An OECD study has found as many as 150 million people in coastal cities worldwide are likely to be reliant on flood defences by the 2070s – more than treble the 40 million people currently exposed to flood.

The study analyses vulnerability to a major flood, on a scale likely to occur once in 100 years. It focuses on 130 international cities and was compiled by Risk Management Solutions and leading scientists.

It reveals climate change and urban development are increasing the world’s exposure to flood catastrophes, and estimates that by 2070 property and infrastructure worth $US35 trillion ($40 trillion) – or 9% of the world’s projected GDP – will be at risk in coastal areas.

The US city of Miami faces the greatest loss of property and infrastructure assets – estimated at $US3.5 trillion ($4 trillion).

Guangzhou in China faces a $US3.3 trillion ($3.8 trillion) loss while losses in New York could run to $US2.1 trillion ($2.4 trillion).

The report suggests insurers should encourage policyholders to adapt to climate change, and warns of the time required to implement major flood protection works – typically up to 30 years.