addressing climate change adaptation in regional transportation plans

Addressing Climate Change Adaptation in Regional Transportation PlansA Guide for California MPOs and RTPAsSelect Climate ScenariosDue to the uncertainty inherent in climate projections, it is advisable to considera range of emissions scenarios and GCMs. Two emissions scenarios—A2(medium-high) and B1 (low)—have been widely applied in statewide analyses ofclimate impacts in California. These scenarios provide the range of climatescenarios which MPOs and RTPAs should consider when evaluating the range ofpotential climate conditions. Of the two scenarios evaluated by California forstatewide climate assessments and used in this guide, the A2 scenario is the morerealistic choice for decision-makers to use for climate adaptation planning.According to the 2009 California Climate Adaptation Strategy, “the world hasfollowed a ‘business as usual’ emissions pathway, which most closely resemblesthe A2 scenario.”A2 versus B1: Which Scenario Should I Use?Two GHG emissions scenarios – referencedas A2 and B1 – were created by theIntergovernmental Panel on Climate Change(IPCC) and used in this guide. These are bothscenarios evaluated by California forstatewide climate assessments. Eachscenario leads to a projection of possibleemissions levels based on population growthrate, economic development, and otherfactors. Ultimately, the effect on climatechange depends on the amount and the rateof accumulation of heat‐trapping gases inthe atmosphere that these scenariossuggest.Emissions ScenariosThe A2 (medium-high) and B1 (low) emissionsscenarios reflect potential reasonable range ofclimate conditions (Cayan et al., 2012). Climatedata for individual stressors and analysis years areprovided for both the A2 and B1 emissionsscenarios. It is preferred for planners to use the A2scenario from an impacts and adaptation point ofview, as it is the more aggressive of the twoprobable scenarios.Projecting potential climate trends and extremesrequires first establishing future scenarios ofpopulation, economic, and technologicalconditions that will influence future climatepatterns. Due to the high level of uncertainty inthe evolution of these factors, a series of qualitativestorylines describing the evolution of possibletrajectories of heat-trapping GHG emissions weredeveloped by the International Panel on ClimateChange (IPCC) 18 to guide climate change modelingefforts (IPCC, 2007). The IPCC’s (2000) specialreport on emissions scenarios (SRES) provides sixscenario groups of plausible global emissionspathways, with no assigned probabilities ofoccurrence. Two of these scenarios, A2 and B1 are often selected to represent,respectively, medium-high and relatively low emissions projections (Cayan et al.,2012). These emissions scenarios represent the world as follows:Of the two options provided, the A2 scenariois the more realistic choice for decisionmakersto use for climate adaptationplanning. Generally, the B1 scenario mightbe most appropriately viewed as a version ofa “best case” or “policy” scenario foremissions, while A2 is more of a status quoscenario incorporating incrementalimprovements.18 Unless otherwise stated, all references to the IPCC report on emission scenarios refer tothe Fourth Assessment Report (AR4).Cambridge Systematics, Inc. 10-9