Asia's massive delta cities have most to fear from catastrophic storm floods driven by climate change, according to an OECD report.

Of 136 port cities assessed around the world for their exposure to once-in-a-century coastal flooding, 38 percent are in Asia and27 percent are located in deltas, the Organisation for Economic Cooperation and Development (OECD) said.

Publication of the report coincided with an 11-day UN conference in Bali, Indonesia, aimed at shaping a long-term response to the climate peril.

Today, around 40 million people around the world are exposed to coastal flooding in large port cities, according to the report.

The top 10 cities most at risk, in terms of exposed population, are Mumbai, Guangzhou, Shanghai, Miami, Ho Chi Minh City, Kolkata, Greater New York, Osaka-Kobe, Alexandria and New Orleans.

The total value of assets exposed in the 136 port cities analysed is 3,000 billion (three trillion) dollars -- or around five percent of the global gross domestic product (GDP) in 2005, it says.

Miami, Greater New York, New Orleans, Osaka-Kobe, Tokyo, Amsterdam, Rotterdam, Nagoya, Tampa-St. Petersburg (Florida) and Virginia Beach (Virginia) are the most valuable pieces of real estate at risk.

By the 2070s, the total population exposed could more than triple, to around 150 million people.

Of the "Top 10" most exposed coastal cities in 2070, nine are in Asia.

The 10 are: Kolkata, Mumbai, Dhaka, Guangzhou, Ho Chi Minh City, Shanghai, Bangkok, Rangoon, Miami and Haiphong (Vietnam).

The total value of assets exposed by the 136 cities in the 2070s is put at 35 trillion dollars, or nine percent of projected global annual GDP.

Ranked according to assets exposed to flooding, the 2070 list is headed by Miami, Guangzhou, Greater New York, Kolkata, Shanghai, Mumbai, Tianjin (China), Tokyo, Hong Kong and Bangkok.

"Climate change is already happening, and concerted action is needed now to prevent its worst impacts," said OECD Secretary General Angel Gurria, noting the December 3-14 Bali meeting.

"A range of economic policy options is available and political commitment is needed to implement them,"

The report was authored by OECD experts working with scientists from Britain's University of Southampton, the US company Risk Management Solutions and France's Meteo-France and Centre for International Climate and Environmental Research (CIRED).

Their assessment projects the damage that would be caused by an extreme but very rare weather event -- a combination of storm surge and high winds that, in purely statistical terms, occurs once a century.

The exposure rises in the 2070s because of coastal subsidence and further population growth in these port cities, and the risk increases due to climate change, which will boost sea levels and is likely to make storms more powerful.

In its estimate, the OECD assumes that sea levels will rise 0.5 metres (19.7 inches), as a result of thermal expansion (when water warms, it expands) as well as from melting ice sheets.

In the first volume of a triple report on global warming, published in February, the UN's Intergovernmental Panel on Climate Change (IPCC) said sea levels would climb between 18 and 59 centimetres (7.2 and 23.2 inches) by 2100.

But in a summary of the overall report, issued in Valencia, Spain, last month, it said it could no longer put an upper limit on this projection because of uncertainties about ice-sheet loss.

The OECD report does not factor in the effectiveness of flood defences in its calculations, saying that exposure to a flood "does not necessarily translate into impact."

But it notes that cities in rich northern countries have -- and are more likely to have in the future -- much better protection than cities in poor tropical countries, where the flood risk from cyclones is greatest.

"London, Tokyo and Amsterdam are protected to better than the one-in-1,000-year standard, while many developing countries have far lower standards, if formal flood defences exist at all," the report said.