Formulating scenarios

The text on this page is taken from an equivalent page of the IEHIAS-project.

Different problems require different forms of assessment. Not all merit a full, integrated impact assessment, either because they are simple enough to be done by other, more traditional means, or because the uncertainties involved would be too large (see IEHIA in relation to other forms of assessment). Whether or not a full integrated assessment is merited may also not be clear when the issue is first identified. During the Design stage, therefore, an important task is to carry out a screening of the issue, to determine whether the likely impacts are sufficient to warrant detailed assessment, and if so how to proceed.

Types of assessment

Integrated assessments may also vary greatly in their scope, structure and design, depending on the type of question that is being addressed and the needs of the users. No formal classification of assessments has been developed and agreed, but a useful distinction can be made between three different approaches:

• diagnostic assessments - aimed at addressing the question of 'is there a problem' and if so 'from where does it derive, or where might we best intervene?'
• summative assessments - aimed at determining how well existing policies are performing;
• prognostic assessments - aimed at answering 'what if' questions, about future impacts (e.g. under different policy scenarios).

Assessment and scenarios

These distinctions are important, for each approach is likely to imply a somewhat different process of assessment. In particular, different types of assessment imply the use of different types of scenario.

All assessments involve at least two scenarios – one describing some reference situation and one describing an alternative set of conditions. In some cases, more scenarios may be defined, presenting further alternatives (e.g. different policy options). The nature of the reference and alternative scenarios varies, depending on the type of assessment being done:

• In a diagnostic assessment, the reference scenario is usually a description of the world as it is now (the status quo), while the alternative scenario represents some counterfactual, ideal state – for example, how the world might look without the hazard of concern (e.g. with zero pollution) or if the hazards were at tolerable levels.
• In a summative assessment, likewise, the reference scenario describes the current situation (i.e. with the policy or technology of interest), while the alternative scenario represents conditions without the policy or technology.
• In a prognostic assessment, the alternative scenarios describe the world as it might look in the future if certain changes are allowed to happen (e.g. new policy developments, technologies or environmental changes). The reference scenario may, again, describe the current situation, but (more strictly) should project this into the future, to show how the world would look if the proposed changes did not occur – i.e. a ‘business-as-usual’ scenario.

The role of scenarios

All assessments are comparative, in that they assess the differences between one situation (e.g. a policy option) and one or more others (e.g. conditions without the policy). These different situations - or the factors that lead to them - have to be prescribed before an assessment can be done. This is done by defining scenarios.

A scenario is a vision of the world as it might be. Scenarios are not intended to be predictions of what will happen, or accounts of what has happened, but plausible descriptions of what the world could look like under a specific set of assumptions. Scenarios thus play a vital role in any assessment, for they determine the nature of the comparisons which will be made, and thus the possible results of the assessment.

As such, the scenarios should reflect the interests and expectations of the stakeholders involved, and once analysis has been started, they should not be changed, for to do so would mean that the assessment is no longer faithful to these expectations. Scenarios thus provide the fixed basis on which the assessment is done.

Scenarios and the causal chain

Assessments of systemic health issues can conveniently be structured on the basis of a causal chain, representing the key links between primary sources and ultimate impacts (see link to The causal chain for more information).

The position of scenarios in relation to the causal chain may vary. Some are exogenous (upper figure), and describe only the external context or driving forces that influence this chain (e.g. policy developments, technological changes). Most, however, are endogenous (lower figure) in that they describe links within the chain – for example, levels of (or changes in) source activities, releases, ambient concentrations and/or exposure.

As these figures show, the position of the scenario in relation to the causal chain fundamentally affects the character and scope of the assessment. This is because the scenarios give the fixed assumptions for the assessment. The assessment itself takes the conditions thus specified as given, and considers only the ‘downstream’ effects, as they work through the causal chain.

• Exogenous scenarios define the external conditions (i.e. context) but tell nothing direct about the changes that might occur within the causal chain. These all have to be deduced as part of the assessment – for example, by modelling the way the external influencers work their way from source to impact.
• Endogenous scenarios, in contrast, define some aspects of the causal chain – those which are specified as part of the scenario. The assessment is only concerned with estimating the subsequent implications . In the case of an emissions scenario, therefore, expected changes in emission are predefined, and the assessment focuses on estimating how these work through to affect environmental concentrations and exposures, and thence health effects. In the case of an exposure scenario (i.e. where changes in exposure are specified), the assessment only has to consider the link from exposure to health impact.

More detailed and encompassing scenarios might seem useful, since they define more clearly the conditions under which the assessment should be done. They do so, however, at the cost of predefining more of the results, and limiting the independence of the assessment. A balance needs to be sought, such that the scenario defines only what can legitimately be taken as given, while the assessment should explore and discover what is not known.

