one baseline. Normative scenarios, on the other hand, develop stories about preferred futures. They provide a vision of a transition to a desired or alternate future.