updated 6/6/2011 7:17:55 AM ET 2011-06-06T11:17:55

As climate warming causes the oceans to rise, subsequent flooding and storm surges are sure to impact people in coastal areas around the world. But estimates for how many people will be inundated have fallen short by many millions of people, suggests a new study.

For the first time, the study combined predictions of sea level rise with forecasts of how many people will live in vulnerable areas 20 years from now. Previous studies have factored in current population numbers, which are only getting bigger.

The new research, which also foresaw a ripple effect as people move away from the coasts, offers a better idea of how many people will be at risk from environmental change. That should help communities better prepare and protect themselves.

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"We're underestimating in potentially very significant ways the magnitude of impact," said Katherine Curtis, a sociologist and demographer at the University of Wisconsin, Madison. "We're missing millions of people."

Among the many potential consequences of climate change, rising sea levels are bound to cause problems, particularly for the more than 10 percent of people around the world who live in low-elevation coastal communities. In those places, flooding along with a spike in severe storms are likely to destroy homes and increase health risks in the years to come.

To find out just how many people are potentially vulnerable, Curtis and colleague Annemarie Schneider began with the Intergovernmental Panel on Climate Change's predictions about sea level rise over the next few decades. Then, they looked geographically to identify four of the most vulnerable regions in the United States as California, Florida, New Jersey and South Carolina.

When the researchers combined sea-level data with projected populations in those areas for the year 2030, they came up with an estimate of 19.3 million people who will be at risk of being inundated by rising sea levels in just the four regions they looked at.

That number, which they reported in the journal Population and Environment, is 35 percent higher than the estimated 12.5 million people who would have been predicted to be at risk if the study had considered population levels in the year 2000. And it's 30 percent higher than the 13.7 million people predicted with 2008 population levels.

In Florida alone, nearly 10 million people are likely to be displaced by storm surges, flooding and inundation, the study found.

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Coastal areas aren't the only places that will feel the effects. As people get pushed out of their homes, they will move elsewhere, putting extra burdens on the communities that receive them.

"These places that are impacted are not social or demographic islands," Curtis said. There are "potential ripple effects throughout the United States."

Studies like this one should help communities decide where and how to build homes, schools, roads and other structures, said Deborah Balk, a demographer at the City University of New York's Institute of Demographic Research.

"In general, doing this kind of work is important because we have to be able to estimate population risk in a systematic way," Balk said. "Communities need to know."

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