The Organization for Economic Cooperation and Development, an international non-governmental organization that collects and studies economic statistics and social data said the value of exposed property could inflate to 35 trillion U.S. dollars. It called for quick action on coastal defenses for cities at risk, saying it can take 30 years to build defenses for vulnerable cities.

    "Climate change is already happening, and concerted action is needed now to prevent its worst impacts," OECD Secretary-General Angel Gurria said in a statement. "A range of economic policy options is available and political commitment is needed to implement them."

    About 150 million people could be exposed to a "1-in-100 year" flooding event by 2070 — up from 40 million now at risk, according to the report. The "1-in-100-year" scenario, which was used because it is a commonly accepted risk assessment standard, also assumed a mean rise in sea level of a half-meter (1.6 feet) by 2070.

    The report also listed cities most likely to be impacted in terms of population and cost, focusing on 136 port cities around the world.

    Calcutta, India, heads the list of the top 10 cities at risk in 2070 in terms of population exposure, with India's Mumbai, or Bombay, second. Miami, Fla., in ninth place, was the only city in a developed country on the report's list of the 10 top cities at risk due to population exposure.

    The other cities on that list are Dhaka (Bangladesh), Guangzhou (China), Ho Chi Minh City (Vietnam), Shanghai (China), Bangkok (Thailand), Rangoon (Myanmar) and Hai Phong (Vietnam), respectively.

    For present-day conditions, the top 10 cities in terms of exposed population are estimated to be Mumbai, Guangzhou, Shanghai, Miami, Ho Chi Minh City, Calcutta, Greater New York, Osaka-Kobe (Japan), Alexandria (Egypt) and New Orleans, respectively.     (Dec.5)