10 Coastal Cities at Greatest Flood Risk as Sea Levels Rise

Posted by News Editor on September 3, 2013

WASHINGTON, DC, September 3, 2013 (ENS) – The World Bank has crunched the numbers and the results are in. Five of the world’s 10 cities most at risk of flooding as climate change causes sea levels to rise are in the United States.

In terms of the overall cost of damages, they are: Miami, which is at greatest risk, followed by New York, New Orleans, Tampa and Boston.

Miami, Florida is built right at sea level. (Photo courtesy NOAA)

The other five are: Guangzhou, China; Mumbai, India; Nagoya, Japan; Shenzen, China; Osaka, Japan.

In its newly published quantification of present and future flood losses in the world’s 136 largest coastal cities, the bank calculates that the costs of flood damage could rise to $1 trillion a year if coastal cities do not adapt.

“Coastal cities face a high risk from increasingly costly flooding as sea levels rise amid climate change. Their current defenses will not be enough as the water level rises,” the bank warned in a statement announcing the new research.

Led by World Bank economist Dr. Stephane Hallegatte and the Organization for Economic Cooperation and Development, or OECD, the study forecasts that average global flood losses will multiply from $6 billion per city in 2005 to $52 billion a year by 2050 with just social-economic factors, such as increasing population and property values, taken into account.

Some of the cities where flood risk will increase the most in the coming years are not the cities where the risk is particularly high today, the study found.

Cities in developing countries move up the list when flood costs are measured as a percentage of city gross domestic product, or GDP. Many of them are growing rapidly, have large populations, are poor, and are exposed to tropical storms and sinking land.

The study lists the 10 most vulnerable cities when measured as percentage of GDP as: 1) Guangzhou; 2) New Orleans; 3) Guayaquil, Ecuador; 4) Ho Chi Minh City; 5) Abidjan; 6) Zhanjing; 7) Mumbai; 8) Khulna, Bangladesh; 9) Palembang, Indonesia; and 10) Shenzen.

In most of these cities, the poor are at greatest risk as rapid urbanization has pushed them into the most vulnerable neighborhoods, often in low-lying areas and along waterways prone to flooding, the study finds.

Port cities that have not been highly vulnerable in the past are among those facing the greatest increase in risk by 2050.

Leading the cities with the greatest increase in risk are: Alexandria, Egypt; Barranquilla, Colombia; Naples, Italy; Sapporo, Japan; and Santo Domingo, Dominican Republic.

“Coastal defenses reduce the risk of floods today, but they also attract population and assets in protected areas and thus put them at risk in case of the defense fails, or if an event overwhelms it,” Hallegatte said. “If they are not upgraded regularly and proactively as risk increases with climate change and subsidence, defenses can magnify – not reduce – the vulnerability of some cities.”

Low-lying areas of Guangzhou, China flood requently. (Photo by Michael Ho)

The research is part of an ongoing OECD project to explore the policy implications of flood risks due to climate change and economic development. This study builds on past OECD work which ranked global port cities on the basis of current and future exposure, where exposure is the maximum number of people or assets that could be affected by a flood.

To estimate the impact of future climate change the study assumes that mean sea level, including contributions from melting ice sheets, will rise between 0.2 meters and 0.4 meters (7.8 inches to 15.7 inches) by 2050.

In addition, about a quarter of the 136 cities are in deltas and exposed to local subsidence and local sea level change, especially where groundwater extraction accelerates natural processes.

Because existing flood defenses have been designed for past conditions, even a moderate rise in sea level would lead to soaring losses in the absence of adaptation, the study warns.

Inaction is not an option as it could lead to losses in excess of $US1 trillion. Therefore, coastal cities will have to improve their flood management, including better defenses, at a cost estimated around US$50 billion per year for the 136 cities.

Even with better protection, the magnitude of losses will increase, often by more than 50 percent, when a flood does occur.

Dr. Hallegatte 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.”

To help cities deal with disasters when they do hit, the study advises policy makers to consider early warning systems, evacuation planning, more resilient infrastructure and financial support to rebuild economies.

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