The report by Nature Climate Change says the cities at greatest risk, as measured by annual average flood losses, are Guangzhou (China), Miami (US), New York (US), New Orleans (US), Mumbai (India), Nagoya (Japan), Tampa-St Petersburg (US), Boston (US), Shenzen (China) and Osaka-Kobe (Japan).
Owing to their high wealth and low flood protection, Miami, New York and New Orleans comprise 31% of total losses. Adding Guangzhou, the four top cities account for 43% of global losses as of 2005, when the cost of worldwide flood damage was an estimated $6bn a year.
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, Guayaquil (Ecuador), Ho Chi Minh City (Vietnam) and Abidjan (Ivory Coast) are among the most vulnerable.
Flood defences were typically designed for past conditions, so even a moderate rise in sea level would lead to soaring losses in the absence of adaptation, the study warns. Inaction, it adds, could lead to losses in excess of $1tn a year.
Coastal cities will, therefore, have to improve their flood management, including better defences, at an estimated cost of $50bn annually for the 136 cities. The estimated adaptation costs are far below estimates of annual aggregate damage losses, should improvement measures not be introduced.
The study echoes a report in May by the UN office for disaster risk reduction (UNISDR) which warns that it will become increasingly untenable for governments and businesses to sweep the risk of disaster under the carpet, citing the examples of hurricane Sandy in the US, the Japanese earthquake and tsunami, and the Thai river floods.
The UNISDR report also says economic losses from disasters have spun out of control, and called on the business community to incorporate disaster risk management into their investment strategies to avoid further losses. The study, entitled Creating shared value: the business case for disaster risk reduction, reviewed disaster losses in 56 countries. It found that direct losses from floods, earthquakes and drought have been underestimated by at least half. This century, losses from disasters have amounted to $2.5tn.
Even with better protection, the magnitude of losses will increase – often by more than 50% – when a flood occurs. Dr Stephane Hallegatte, author of the Nature Climate Change study, said: "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, policymakers should consider early warning systems, evacuation planning, more resilient infrastructure, and financial support to rebuild economies, the report says.
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 metres by 2050. About a quarter of the 136 cities are in deltas and exposed to local subsidence and sea-level change, especially where groundwater extraction accelerates natural processes.