"Scenarios are one of the main tools for assessment of future developments in complex systems that often are inherently unpredictable, are insufficiently understood, and have high scientific uncertainties."⁽¹⁷⁾

Scenarios play an important role in impacts and adaptation research. As discussed in the previous section, scenarios are the only tool available for projecting future conditions, and future conditions are a key factor influencing vulnerability. In addition to changes in climate, changes in social, economic and political conditions will strongly influence the net impacts of climate change and our ability to adapt. It is important to recognize that climate and socio-economic scenarios are strongly interrelated, in that future changes in global greenhouse gas emissions will reflect evolving social and economic conditions.

This section provides a brief overview of the different types of scenarios available to the impacts and adaptation research community, while highlighting recent developments and future directions.

What are Scenarios?

Scenarios are used to determine how conditions may change in the future. A scenario can be defined as "...a coherent, internally consistent and plausible description of a possible future state of the world."⁽³³⁾ It is important to note that a scenario is not a prediction of the future, since use of the term "prediction" or "forecast" implies that a particular outcome is most likely to occur. Rather, a scenario represents one of any number of possible futures, which can be used to provide data for vulnerability, impacts and adaptation studies; to scope the range of plausible futures; to guide and explore the implications of adaptation and mitigation decisions; and to raise awareness of climate change issues. They provide a range of possible futures that allow consideration of the uncertainty relating to the different pathways that exist for future social, economic and environmental change.

Leadership regarding the construction of climate scenarios is provided by the IPCC Task Group on Scenarios for Climate Impact Assessment (IPCC-TGCIA). Much of the material presented here is based on the IPCC-TGCIA General Guidelines on the Use of Scenario Data for Climate Impact and Adaptation Assessment,⁽³⁴⁾ as well as on the chapter of the IPCC's Third Assessment Report that examines scenario development.⁽³⁵⁾

Types of Scenarios

Global Climate Models

The most common and widely accepted method of scenario construction involves the use of the output of Global Climate Models (GCMs), also known as General Circulation Models. GCMs are mathematical representations of the large-scale physical processes of the Earth-atmosphere-ocean system that provide a complete and internally consistent view of future climate change. Background information on GCMs can be obtained from the Canadian Climate Impacts Scenarios Web site.

The most recently developed GCMs contain a representation of the changes in atmospheric composition on a year-by-year basis from about 1860 to 1990, and are therefore able to simulate global-average conditions over this time period with much more reliability than earlier models. Recent GCMs are also able to model the effects of sulphate aerosols, which generally have a cooling effect on climate, as well as the warming effects of increased greenhouse gas concentrations. Overall, these newer models tend to be more reliable than earlier ones, since they incorporate more processes and feedbacks and are usually of a higher spatial resolution.

Despite the improvements in GCM resolution and in the representation of some of the climate processes during the last few years, there remain limitations. For example, GCM scenario development is very time-consuming; running a single climate change experiment with a GCM for a particular emissions scenario takes several months to a year, depending on the resolution and complexity of the model.

In addition, GCM output is still not at a fine enough resolution to enable it to be used directly by most impacts researchers. Therefore, GCM data are generally downscaled to produce gridded datasets of higher spatial resolution. This downscaling requires considerable time, and may introduce additional sources of error and uncertainty. Developments are currently under way, however, to improve model resolution and better represent land-surface conditions. There are also a number of recent and ongoing studies that focus on manipulating scenario data to build datasets of projections for specific regions or sectors in Canada (see Table 3). The results of these studies will be useful for the impacts and adaptation research community.

Table 3: Examples of recent and ongoing scenarios research using GCMs (funded by Climate Change Action Fund, Science Component)

Research using GCM-derived scenarios has been ongoing for the past 15 or so years. Although early impacts and adaptation research projects tended to apply only one climate scenario, it is now recommended that multiple scenarios be used to better represent the range of possible future climates. Two recent examples of studies in Canada that have used a range of climate change scenarios focused on water management and climate change in the Okanagan Basin,⁽³⁶⁾ and on conservation and management options for maintaining island forests within the prairie ecosystem.⁽³⁷⁾

The IPCC-TGCIA established the IPCC Data Distribution Centre (IPCC-DDC) in 1998 to facilitate access to GCM output and climate change scenarios by the vulnerability, impacts and adaptation research community. One limitation of the IPCC-DDC is that it is only possible to access the complete global fields for the GCM output and climate change scenarios, which means that researchers must be able to cope with and manipulate large volumes of data. This may be problematic for some researchers.

