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Climate Change Is Turning 500-Year Floods Into 24-Year Ones

Rising sea levels and an increase in large storms will continue to threaten the Atlantic coast

(LEONHARD FOEGER/Reuters/Corbis)
smithsonian.com

People have long referred to the severity of floods in terms of their recurrence interval: the probability that a flood might reach a certain level within a certain number of years. Now, writes Tim Darragh for NJ.com, those designations are getting even more confusing — and more dire — as scientists warn that residents of the Atlantic coast could witness storms with the magnitude of what were once “500-year floods” every 24 years.

New research shows that flood risks in New York City and along the Atlantic coast has “increased significantly” during the past millennium. The change is due to a combination of rising sea levels and an increase in the kinds of storms that produce widespread flooding.

Scientists compared prehistoric and modern-day storms and floods with the help of ground core samples, carbon dating and a variety of data on hurricanes. They found that not only are floods more likely to be severe in the future, but that they’ve become more intense in the recent past. Mean flood heights have increased by nearly four feet over the past 1,200 years, says the team — and ocean levels rise, that increases the risk of even more severe floods.

“We need to act quickly,” lead author Benjamin Horton tells Darragh. “A storm that occurred once in seven generations is now occurring twice in a generation.”

That’s sobering news, but Horton and his team hope it will spur action and planning for the vulnerable Atlantic coast. In an interview with New Scientist, Horton notes that the dire predictions could spur better prediction models, cuts in greenhouse gas emissions and plans to mitigate future floods. Scientists may continue to argue about whether it makes sense to refer to floods as “500-year events,” but the new study could help bring them together as they look for ways to predict and prepare for the worst.

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