Types of scenario

Scenarios may be described and represented in different ways (see Table below). Which is used depends on the type of issue being considered (e.g. how complex it is), the needs and skills of the stakeholders involved in scenario development, and the type of assessment (i.e. diagnostic, summative or prognostic). Follow the links below to find more information on each type.

Charasteric of approach	Prescriptive	Descriptive	Predictive	Probabilistic
Defined in terms of:	Outcomes	Contexts	Trends	Processes
Approach to system specification	Normative	Qualitative	Quantitative	Quantitative
Method of formulation	Expert judgement	Stakeholder consultation	Deterministic models or extrapolation	Stochastic models
Form of information	Assumptions (fixed states)	Narratives, pictures	Predictions of future states	Predictions of system behavioun

Creating scenarios for IEHIA

Different methods for scenario construction will obviously be necessary, 413depending on the type of scenario concerned. Descriptive scenarios may be generated, for example, by generating a narrative based on the ideas of (and discussions with) the stakeholders. Predictive or probabilistic scenarios usually require a significant amount of data gathering and analysis. In each case, however, it is important to recognise that the scenarios, however constructed, represent a precursor to the stage of impact assessment; they define the assumptions and preconditions for the assessment that cannot, subsequently be changed (except by a full reiteration of the assessment). For this reason it is essential that the conditions set down by the scenario are clearly and unambiguously specified and are meaningful (and plausible) within the context of the study. If the assessment is to be informative, considerable care thus needs to be given to the process of scenario development, and scenario development can take up a considerable amount of time and effort.

To make the process of scenario construction easier, it is often useful to start with existing, ready-made scenarios. This avoids duplication of effort, and it also helps to ensure greater consistency between different assessments. It is therefore advisable to begin by reviewing previous studies and seeing what scenarios they have used, and how they were developed (see links to examples under See also, below).

In some areas, well-established scenarios have already been generated, often to address large, international issues, such as climate change, or as a framework for national planning and strategic development (e.g. national waste management scenarios). Links to several European policy scenarios are given to the left. In many cases, these existing scenarios provide a ready-made basis for assessment. Nevertheless, they will often have to be adapted or further developed to match the specific issue being addressed; for example, by incorporating additional elements, by extending the timescale, or by rescaling the scenarios to a different geographic level. What is most important is to retain, as far as possible, the broader context given by the existing scenarios so that the results of the assessment are consistent with what has been done before.

Various tools also exist to help users generate scenarios. These range from relatively simple check-lists of things not to forget to rather sophisticated modelling and visualisation systems, that allow users to change key elements within the system, and estimate what the consequences might be. Again, links to some of these scenario construction tools are given to the left.

European policy scenarios

Scenarios have become important tools for policy development. As a consequence, scenarios have been developed for a wide range of policy areas. The scenarios differ greatly in the way they have been devised, their geographic extent, the time period they cover and their level of detail. Many are ony broad descriptive storylines, outlining how the world may change under different sets of assumptions; others translate these into a series of indicators for key outcomes; a few provide detailed estimates of critical variables, based on modelling of the processes involved. All can offer useful starting points for scenario building, though for many applications further work may be necessary to fill in the details needed for the assessment.

The list below includes scenarios for some of the key themes relevant for environmental health, covering the EU area. It is restricted to recently developed scenarios that offer some degree of quantification of the outcome variables. Some of these are also available in the form of interactive tools, which allow users to select or amend specific elements of the scenario and run them for the area and/or time-period of their choice.

Scenario name	Main policy area/compartment	Drivers of change	Geographical extent	Spatial resolution	Time-period	Sponsor/ owner
IPCC-SRES	Climate change	Greenhouse gas emissions	Global	Regional	2000-2050	IPCC
PRELUDE	Agricultural land use change	Regional/global trade, environmental attitudes/policies	EU27	NUTS-2	2000-2030	EU
PLANETS	Energy technologies and markets	Environmental policies, technology standards	Global, EU	Regional	2010-2050	EU
SULTAN	Transport volumes, composition	Environmental and transport policies	EU27	EU27	2010-2050	EU
SCENES	Water resources and sectoral use	Population change, economic change, EU policies, technology	Wider Europe/ N Africa	5o x 5o	2000-2025	EU
DEMIFER	Population distribution and demographic structure	External/internal migration, demographic change	EU27	NUTS-2	2005-2050	EU
GEO-4	Emissions, land use/biodiversity, water resources, technology, population	Economic/environmental policy	Global	Regional	2000-2050	UNEP
FORWAST	Waste generation (by sector), monetised enviornmental impact	Waste management policy/technology; population growth; economic change	EU27	EU (and national)	2000-2050	EU