Many coastal cities in the United States, already threatened by flooding during high tides, will find the problem becoming "chronic" in the next 15 years with ongoing global warming, scientists say.
Rising sea levels combined with high tides are routinely flooding streets and other low-lying zones close to the waterfront neighborhoods in cities along America's coastline, they say.
The Union of Concerned Scientists, in a report analyzing 52 coastal locations, says increased flooding can be expected in 14 of them -- including Annapolis, Md., Washington, D.C. and Atlantic City, N.J. -- along the U.S. East Coast and Gulf Coast.
The nation's capital could experience 388 tidal floods a year, often two a day with tides coming in and out twice daily, the report says.
"The shock for us was that tidal flooding could become the new normal in the next 15 years; we didn't think it would be so soon," says Melanie Fitzpatrick, one of the researchers at the nonprofit group who prepared the report. "If you live on a coast and haven't seen coastal flooding yet, just give it a few years. You will."
The group began preparing the report as an analysis of the risks presented by storm surges associated with hurricanes, but alarming data from tide gauges combined with known sea level projections turned the researchers' attention in a different direction, Fitzpatrick says.
"We realized before we even got through the statistics of the last 40 years that tidal flooding is a much bigger story," she says. "But nobody's really telling that story."
Researchers predicted future coastal flooding rates based on sea level projections coming from a recent National Climate Assessment, which said levels are expected to rise 5 inches from 2012 to 2030 and to have increased by a full foot by 2045.
As melting polar ice sheets and glacier add to sea levels, the threat to humans becomes greater as the world's shorelines become ever more densely populated, the Union of Concerned Scientists report points out.
Almost one-third of the population of the United States, around 100 million people, lives in coastal communities.
Tidal floods, while not as catastrophic as hurricane-generated storm surges, can damage key infrastructure such a roads and bridges and can cause significant property damage, the researchers say.
Earlier this year the National Oceanic and Atmospheric Administration released a report indicating flooding on all three U.S. coasts -- Atlantic, Pacific and Gulf -- has increased by between 300 percent and 925 percent since the 1960s.
