Miami? Underwater. Norfolk? Swamped. New York City? Fuggedaboutit.

Top experts say that in a worst-case scenario, portions of these and other U.S. coastal cities could be lost to the sea by the end of the century as ocean levels rise because of global warming, a study released Monday said. In fact, on average, seas around the world could be as much as 6.5 feet higher by the end of the century if climate change continues unchecked.

This is an estimate that a group of 22 top scientists came up with by using new techniques to measure how the ice will melt in the world's polar regions. The primary cause of the rising seas will be the ongoing melt of the gigantic ice sheets on Greenland and Antarctica.

While admittedly a high-end estimate, "we should not rule out a sea-level rise of over two meters (6.5 feet) if we continue along a business-as-usual emissions trajectory," said study lead author Jonathan Bamber of the University of Bristol in the United Kingdom.

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This is potentially twice the upper limit suggested by the United Nations' climate science panel’s last major report, according to New Scientist.

Man-made climate change, aka global warming, is caused by the burning of fossil fuels such as gas, coal and oil, which release greenhouse gases such as carbon dioxide (CO2) and methane into the atmosphere. This extra CO2 causes temperatures of the atmosphere and oceans to rise to levels that can't be explained by natural causes.

Low-lying coastal cities are most vulnerable to rising seas, Bamber said. This includes cities in Florida and Louisiana, along with California cities such as Los Angeles and the Bay Area of San Francisco.

Worldwide, “such a rise in global sea level could result in a land loss of almost 700,000 square miles," he said. That's equivalent to an area over three times as large as the state of California.

Other big global cities partially lost to the sea include London and Rio de Janiero.

Such a rise in water levels would also swamp "critical regions of food production, and potential displacement of up to 187 million people," which is about 2.5% of the Earth's population.

"A sea-level rise of this magnitude would clearly have profound consequences for humanity," Bamber said.

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And yes, the seas have risen and fallen many times before. What's new is the enormity of coastal development that will need to be protected, moved or abandoned.

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 land subsidence – 11 inches in New York and Boston, 12 in Charleston, 16 in Atlantic City, 18 in Norfolk and 25 in Galveston, Texas, according to a recent USA TODAY analysis of tide gauge data collected by the National Oceanic and Atmospheric Administration.

Here's why: As the Earth's temperature warms, so do the seas. 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. Their greatest uncertainty is how quickly the massive West Antarctic ice sheet will melt.

Referring to the new study, Bamber said it's "important to consider the relatively low probability – but high-impact consequences – of ice sheet melt in the future."

The study was published Monday in the peer-reviewed journal Proceedings of the National Academy of Sciences.