LONDON — More than two-thirds of the world's large cities are in areas vulnerable to global warming and rising sea levels, and millions of people are at risk of being swamped by flooding and intense storms, according to a new study released Wednesday.
Based on new computer population models and NASA satellite data, more than 634 million people live in the threatened coastal areas — defined as those lying at less than 10 meters (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 those zones, and two thirds 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.
Twenty-one nations have more than half of their population in the zone — and 16 of those are small, island states. More than 90 percent of the Maldives, the Marshall Islands, Tuvalu, the Cayman Islands and the Turk and Caicos Islands are less than 33 feet above sea level.
The study gives no time frame for rising sea levels or the potential flooding in individual countries. It warns, however, that responses 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.
Asia at greatest risk
The peer-reviewed scientific study said 75 percent of all people living in vulnerable areas are in Asia, with poorer nations most at risk. The five nations with the largest total population living in endangered coastal areas are all in Asia: China, India, Bangladesh, Vietnam and Indonesia.
Between 1994 and 2004, about one-third of the world's 1,562 flood disasters occurred in Asia, with half of the total 120,000 people killed by floods living in that region, the study said.
Globalization is promoting a shift towards coasts in countries including China and India by fostering a world trade largely dependent on shipping.
China, for example, has created special economic zones in coastal locations such as Shanghai. Fast economic growth has been associated with very rapid coastward migration, with the population in low-lying areas growing at almost twice the national population growth rate between 1990 and 2000, the study said.
"Unless something is done, there is the possibility that, as well as the people living in the low-elevation coastal zone, China's economic success will be placed at risk," it said.
Megafloods more often?
Separately, a draft copy of a report to be released next week by the U.N. Intergovernmental Panel on Climate Change said 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.
Some experts consider that scenario conservative since the IPCC review did not take into account more recent findings on melting ice sheets in Greenland and Antarctica.
The Associated Press and Reuters contributed to this report.