In Canada, impacts and adaptation researchers are able to access climate change scenarios through the Canadian Climate Impacts Scenarios (CCIS) project. This project provides climate change scenarios for Canada and North America, as well as related information concerning the construction and application of climate change scenarios in impacts studies (see Figure 2). It is designed to assist climate change impacts research in Canada by enabling the visualization of the scenarios and providing access to data via download from the project Web site. In addition, the project provides scenario tools that help users select which scenarios to use in their research and enable them to construct scenarios with finer spatial and temporal resolution than is currently provided by the GCM-derived scenarios.

FIGURE 2: Example of some of the scenario-related information available to impacts researchers from the Canadian Climate Impacts Scenarios (CCIS) Project.

Regional Climate Models

Over the past 10 years, significant work has been completed in the development of Regional Climate Models (RCMs).⁽³⁸⁾ RCMs provide higher spatial resolution data than GCMs by nesting the high- resolution RCM within the coarse resolution GCM. This means that RCMs are susceptible to any systematic errors present in the GCM used.⁽³⁹⁾ An advantage of RCMs is their ability to provide information that is more spatially detailed, and at a more appropriate scale for climate impact studies.⁽⁴⁰⁾ There is a high degree of interest among impacts and adaptation researchers for data from RCMs. Canadian researchers have access to a limited amount of RCM data from the Canadian Regional Climate Model (CRCM), through the Canadian Centre for Climate Modelling and Analysis (CCCma). Output from time-slice simulations (1975-1984, 2040-2049 and 2080-2089) is available on the CCCma Web site. The Ouranos Consortium, based in Montréal, provides support for the development of the CRCM and also runs climate simulations at the geographic scales most often needed for impacts and adaptation research.⁽⁴¹⁾ Regional climate models have been used in some recent studies, including a Canadian study that investigated the effect of climate change on fires in the boreal forest.⁽⁴²⁾ As work continues to improve the models and increase the availability of RCM scenario data, use of these models in impacts and adaptation research will likely increase.

Other Types of Climate Scenarios

Synthetic Scenarios

Synthetic scenarios, sometimes also called "arbitrary" or "incremental" scenarios, are the simplest climate change scenarios available. Their main use is in sensitivity analysis: determination of the response of a particular system (e.g., crops, streams) to a range of climatic variations. A synthetic scenario is constructed by adjusting a historical record for a particular climate variable by an arbitrary amount (e.g., increasing precipitation by 10%). Most studies using synthetic scenarios tend to apply constant changes throughout the year, although some have introduced seasonal changes.

Analogue Scenarios

Analogue scenarios make use of existing climate information, either at the site in question (temporal analogues) or from another location that currently experiences a climate anticipated to resemble the future climate of the site under study (spatial analogues). Temporal analogues may be constructed from paleoclimate information derived from either the geological record (e.g., from fossil flora and fauna remains, sedimentary deposits, tree rings or ice cores) or from the historical instrumental record. Analogue scenarios have the advantage of representing conditions that have actually occurred, so we know that they are physically plausible, and there are generally data available for a number of climate variables. Nevertheless, since the causes of changes in the analogue climate are generally not triggered by greenhouse gases, some have argued that these types of scenarios are of limited value in quantitative impact assessments of future climate change.⁽⁴³⁾

Socio-economic Scenarios

Scenarios are also used to provide information on projected changes in social and economic conditions. Information concerning population and human development, economic conditions, land cover and land use, and energy consumption is included in socio-economic scenarios.

To date, the main role of socio-economic scenarios has been to provide GCMs with information about future greenhouse gas and aerosol emissions. Future levels of greenhouse gas and aerosol emissions are clearly dependent on a wide range of factors, including population growth, economic activity and technology. The resulting range of possible emissions futures is captured through a suite of emissions scenarios. For its Third Assessment Report,⁽⁴⁴⁾ the IPCC commissioned a Special Report on Emissions Scenarios (SRES),⁽⁴⁵⁾ which describes about 40 different emissions scenarios. Six of these scenarios have been identified as "marker scenarios" and are recommended for use by the climate modelling community. These emissions scenarios indicate that the global-average temperature may increase by 1.4-5.8°C by 2100.

More recently, socio-economic scenarios have also been used to study the sensitivity, adaptive capacity and vulnerability of social and economic systems in relation to climate change.⁽¹⁷⁾ There are, however, a number of difficulties associated with this use of socio-economic scenarios. For example, in addition to the uncertainty in projections of future estimates of population, energy use and economic activity, estimates for many of these components are generally only available for large regions and must therefore be adjusted for assessments of smaller geographic areas, thus compounding the uncertainty.

The IPCC Data Distribution Centre provides links to the Center for International Earth Science Information Network (CIESIN) at Columbia University in New York, from which national-scale estimates of population and gross domestic product (GDP) are available. Other groups working on global-scale socio-economic scenarios include the World Business Council for Sustainable Development and the World Energy Council. Within Canada, scenarios of socio-economic variables, such as population projections, for future time periods up to 2026 have been developed by Statistics Canada.