Climate change combined with rapid population increases, economic growth and land subsidence could lead to a more than nine-fold increase in the global risk of floods in large port cities between now and 2050.

'Future Flood Losses in Major Coastal Cities', published in Nature Climate Change, is part of an ongoing project by the Organisation for Economic Co-operation and Development (OECD) 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.

The authors estimate present and future flood losses - or the global cost of flooding - in 136 of the world's largest coastal cities, taking into account existing coastal protections. Average global flood losses in 2005, estimated at about US$6 billion per year, could increase to US$52 billion by 2050 with projected socio-economic change alone.

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 per cent of the losses across the 136 cities. Adding Guangzhou, the four top cities explain 43 per cent of global losses as of 2005.

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 defences 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 defences, at a cost estimated around US$50 billion per year for the 136 cities.

Robert Nicholls, Professor of Coastal Engineering at the University of Southampton and co-author of the study, says: "This work shows that flood risk is rising in coastal cities globally due to a range of factors, including sea-level rise. 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 per cent, when a flood does occur. According to Dr Stephane Hallegatte, from the World Bank and lead author of the study: "There is a limit to what can be achieved with hard protection: populations and assets will remain vulnerable to defence failures or to exceptional events that exceed the protection design."

To help cities deal with disasters when they do hit, policy makers 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.

The abstract and full article can be downloaded from the Nature Climate Change website