The Coming $50 Billion a Year Cost of Coastal Floods
Policy + Politics

The Coming $50 Billion a Year Cost of Coastal Floods

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The major coastal cities most at risk for future flood losses can expect to spend around $50 billion a year to manage coastal floods, experts warn.

A new study, published in Nature Climate Change, estimates present and future flood losses in 136 of the world’s largest coastal cities, taking into account existing coastal protections.


The report builds on past work by the the Organization for Economic Co-operation and Development, 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.

The cities ranked most “at risk” today, as measured by annual average losses due to floods, span developed and developing countries: Guangzhou, Miami, New York, New Orleans, Mumbai, Nagoya, Tampa-St. Petersburg, Boston, Shenzen, Osaka-Kobe, and Vancouver. The countries at greatest risk from coastal city flooding include the United States and China.

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 losses across the 136 cities. Adding Guangzhou, the four top cities explain 43 percent of global losses as of 2005. (Read the original study.)

Total dollar cost is one way to assess risk. Another is to look at annual losses as a percentage of a city’s wealth, a proxy for local vulnerability. Using this measure, Guangzhou, China; Guayaquil, Ecuador; Ho Chi Minh City, Vietnam; and Abidjan, Ivory Coast are among the most vulnerable.

To estimate the impact of future climate change the study assumes that mean sea-level, including contributions from melting ice sheets, will rise 0.2-0.4 meters 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 accelerate natural processes.

An important finding of this study is that because 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. Inaction is not an option as it could lead to losses in excess of $US 1 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.


“This work shows that flood risk is rising in coastal cities globally due to a range of factors, including sea-level rise,” says Robert Nicholls, professor of coastal engineering at the University of Southampton and co-author of the study. “Hence there is a pressing need to start planning how to manage flood risk now.”

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

“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,” says lead author Stephane Hallegatte from the World Bank.

To help cities deal with disasters when they do hit, policymakers should consider early warning systems, evacuation planning, more resilient infrastructure, and financial support to rebuild economies. The report also notes that large increases in port city flood risk may occur in locations that are not vulnerable today, catching citizens and governments off-guard.

The five cities with the largest estimated increase in flood risk in 2050 are Alexandria, Egypt; Barranquilla, Colombia; Naples, Italy; Sapporo, Japan; and Santo Domingo, Dominican Republic.

This article originally appeared in Source: University of Southampton.