Doyle Rice

USA TODAY

Sea-level rise, one of the planet's most dire side effects of man-made global warming, is accelerating nearly three times as rapidly as during the 20th century, according to a new study.

Sea-level rise recently "accelerated significantly,” said study lead author Sönke Dangendorf of the University of Siegen in Germany. Before 1990, the Earth's oceans were rising less than a half-inch per decade. But from 1993 through 2012, the seas rose about 1.22 inches per decade, the research found.

Here's how sea-level rise works: As the Earth's temperature warms, so do the oceans. Heat-trapping greenhouse gases cause more land ice — glaciers and ice sheets — to melt and water to expand. Warmer water simply takes up more room than cooler water.

Scientists say global warming will be the primary cause of future sea-level rise. The greatest uncertainty is how quickly the massive West Antarctic ice sheet will melt.

Sea level has risen nearly 8 inches worldwide since 1880 but, unlike water in a bathtub, it doesn't rise evenly. In the past 100 years, it has climbed about a foot or more in some U.S. cities because of ocean currents and naturally sinking land.

New visualizations show worst-case scenario for sea levels by 2100

"Ice sheets in Greenland and the Antarctic store some hundred times more water than glaciers," Dangendorf said. "As a result, further melting of the ice sheets poses a particular risk for low coastal areas."

Scientists don't know exactly how much higher the oceans can rise, but, depending on fossil fuel emissions, they project global sea level will rise about 1 foot to slightly more than 3 feet, or 39 inches, by 2100, according to the Fifth Assessment Report by the United Nations' Intergovernmental Panel on Climate Change.

The National Oceanic and Atmospheric Administration projected sea level could rise as much as 6½ feet by century's end, potentially inundating many coastal cities.

The study appeared in the Proceedings of the National Academy of Sciences, a peer-reviewed journal.