Climate change, population growth and urban development will mean the number at risk will rise from the current 40 million.
The report analysed the vulnerability now and in the future of 130 port cities to a major flood, on a scale likely to occur once in 100 years.
Miami in Florida will remain the city with the highest value of property and assets exposed to coastal flooding, the report said.
However, Guangzhou in China will be the second most exposed city in terms of assets in 2070, followed by New York, Kolkata, Shanghai, Mumbai, Tianjin, Tokyo, Hong Kong and Bangkok, the report added.
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By Ben Jacob