The authors focus on a scenario that stabilizes concentrations at 450 ppm-CO2-eq, leading to 42 cm of global mean sea-level rise in 1995–2100 (GMSLR) and an unmitigated one leading to 63 cm of GMSLR. They also consider sensitivity scenarios reflecting increased tropical cyclone activity and a GMSLR of 126 cm.
The only adaptation considered is upgrading and maintaining dikes.
Under the unmitigated scenario and without adaptation, the number of people flooded reaches 168 million per year in 2100. Mitigation reduces this number by a factor of 1.4, adaptation by a factor of 461, and both options together by a factor of 540.
The global annual flood cost (including dike upgrade cost, maintenance cost and residual damage cost) reaches USD 210 billion per year in 2100 under the unmitigated scenario without adaptation. Mitigation reduces this number by a factor of 1.3, adaptation by a factor of 5.2, and both options together by a factor of 7.8.
When assuming adaptation, the global annual flood cost relative to GDP falls throughout the century from about 0.06% to 0.01–0.03% under all scenarios including the sensitivity ones. From this perspective, adaptation to coastal flood impacts is meaningful to be widely applied irrespective of the level of mitigation. From the perspective of a some less-wealthy and small island countries, however, annual flood cost can amount to several percent of national GDP, and mitigation can lower these costs significantly.
The authors conclude that adaptation and mitigation are complementary policies in coastal areas.
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