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Rising Sea Levels May Disrupt Lives of Millions, Study Says

Rodney Clement gingerly stepped from the sidewalk to the street through tidal flooding around his home in Charleston, S.C., last year. (Credit Grace Beahm/The Post and Courier, via Associated Press)

Sea-level rise, a problem exacerbated by greenhouse gas emissions, could disrupt the lives of more than 13 million people in the United States, three times the most current estimates, according to a study published Monday.

Rising seas, which already endanger coastal communities through tidal floods and storm surges, could rise three feet or more over the next century if emissions continue at a high level, threatening many shoreline communities. The study, published in Nature Climate Change, argues that most projections vastly underestimate the number of people at risk because they do not account for population growth.

For the study, the authors combined future population estimates with predicted sea-level rise, using data from the National Oceanic and Atmospheric Administration, to demonstrate that millions are at risk: 4.2 million if seas rise by three feet, or 13.1 million with a six-foot increase, a high-end estimate.

Mathew E. Hauer, one of the study’s authors and a doctoral student in geography at the University of Georgia, said, “We could see a huge-scale migration if we don’t deploy any protection against sea level rise.”

Recent studies have shown that the sea levels are rapidly increasing, probably at the fastest rate in 28 centuries, and its accompanying tidal flooding, increasingly frequent, is already causing headaches in low-lying places, especially in the South. Though sea levels have risen and fallen significantly in the past, scientists say they have been fairly constant for the last several thousand years.

Mr. Hauer and his co-authors also found a highly regional effect of sea-level rise. Of the projected population at risk, nearly 50 percent will be in Florida, and an additional 20 percent in other parts of the Southeastern United States. In 30 counties, more than 100,000 people would be at risk if the sea level rose by about six feet.

None of the 22 coastal states in the continental United States, as well as Washington, D.C., will be immune from the effects of sea-level rise, the authors predict. If the seas were to rise by about six feet by 2100, more than one million people in California, and almost as many in New York and New Jersey, would be affected, the study shows.

The researchers estimated that the cost of relocating the 13.1 million people displaced by sea-level rise would be $14 trillion, based on relocation estimates for residents of Alaskan coastal villages.

Mr. Hauer said the study could be useful on a local policy level as well.

Predictions for sea-level rise are often done on a very small scale, while population forecasting is often done at a county or state level, Mr. Hauer said. Using census data, Mr. Hauer and his co-authors grouped units similar to city blocks to develop their forecast, and assumed that housing development patterns would continue at the same rate.

Benjamin H. Strauss, an expert on sea-level rise at Climate Central, a climate change research organization, said he believed that the new study overstated the number of people at risk, though he agreed that most estimates were too conservative.

The continuation of “current development patterns through the rest of the century seems like an unlikely future,” Dr. Strauss said, “because as sea levels continue to rise and coastal problems become glaringly obvious, coastal development and real estate will have to change.”

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