ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario22et al., 2008), or whether the assumption that GCMs thatreplicate current climate well will also perform well inprojecting future climate.GCMs have been one of the most commonly usedresources for developing changed climate input datafor water resource impact assessments. Methods usedrange from those that rely entirely on the GCM tomodify historic time series on a monthly average basis(i.e., change field or delta method) through to those thatuse the GCM output to inform local climate modellingtools/approaches (see Chapter 4). In any case, griddedoutput from GCMs is not suitable for direct use inhydrologic models. The GCM resolution, usually on theorder of several hundred kilometres and poor reliabilityat a single time step at a particular grid point, precludeusing these data directly to represent present-day andfuture climates. The change field method is outlined inSections 4.1 and 6.5.2.“Our understanding of climate is still insufficient tojustify proclaiming any one model “best” or evenshowing metrics of model performance that implyskill in predicting the future. More appropriate in anyassessments focusing on [climate change] adaptationor mitigation strategies is to take into account, in apertinently informed manner, the products of distinctmodels built using different expert judgements atcenters around the world” (Bader et al., 2008 11).Essentially, the current recommendation is that climatechange impact assessment should use as manyscenarios of climate change as possible to cover awide range of potential outcomes; scenarios can bederived from differences in climate model formulationsor differences in emission scenarios that drive theclimate response. This point is crucial to developing anunbiased assessment, understanding the uncertaintiessurrounding future climate estimation and developingadaptation strategies that improve resilience to a widerange of future stresses.3.2 GHG Emission Scenarios“Future greenhouse gas (GHG) emissions are theproduct of very complex dynamic systems, determinedby driving forces such as demographic development,socio-economic development, and technologicalchange. Their future evolution is highly uncertain.Scenarios are alternate images of how the future mightunfold and are an appropriate tool to analyse howdriving forces may influence future emission outcomesand to assess associated uncertainties” (Nakicenovicet al., 2000 3).defEmission scenario • plausible representationof the future development of emissions ofsubstances that are potentially radiatively active(e.g., GHGs, aerosols), based on a coherent andinternally consistent set of assumptions aboutdriving forces (such as demographic and socioeconomicdevelopment, technological change)and their key relationships; concentrationscenarios, derived from emission scenarios, areused as input in a climate model [e.g., GCM] tocompute climate projections (Baede et al., 2007)Early modelling of the sensitivity of the climate systemto increasing GHGs was based on equilibrium doublingor quadrupling of carbon dioxide (2xCO 2, 4xCO 2)experiments. As the climate models developed intothe transient response, coupled atmosphere-oceanGCMs, the emission scenarios also evolved. In 1992,the IPCC developed six alternative emission scenariosthat included GHG increase only and GHG (CO 2) andsulphate aerosols combined (the IS92). The IS92a(“business as usual”) scenario represented a mid-rangescenario; emissions were based on historical increasesuntil 1990 then compounded annually at a rate of 1% peryear (Leggett et al., 1992). Although IS92a was the mostfrequently used emission scenario for climate modelsimulations, the IPCC recommended using all six IS92emission scenarios to capture the range of uncertainty infuture emissions (Barrow, 2002).In 2000, the IPCC Special Report on Emission Scenarios(SRES) published a set of new emission scenarios(Nakicenovic et al., 2000). The SRES scenarios werethe “most comprehensive, ambitious, and carefullydocumented emission scenarios produced to date” andpresented a “substantial advance from prior scenarios”(Parson et al., 2007 34). There were 40 SRES scenariosin total, each covering a range of GHG and sulphuremissions identified in the literature and the SRESscenario database (Nakicenovic et al., 2000). These newscenarios contributed to both the IPCC Third and Fourth