JICA Research InstituteClimate models supporting the IPCC Fourth Assessment Report predict that climate change willincrease local temperatures and precipitation in monsoon regions in Asia, where the number of largecities is increasing and existing urban areas are expanding, particularly along the coasts. In this study,Metro Manila, typical of Asian coastal megacities, is used as a case study to comprehensivelysimulate impacts of future climate change and identify necessary adaptation measures.In spite of the various uncertainties inherent in predictions, this study translates future global climatescenarios to regional climate scenarios, a process called “downscaling”. The B1 and A1FI scenariosof the IPCC SRES framework provide a basis for discussing local temperature and precipitationchanges in Metro Manila. Based on these scenarios, hydrological conditions such as river overflowand storm surge were projected. Flood simulation maps were then constructed showing the range ofpotential spatial spreads, inundation depths and flooding durations anticipated in the metropolis.Based on the flood simulation maps, socio-economic impact analyses were applied to understand thecharacteristics and magnitude of flood damage anticipated in the year 2050. The benefit side of theanalysis calculated avoided damage at the aggregate level. Tangible direct losses were assessed as inconventional flood control project analyses. Incremental costs to transportation (VOC and timecosts), and lost wages and income (sales) due to flooding were combined for tangible indirect costs.Note that the simulated flood maps and impacts are some cases among a wide range of futurepossibilities resulting from a “cascade of uncertainties” inherent in the various steps of themethodology.If flood control infrastructure improvements were stopped now, and the A1FI climate scenario isassumed, a 100-year return period flood could cause aggregate damages of up to 24% of the GRDP,while damages from a 30-year return period flood would be about 15% of the GRDP. If, however,infrastructure improvement based on the 1990 Master Plan is continued and climate scenario B1 isassumed, the projected damages would be only 9% of the GRDP for a 100-year return period flood,and 3% for a 30-year return period flood.Finally, options for adapting to the scenarios were selected, with the objective of eliminating asmuch as possible of the flooding projected in the flood simulations. Economic evaluations usingeconomic internal rate of return (EIRR) and net present value (NPV) were conducted by combiningthe costs of the adaptation options with the damages avoided by implementing those options. TheEIRR and NPV evaluations yielded different results, but they both suggest that filling theinfrastructure gap identified under the current Master Plan (for status quo climate) is the first andforemost priority
