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Real estate agents know this: People love to live near water. A new study estimates that one tenth of the world's population lives in low-lying coastal zones. Those locations have always carried risks and now they could be hit hard by climate change.

NPR's Nell Boyce has more.

NELL BOYCE: The world at night makes for some very interesting satellite images. You can see the continents outlined with bright points of light. That's because people have always built cities and towns along the coasts.

Ms. DEBORAH BALK (Acting Associate Director, Institute for Demographic Research): Anecdotally, we know that that's true. That people do live near oceans. I mean we know that the major cities in the world are located near the mouths of rivers.

BOYCE: That's Deborah Balk, a demographer who works at the City University of New York.

Ms. BALK: You know, it's nice to live near water. But I think historically people have also known that there are risks.

BOYCE: Those risks may change as the global climate warms. Last month, an international panel of researchers warned that sea levels could rise by more than a foot over the next century. Other scientists believe it could be much more than that. And hurricanes may become more severe as well.

All of this made Balk wonder exactly how many people live in places that could potentially face flooding and storm damage. That information wasn't available. There were just vague estimates.

Ms. BALK: For a while it was said, oh, two thirds of all the population lives within a hundred kilometers of a coastline.

BOYCE: So Balk and some colleagues did a more precise study. They used satellite data to map out places along the coast that have low elevations -less than 30 feet above sea level. Then, to find out who lived there, they looked at census figures from over 200 countries. The numbers showed that low areas are home to 634 million people.

Ms. BALK: Roughly one in 10 persons in the world lives in this low-elevation coastal zone.

BOYCE: And she says some countries have lots of people living there.

Ms. BALK: China, India, Bangladesh, Vietnam, Indonesia, Japan, Egypt, the United States, Thailand and the Philippines. Those are your top 10.

BOYCE: It turns out that most of the world's largest cities are at least partially in these low areas. That's important because people are moving to the cities more and more.

Ms. BALK: Over the next hundred years, where we anticipate seeing sea-level rise, we will also see most of our population growth occurring in urban areas.

BOYCE: Now, Balk says this study is just a start. Some low-lying places will be more at risk than others because of weather patterns and geography. For example, here in the U.S., the southern Gulf Coast is more vulnerable than the West Coast. Still, she hopes the study gets more people thinking.

Ms. BALK: You know, some of these countries are poor, very poor, and thinking about climate change is not a high priority.

BOYCE: Thomas Dietz is a professor at Michigan State University who studies the human side of climate change. He says this is a great study. But he thinks we need to get even more detail on who lives in the very lowest areas along the coast.

Professor THOMAS DIETZ (Director, Environmental Science and Policy Program, Michigan State University): We just don't have good enough satellite imagery at this point to go further than what they did in this analysis. Nonetheless, I think the analysis is really an important step forward because it highlights the need to really incorporate these kinds of factors in planning.

BOYCE: The study appears in the current issue of the journal Environment and Urbanization.

Nell Boyce, NPR News.

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