Cities at Risk of Rising Sea Levels

In all, 634 million people live in the threatened coastal areas worldwide - defined as those lying at less than 33 feet above sea level - and the number is growing, said the study published in the journal Environment and Urbanization.

More than 180 countries have populations in low-elevation coastal zones, and about 70 percent of those have urban areas of more than 5 million people that are under threat. Among them: Tokyo; New York; Mumbai, India; Shanghai, China; Jakarta, Indonesia; and Dhaka, Bangladesh.

The peer-reviewed scientific study said it is the first to identify the world's low-lying coastal areas that are vulnerable to global warming and rising sea levels. It said 75 percent of all people living in vulnerable areas are in Asia, with poorer nations most at risk.

The study gives no time frame for rising sea levels or the potential flooding in individual countries. It warns, however, the solution to the problem will not be cheap and may involve relocating many people and building protective engineering structures. And, it adds, nations should consider halting or reducing population growth in coastal areas.

"Migration away from the zone at risk will be necessary but costly and hard to implement, so coastal settlements will also need to be modified to protect residents," said Gordon McGranahan of the International Institute for Environment and Development in London, a co-author of the study.

IIED publishes the journal Environment and Urbanization. The other two co-authors of the study are Deborah Balk of the City University of New York and Bridget Anderson of Columbia University.

Separately, the authoritative Intergovernmental Panel on Climate Change said in a draft copy of a report expected to be released next week that coastlines are already showing the impact of sea-level rise. The draft copy, which was obtained by The Associated Press, said about 100 million people each year could be flooded by rising seas by 2080.

The draft copy warned that two biggest cities in North America - Los Angeles and New York - are at risk of a combination of sea-level rise and violent storms. By 2090, under a worst-case scenario, megafloods that normally would hit North America once every 100 years "could occur as frequently as every 3-4 years," the draft said.

In February, the IPCC warned of sea-level rises of 7-23 inches by the end of the century due to global warming, making coastal populations vulnerable to flooding and more intense hurricanes and typhoons.