Climate Scenario Development
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 modeled 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.
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 analogs, incremental scenarios for sensitivity studies, and expert judgment. 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. 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.
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. 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.
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.
