Stephane Hallegatte, an economist at the World Bank, and his coauthors tallied up estimated flood damage losses for the world's 136 largest coastal cities, on the basis of local population and real estate and infrastructure values crunched with data on each location's elevation, exposure to extreme weather like hurricanes, and existing coastal protection infrastructure. Then he extrapolated these costs into the future using UN population and urbanization models, economic models from the Organization for Economic Cooperation and Development, and climate models of future sea level rise.

Flood exposure is increasing in coastal cities owing to growing populations and assets, climate change, and subsidence.  Average global flood losses in 2005 are estimated to be approximately US$6 billion per year, increasing to US$52 billion by 2050 with projected socio-economic change alone. With climate change and subsidence, present protection will need to be upgraded to avoid unacceptable losses of US$1 trillion or more per year. Installing robust protective infrastructure that accounts not just for sea level rise but also population growth and future shoreline development could reduce annual losses to $52 billion.  

The list of 20 cities most at risk, based on average annual losses due to floods, if no adaptation is undertaken is topped by Guangzhou, Miami, New York, New Orleans, Mumbai, Nagoya, Tampa-St. Petersburg, Boston, Shenzen, Osaka-Kobe and Vancouver.  However, the authors note that when defense measures are taken into account, Vancouver would fair significantly better than cities in developing countries with fewer resources to guard against flooding.

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