ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario62GCMs used in the Third Assessment (TAR), SecondAssessment (SAR) and First Assessment (FAR) Reportsof the IPCC are now obsolete and should no longer beused for scenario development.6.4.2.2 Type of GHG Emission ScenarioGuidance:The most recent GHG emission scenarios shouldbe used.Emission scenarios describe the key forcings for GCMsimulations to assess the sensitivity of the climatesystem to increasing concentrations of GHGs. TheSRES emission scenarios used for the 2007 IPCC AR4remain the most up-to-date. They are fundamentallythe same as what was developed for the 2001 IPCC TARand summarized in the Special Report on EmissionsScenarios (Nakicenovic et al., 2000). Other non-SRESscenarios were also used in the AR4 but they should notbe used in impact and adaptation assessments. The fullsuite of emission scenarios used as “forcings” in theupdated GCMs for the AR4 is described in Table 6.1. Notall SRES emission scenarios were run in all GCMs dueto resource constraints; the most widely used scenarioswere the A2, A1B and B1 and represent three differentemission pathways - low, intermediate and high cases(see Table 6.2 and Figure 6.2). All emission scenarios areconsidered equally plausible; therefore, all can be usedfor scenario development. However, the current globalGHG emissions estimates are greater than the highestemission scenario - the A1FI - suggesting that theseemission scenarios may be conservative.Earlier GCM-based modelling, scenario developmentand associated impact and adaptation assessments usedsix future greenhouse gas emissions scenarios (IS92a-f)reported in the 1992 Supplementary Report (Leggett etal., 1992) to the IPCC First Assessment Report. Althoughthe IPCC recommended using all scenarios, the mostcommonly used was the IS92a “business-as-usual”scenario (1% per year CO2 increase). The first GCM runs- equilibrium model runs - used an instantaneous doubleof carbon dioxide (2xCO2) or quadrupling of carbondioxide (4xCO2) scenarios. The IS92 family of scenariosand associated GCM runs as well as the equilibrium2xCO2 and 4xCO2 GCM runs should no longer be usedfor scenario development.Table 6.1 Emission scenarios used as “forcings” in the GCM runs for the IPCC AR4SRES ScenariosA1A2B1B2A1FI (fossil intensive)A1T (non-fossil energy)A1B (Balanced)Use for climatechange scenariodevelopmentYESYESYESYESNon- SRES ScenariosPICTLExperiments run with constant pre-industrial levels of greenhouse gases.(Essentially a control run without greenhouse gas forcing)Use for climatechange scenariodevelopmentNO20C3MCOMMITExperiments run with greenhouse gases increasing as observed through the20th century.An idealised scenario in which the atmospheric burdens of long-livedgreenhouse gases are held fixed at AD2000 levels.NONOTable 6.1 continued on next page