Climate change, population and economic growth may increase flood losses in major coastal cities to nine times current levels by 2050, U.K. researchers say.

Estimating present and future flood losses -- or the global cost of flooding -- from a combination of climate change and economic development in 136 of the world's largest coastal cities suggests a significant increase in the risk between now and 2050, they said.

The scientists, including coastal engineering specialists at Britain's University of Southampton, said because flood defenses have been designed for past conditions even a moderate rise in sea levels would lead to soaring losses in the absence of adaptation.

Coastal cities will need to improve their flood management, including better defenses, at a cost estimated at about $50 billion per year for the 136 cities, they said.

The countries at greatest risk from coastal city flooding include the United States and China, the researchers said, and due to their high wealth and low protection level, three American cities -- Miami, New York City and New Orleans -- are responsible for 31 percent of the estimated losses across the 136 cities.

"This work shows that flood risk is rising in coastal cities globally due to a range of factors, including sea-level rise," Southampton researcher Robert Nicholls, the study's co-author, said in a university release Monday. "Hence there is a pressing need to start planning how to manage flood risk now."

Even with better protection, the magnitude of losses is likely to increase, the researchers said.

"There is a limit to what can be achieved with hard protection: populations and assets will remain vulnerable to defense failures or to exceptional events that exceed the protection design," lead study author Stephane Hallegatte of the World Bank said.

The study has been published in the journal Nature Climate Change.