As sea levels climb, even Washington, D. C. could see more frequent and widespread flooding at high tide.

    Strong storms and high winds sometimes bring floods to coastal areas. But cities and towns along the US East Coast are flooding even in calm and sunny weather. Among them is Maryland's capital Annapolis. Tourists there must sometimes walk through water flooding downtown streets surrounding the habor. Sometimes water covers roads in Atlantic City, N. J. Even the yards and basements of coastal homes near New York City sometimes flood.

All are suffering from this trend—normal high tides surpass (超过) a rising sea level.

    Sea levels have climbed, on average, about 20 centimeters (8 inches) over the past 135 years. And a sharp increase in tide flooding is one of the more visible impacts affecting many coastal areas, notes William Sweet, an oceanographer.

    Sweet and his co-workers recently analyzed data from 45 tide gauges (计量表) along the US coasts. These tools record the changing heights of tides as they rise and fall. From these data, Sweet's team calculated the number of : “troublesome floods” in various coastal cities. These floods, Sweet explains, typically occur when water level is about 30 centimeters (1foot) above the historic level of the highest tides. In June 2014, the team reported finding a growing rise in these flooding events.

    In Charleston, S.C. from 1957 to 1963, the city experienced troublesome floods an average of 4. 6 days per year. Bur from 2007 to 2013, the occurring rate of troublesome floods jumped five times. Annapolis city was even more serious. From1957 to 1963，troublesome floods were roughly 3. 8 days per year. From 2007 to 2013, the average was 10 times higher- 39. 3days a year.

    “Over the next few decades, climate change probably drives sea levels even higher,” Sweet says. “So today's flooding problems promise to become only more widespread and frequent.”