ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario28amounts raises questions regarding their level ofuncertainty in simulating this important parameter.These spatial and temporal limitations of GCMs andtheir output should be kept in mind while conductinghydrologic studies of the potential impacts of climatechange and in interpreting results, whether utilizing thechange field method or another type of downscalingmethodology (Chapter 4).Since all SRES scenarios (see Table 3.1) are developedfrom plausible versions of the future from ademographic, economic, social, cultural, environmental,technological, and governance perspective, they are allcandidates for use in an assessment. Ideally all scenarioswould be used in each assessment. The advantage ofthis approach is that it results in a range of outcomes.The range creates a distribution of possible outcomesand allows the likelihood and the uncertainty associatedwith the simulation results to be estimated. However,the level of effort required makes this approachchallenging if not impractical in most cases. Mortsch etal. (2005) outline one process for selecting GCM scenariocombinations for climate assessments that attempts tobracket the range of possible outcomes using a fourGCM scenario set. A rationale for scenario selectionis included as part of the guidance offered in thisdocument (see Section 6.4).Many climate change impact assessments requireinformation at a smaller spatial scale than the GCMsprovide to reflect local conditions and may also requirea better attempt to capture changes in future variability(i.e., extreme events) for key variables. Several optionalmethods and many variations on these methods are thesubject of active research in Canada and worldwide. Theremainder of this chapter focuses on the major findingsrelating to the development of high resolution futureclimate data that are useable for studies at the local scale.4.2 Synthetic and Analogue Scenarios4.2.1 Synthetic ScenariosThe simplest future climates to construct involvesynthetic scenarios. Synthetic scenarios are created byadjusting meteorological parameters in a time series byarbitrary amounts or by amounts loosely-based on GCMoutputs or paleoclimatological reconstructions. Oftenthe parameter adjustments are made to the monthly orannual averages in the time series. The objective of thistype of scenario is both to provide a basis for testingthe sensitivity of a system to change and to identifycritical climate related thresholds in terms of systembehaviour. Synthetic scenarios are easy and inexpensiveto apply, require minimal resources, and can be setupwith incremental degrees of change (e.g., +1C o, +2C o,or ±10%, ± 20% change in precipitation) to test variouslevels of climate change and to potentially identifythresholds (Willows and Connell, 2003). These syntheticscenarios are, however, not consistent with estimates ofchange made using GCMs with SRES scenarios (Feenstraet al., 1998) and may not be physically plausible (Mearnset al., 2001). Synthetic scenarios have been used as anexploratory step in early impact assessments to identify ifthe system or region of interest is climate sensitive. Theyare not suitable for continued use in Ontario for climatechange impact assessment of hydrologic systems.4.2.2 Analogue ScenariosAn analogue scenario is one that relies upon climateinformation “borrowed” from a different time period(i.e., temporal analogue) or place (i.e., spatial analogue).In applying analogues, a period in history or anotherplace on the planet that may have a similar climate tothat anticipated for the study area at a future time islocated. Alternatively, extreme events can be transposedfrom a region and superimposed on the study area toassess the sensitivity of the study area to that event.In one study, a climate from Virginia was substitutedfor a Southern Ontario climate to reflect anticipatedwarming trends (Kling et al., 2003). In another study, thewater balance, flows and lake levels in the Great Lakeswas assessed using four spatial transposition scenarios(Croley et al., 1996). The selection of the four climateswas guided by GCM projections and they representedwarmer and wetter or warmer and drier conditions to thesouth of the Great Lakes basin. The climates from theseregions were input to Great Lakes hydrologic models toassess the impacts.Analogue scenarios developed using extreme eventshave also been proposed; these emulate the 2003European heat wave and flooding events relatingto intense precipitation in Bangladesh (Mirza, 2003).Paleoclimatic information for a historical warm periodmay be used to construct a climate time series for thestudy area to represent the future. These scenarios havethe advantage that they may be easy to apply and caninclude a wide array of physically realistic and consistent