Rising seas spurred record number of 'high-tide' floods in U.S. last year

Doyle Rice | USA TODAY

The U.S. endured a record number of days of high-tide flooding last year largely due to rising seas from man-made climate change, the National Oceanic and Atmospheric Administration said.

Across the USA, coastal cities and towns racked up a combined 520 days with high-tide floods, far above the annual average of 275 days over the past couple decades, NOAA oceanographer William Sweet said. That broke the previous record set only a year earlier when 513 flood days were tallied in 2015, he added.

The report only examined coastal flooding, not inundation brought on by sudden, heavy rain or overflowing rivers. As sea levels rise, it no longer takes a strong storm or hurricane to cause coastal flooding. It now occurs with high tides in many locations.

Also known as nuisance or clear-sky flooding, the phenomenon has increased by as much as 1,000% in some areas since the 1960s. NOAA tracks high-tide floods in 28 coastal cities.

Charleston, S.C., with 50 days, and Savannah, Ga., with 38 days, both broke their records in 2016 for number of days in a year with high-tide flooding. Key West, Fla., with 14 days, tied its record.

Other cities that dealt with extensive flooding included Wilmington, N.C., (84 days); Honolulu (45 days); Annapolis, Md., (42 days), and Washington D.C. (33 days).

These floods lead to road closures, overwhelmed storm drains and damaged property but are seldom life-threatening. They're mostly caused by climate-related sea-level rise, NOAA said.

Heat-trapping greenhouse gases from burning fossil fuels causes glaciers and ice sheets in Greenland and Antarctica to melt. Warmer water takes up more space that cooler water or ice, causing sea levels to rise.

Since 1880, the ocean has risen nearly 8 inches worldwide, according to the Union of Concerned Scientists, but it doesn't do so evenly. In the past 100 years, it's climbed about a foot or more in some U.S. cities.

In addition to sea-level rise, the loss of natural barriers and land subsidence — a gradual settling or sudden sinking of the Earth's surface due to underground movement of soil, rock and other materials — also contribute to the flooding.

The effects of rising sea levels along most of the continental U.S. coastline are expected to become more noticeable and severe in the coming decades, likely more so than any other climate-change related factor, NOAA says.