As policymakers plan ahead in Hong Kong, Shanghai, Guangzhou and other major port cities, one of the most vexing questions they face is: how much will sea levels rise in coming decades? The stakes are high. By the 2070s, the number of people in the 136 big port cities around the world who are exposed to coastal flooding is projected to grow more than threefold, to around 150 million. If the predictions of scientists advising the United Nations on global warming are correct, port cities will have to endure more intense effects of climate change, including sea level rises, increasingly violent storms, land subsidence and flooding, according to a recent study for the Organisation for Economic Co-operation and Development (OECD). The study, published in July, forecast that the value of flood-exposed economic assets in these port cities - in the form of buildings, transport networks, utilities and other infrastructure - could grow even more dramatically than population exposure. It could reach US$35 trillion by the 2070s - some 9 per cent of projected global gross domestic product. What is the precise threat from rising sea levels to this accumulated wealth and power, and to the international maritime trade that these port cities dominate? Summarising the available scientific evidence on global warming and its effects, the Intergovernmental Panel on Climate Change advising UN member states concluded late last year that sea levels would probably be somewhere between 18cm and 59cm higher by the end of this century, after a rise of 17cm in the 20th century. There has been a range of predictions from scientists about how rapidly Greenland and Antarctic ice sheet melting could occur - as air and sea temperatures increase more sharply in polar regions than most other places. But, last week, a team of American glaciologists reached a more definitive conclusion. The team took account of the sea level rises expected from ice melting on Greenland, Antarctica and the world's smaller glaciers and ice caps, as well as thermal expansion of the seas and oceans. The most likely scenario, it found, was a total sea level rise of about 1 to 2 metres by 2100. If these glaciologists are right, what will the impact be? The Tyndall Centre for Climate Change Research in Britain has warned that Asia - because of its extensive coastline, many mega deltas, big port cities and dense population - will be the region by far the most seriously affected by such a rise. A 1-metre increase in sea levels, it calculated, would flood more than 800,000 sq km of land, displacing over 100 million people and causing a potential loss of nearly US$450 billion in economic production. The World Bank commissioned a group of specialists to assess the consequences of a sea level rise of 1 to 5 metres for 84 coastal developing countries. In relative terms, China was among the least affected of the 13 East Asian economies covered in the study, published last year. But in absolute terms, the damage to China would still be significant. The OECD report found that 15 of the 20 most populous port cities exposed to coastal flooding in the 2070s would be in Asia, with four of them in China - Guangzhou, Shanghai, Tianjin and Ningbo (with a total flood-exposed population of 23 million). Thirteen of the top 20 port cities with assets exposed to coastal flooding in the 2070s would be in Asia, six of them in China - the four above, plus Qingdao and Hong Kong. Total flood-exposed assets of the six: almost US$9.2 trillion. The OECD report concluded that the likely impact of climate change and sea level rises needs to be integrated into the coastal flood-risk management and urban development strategies of vulnerable countries. Advanced planning and investment are required as big flood- defence projects can take 30 years or more from planning to completion. Michael Richardson is an energy and security specialist at the Institute of Southeast Asian Studies in Singapore. mriht@pacific.net.sg