The Purpose of Climate Scenarios
A climate scenario is a plausible representation of future climate that has been constructed for explicit use in investigating the potential impacts of anthropogenic climate change. Climate scenarios often make use of climate projections (descriptions of the modelled response of the climate system to scenarios of greenhouse gas and aerosol concentrations), by manipulating model outputs and combining them with observed climate data.

This new chapter for the IPCC assesses the methods used to develop climate scenarios. Impact assessments have a very wide range of scenario requirements, ranging from global mean estimates of temperature and sea level, through continental-scale descriptions of changes in mean monthly climate, to point or catchment-level detail about future changes in daily or even sub-daily climate.

The science of climate scenario development acts as an important bridge from the climate science of Working Group I to the science of impact, adaptation and vulnerability assessment, considered by Working Group II. It also has a close dependence on emissions scenarios, which are discussed by Working Group III.

Methods for Constructing Scenarios
Useful information about possible future climates and their impacts has been obtained using various scenario construction methods. These include climate model based approaches, temporal and spatial analogues, incremental scenarios for sensitivity studies, and expert judgement. This chapter identifies advantages and disadvantages of these different methods (see Table 13.1).

All these methods can continue to serve a useful role in the provision of scenarios for impact assessment, but it is likely that the major advances in climate scenario construction will be made through the refinement and extension of climate model based approaches.

Each new advance in climate model simulations of future climate has stimulated new techniques for climate scenario construction. There are now numerous techniques available for scenario construction, the majority of which ultimately depend upon results obtained from general circulation model (GCM) experiments.

Representing the Cascade of Uncertainty
Uncertainties will remain inherent in predicting future climate change, even though some uncertainties are likely to be narrowed with time. Consequently, a range of climate scenarios should usually be considered in conducting impact assessments.

There is a cascade of uncertainties in future climate predictions which includes unknown future emissions of greenhouse gases and aerosols, the conversion of emissions to atmospheric concentrations and to radiative forcing of the climate, modelling the response of the climate system to forcing, and methods for regionalising GCM results.

Scenario construction techniques can be usefully contrasted according to the sources of uncertainty that they address and those that they ignore. These techniques, however, do not always provide consistent results. For example, simple methods based on direct GCM changes often represent model-to-model differences in simulated climate change, but do not address the uncertainty associated with how these changes are expressed at fine spatial scales. With regionalisation approaches, the reverse is often true.

A number of methods have emerged to assist with the quantification and communication of uncertainty in climate scenarios. These include pattern-scaling techniques to inter-polate/extrapolate between results of model experiments, climate scenario generators, risk assessment frameworks and the use of expert judgement. The development of new or refined scenario construction techniques that can account for multiple uncertainties merits further investigation.

Representing High Spatial and Temporal Resolution Information
The incorporation of climate changes at high spatial (e.g., tens of kilometres) and temporal (e.g., daily) resolution in climate scenarios currently remains largely within the research domain of climate scenario development. Scenarios containing such high resolution information have not yet been widely used in comprehensive policy relevant impact assessments.

Preliminary evidence suggests that coarse spatial resolution AOGCM (Atmosphere-Ocean General Circulation Model) information for impact studies needs to be used cautiously in regions characterised by pronounced sub-GCM grid scale variability in forcings. The use of suitable regionalisation techniques may be important to enhance the AOGCM results over such regions.

Incorporating higher resolution information in climate scenarios can substantially alter the assessment of impacts. The incorporation of such information in scenarios is likely to become increasingly common and further evaluation of the relevant methods and their added value in impact assessment is warranted.

Representing Extreme Events
Extreme climate/weather events are very important for most climate change impacts. Changes in the occurrence and intensity of extremes should be included in climate scenarios whenever possible.
Some extreme events are easily or implicitly incorporated in climate scenarios using conventional techniques. It is more difficult to produce scenarios of complex events, such as tropical cyclones and ice storms, which may require specialised techniques. This constitutes an important methodological gap in scenario development. The large uncertainty regarding future changes in some extreme events exacerbates the difficulty in incorporating such changes in climate scenarios.

Applying Climate Scenarios in Impact Assessments
There is no single �best� scenario construction method appropriate for all applications. In each case, the appropriate method is determined by the context and the application of the scenario.

The choice of method constrains the sources of uncertainty that can be addressed. Relatively simple techniques, such as those that rely on scaled or unscaled GCM changes, may well be the most appropriate for applications in integrated assessment modelling or for informing policy; more sophisticated techniques, such as regional climate modelling or conditioned stochastic weather generation, are often necessary for applications involving detailed regional modelling of climate change impacts.

Improving Information Required for Scenario Development
Improvements in global climate modelling will bring a variety of benefits to most climate scenario development methods. A more diverse set of model experiments, such as AOGCMs run under a broader range of forcings and at higher resolutions, and regional climate models run either in ensemble mode or for longer time periods, will allow a wider range of uncertainty to be represented in climate scenarios. In addition, incorporation of some of the physical, biological and socio-economic feedbacks not currently simulated in global models will improve the consistency of different scenario elements.

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