ClimateChange Assessment Guide.pdf - University of Waterloo

Future Climate Scenarios3. Future Climate Scenarios19Scenarios support the understanding of, and decisionmakingfor, global environmental issues such asclimate change (Parson et al., 2007). Scenarios havefigured prominently in strategic planning, analysis andassessment as they describe a range of potential futureconditions that assist when dealing with uncertainty(Parson et al., 2007). Scenarios can inform adaptationand mitigation by characterizing potential impacts andthe severity of these impacts; scenarios can assist inidentifying key vulnerabilities or priorities, or providea framework for evaluating alternative adaptationmeasures (Parson et al., 2007).This section provides background information on howthe sensitivity of the climate to increasing concentrationsof greenhouse gases (GHGs) is assessed using globalscale modelling. This section describes Global ClimateModels (GCMs); the primary tool used to deriveprojections of future climates due to anthropogenicinfluences. Also discussed are the GHG emissionscenarios resulting from plausible future economic,technological and social conditions, and that influencethe evolution of climatic conditions in the future.3.1 Global Climate Models (GCMs)A GCM is a complex computer program that is usuallyexecuted on a state-of-the-art super computer. Physicallaws governing the behaviour of the earth’s atmosphere,oceans, sea-ice and land surface, as well as, theinteracting physical processes that determine weatherand climate are represented as mathematical algorithms(see Figure 3.1). The computer program simulates short(approximately 15-minute) time steps on a global threedimensionalgrid. Questions with respect to the past,current and future climate (long-term average of dailyweather patterns) are explored through simulationsusing these models. The focus is on the long-termenergy budget of the Earth’s atmosphere – balance ofabsorbed incoming short-wave radiation from the sunand losses of long-wave radiation from the Earth tospace. The climate is recreated by running the model forvery long periods of time, simulating the slow changesof past climate and then projecting it decades or evencenturies into the future. A typical 100-year simulationcan take several months to complete. Derived productssuch as daily, monthly and seasonal statistical summariesor maps are usually used for climate change impactassessments. Direct output of many meteorologicalvariables (e.g., air temperature, winds, surface pressureand precipitation amount) is available on this grid (seeChapter 6).defScenario • 1. coherent, internally consistent andplausible description of a possible future state ofthe world (Parson et al., 2007); 2. plausible andoften simplified description of how the futuremay develop, based on a coherent and internallyconsistent set of assumptions about drivingforces and key relationships; scenarios may bederived from projections, but are often basedon additional information from other sources,sometimes combined with narrative storylines(Baede et al., 2007).Climate scenario • plausible and often simplifiedrepresentation of the future climate, based onan internally consistent set of climatologicalrelationships that has been constructed forexplicit use in investigating the potentialconsequences of anthropogenic climate change,often serving as input to impact models; climateprojections often serve as the raw material forconstructing climate scenarios, but climatescenarios usually require additional informationsuch as the observed current climate (Baedeet al., 2007).Projection • used in two senses in the climatechange literature. In general usage, a projectioncan be regarded as any description of thefuture and the pathway leading to it. However, amore specific interpretation has been attachedto the term “climate projection” by the IPCCwhen referring to model-derived estimates offuture climate (http://www.ipcc-data.org/ddc_definitions.html).Forecast/Prediction • When a projection isbranded “most likely” it becomes a forecast orprediction. A forecast is often obtained usingmultiple deterministic models. Where multiplemodels produce similar output, some level ofconfidence can be attached to the projections(http://www.ipcc-data.org/ddc_definitions.html).