Rising sea levels caused by climate change could threaten hundreds of millions of people living in low-lying coastal areas, and the need for public action is urgent, according to the co-author of a report that shows global sea levels could rise by over four feet by the end of the century if carbon emissions are not reduced.

"If you knew what I know about climate change you would be very worried," Ben Horton, professor at Rutgers University Institute of Marine and Coastal Sciences, told Al Jazeera.

Horton co-authored a Rutgers University-led study published Monday in Proceedings of the National Academy of Sciences which, for the first time, used data instead of models to find the rate of sea level rise over the last 28 centuries and to predict its effect by 2100. That level of rise could cause entire islands to disappear.

The data, based on factors that contribute to rising seas including temperature and melting ice sheets, showed that seas are rising at their fastest rate in 28 centuries. Without fossil fuels, it is believed that oceans would have risen half as much and may have even fallen during the 20th century.

"It clearly illustrates we're living in a very unusual time," Horton said.

If the world takes no action on climate change, the data showed up to 4.2 feet of sea level rise by 2100, Horton said. Conversely, if fossil fuel use peaks in 2040 and is phased out afterward, sea level rise could be below three feet by the end of the century.

That rise will be variable by location, Horton said. The Indonesian capital of Jakarta, for example, has seen nearly a foot of sea level rise per year because the land is sinking due to the withdrawal of water from aquifers.

Four feet of sea level rise could affect hundreds of millions of people who live in coastal and low-lying communities, according to Climate Central. The environmental advocacy group released a companion report to Rutgers' that found sea level rise caused over half of the 8,000 nuisance floods in the United States since 1950.

“There’s a definite recognition among people who weren’t talking about sea level rise five years ago that it’s something to be concerned about,” Laura Tam, a policy director at SPUR, an urban planning think tank based in San Francisco, said in a Monday press release by Climate Central.

Some islands could disappear entirely. The lowest country on the planet, the Maldives, an atoll nation in the Indian Ocean, has a top elevation of nearly five feet. Pacific Ocean atolls, ring-shaped coral islands that include Tuvalu, Kiribati, Tokelau, Torres Straight Islands, Carteret Islands, and the Marshall Islands, could all see most of their land go under if sea levels were to rise by four feet.

In Majuro, the capital of the Marshall Islands, residents have already begun building sea walls around their homes to keep out floods during high tides. Some have begun emigrating because their communities have already become uninhabitable due to drought, flooding, and saltwater intrusion.

Nearly 200 nations came together in Paris in December to sign a global climate treaty aimed at reducing carbon emissions and transitioning to renewable energy. But the only country to ratify the agreement so far is Fiji — where more than 20 people were killed on Saturday night after one of the strongest storms on record slammed into the island.

“The new sea level data confirm once again just how unusual the age of modern global warming, due to our greenhouse gas emissions, is,” Stefan Rahmstorf, a physics professor at Potsdam University in Germany, said in the Climate Central release.

“They also demonstrate that one of the most dangerous impacts of global warming, namely rising seas, is well underway,” said Rahmstorf, who is one of 10 authors of the paper.