PARIS - The world's 136 largest coastal cities could face combined annual losses of US$1 trillion (S$1.3 trillion) from floods by 2050, unless they drastically improve their defences, a study warned on Sunday.
Current losses are about US$6 billion per year, with four cities - Miami, New York and New Orleans in the United States and Guangzhou in China - incurring 43 per cent of the costs, according to a report in the journal Nature Climate Change.
World Bank economist Stephane Hallegatte and his colleagues composed a loss-risk scenario based on city-population growth as well as different levels of sea-level rise, protection upgrades and subsidence, which is the sinking of surface areas, often linked to the extraction of oil or other ground resources.
Assuming cities improve their protection to maintain the flood risk at current levels, and based purely on the projected growth of city populations and the assets accumulated there, the team warned of a nine fold increase in losses to US$52 billion per year by 2050.
When the team adds the effects of climate change-induced sea-level rise and subsidence, the figure increases to between US$60 billion and US$63 billion per year.
"With no adaptation (of flood protection), the projected increase in average losses by 2050 is huge, with aggregate losses increasing to more than US$1 trillion per year", a worst-case-scenario outcome, said the study.
With protection upgrades, the cities with the highest projected annual losses by 2050 were Guangzhou (US$13.2 billion), Mumbai (US$6.4 billion) and Kolkata (US$3.4 billion) in India, Guayaquil (US$3.2 billion) in Ecuador and Shenzhen (US$3.1 billion) in China.
For Guangzhou, this represented an 11 per cent rise on 2005 losses and, for Kolkata, 24 per cent, said the authors.
No. 6 on the list was Miami, with projected annual losses of US$2.5 billion, followed by Tianjin in China with US$2.3 billion, New York with US$2 billion, Ho Chi Minh City in Vietnam with US$1.9 billion and New Orleans with US$1.9 billion.
According to Mr Hallegatte, the team has calculated that about US$50 billion per year would be required to boost flood protection for the 136 cities in the report - "far below" the estimated losses.
"Failing to adapt is not a viable option in coastal cities," added the report.