‘Clear-sky’ flooding worsens across U.S. as sea levels rise, report says

Doyle Rice | USA TODAY

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High-tide flooding is happening across the USA at twice the rate it was just 30 years ago, according to a new report released Wednesday.

And this flooding isn't necessarily caused by a storm, but by rising seas: As ocean levels rise because of global warming, flooding can now occur with high tides in many locations.

Also known as "sunny-day," "clear-sky" or "nuisance" flooding, 27 locations across the nation set or tied records for most days with floods from May 2017 to April 2018, the report said. Cities such as Boston and Atlantic City both had 22 days with high-tide floods.

"Due to sea level rise, the national average frequency of high-tide flooding is double what it was 30 years ago," the report said. William Sweet, a NOAA oceanographer and co-author of the report, said at a press briefing that "what used to be uncommon is now becoming fairly common."

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Nationwide, the average of six flood days for each location was the highest on record.

Ben Horton. a Rutgers University researcher who was not involved in the study, called it "a warning, a shot across the bow. Across the whole of the U.S. coastline, we are in dire need of action," he said.

The report found tidal flooding was at record levels last year along parts of the southeast Atlantic and eastern Gulf coasts. It examined only coastal flooding, not inundation brought on by sudden, heavy rain or overflowing rivers.

"As they examine their risk, communities can use this information to help better mitigate and prepare for high tide flooding from long-term sea-level rise," according to NOAA.

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 because of underground movement of soil, rock and other materials — also contributes to 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 said.

Contributing: The Associated Press