NEW YORK — Global warming in this century might raise sea levels more than expected in future centuries, says a study that looked at what happened at a time when Neanderthals roamed Europe.

Unless global warming is curbed or expensive measures are taken to hold back rising water, the projected sea level rise could submerge about one-third of Florida, southern Manhattan, much of Bangladesh and almost all the Netherlands, for example, researchers said.

An expert praised the work but cautioned that such projections can’t be made with precision.

Earth naturally alternates between ice ages and warmer times, due to changes in the tilt of the planet and its orbit around the sun. It is now in a warmer spell that began some 10,000 years ago. But scientists say that man-made, heat-trapping gases are driving the warming beyond the natural amount.

Warmth can raise sea levels by expanding water volume and melting huge sheets of ice in Greenland and Antarctica. To get an idea of what future warming might do to sea levels, scientists at Princeton and Harvard universities looked at Earth’s last warm period, which peaked some 125,000 years ago. It’s sometimes called the Eemian stage.

During this time, Neanderthals lived in Europe and elephants roamed what is now southern Britain and New York state. Lions prowled and hippos bathed in France, Spain and Italy. But such animals were different species from their cousins in Africa today, adapted to different temperatures.

So what happened to sea level during the warm Eemian stage? Previous studies have estimated that the global sea level was maybe 13 feet to 20 feet higher than today.

The new work, reported in Thursday’s issue of the journal Nature, says it almost certainly peaked at more than 22 feet higher than today. In fact, it probably rose between 26 feet and 30 feet, researchers concluded.

Temperatures at the North and South Poles — critical for triggering ice melt — could return to Eemian levels again if the global temperature rises about 4 degrees (2 degrees Celsius), the researchers said.

Scientists project that without concerted action, as is now being discussed in Copenhagen, Earth could add that much heat in this century from the buildup of greenhouse gases.

If the polar regions once more reach Eemian-like temperatures, the ice sheets of Greenland and Antarctica “are at risk of large-scale disintegration,” said Michael Oppenheimer of Princeton, an author of the study.

“We may be locking in this (future) event by the temperatures we reach this century,” said Oppenheimer in a telephone interview from Copenhagen.

He said it’s not clear how long such temperatures would have to continue in the future to set off large-scale melting; it could take centuries or a much briefer time, he said.

Nor can the study tell how fast the water rose per century during the Eemian, said Robert Kopp of Princeton, another study author. It estimates a rate of about 20 to 30 feet per 1,000 years.

The researchers estimated Eemian sea levels by looking at data from fossil corals and ancient sediments from nearly 50 sites around the world.

“It’s a very impressive piece of work,” said Richard Alley of Pennsylvania State University, who didn’t participate in the study. “I really don’t expect this is going to be the last word (about the Eemian) … but I think this is the best word at this point.”

He cautioned that scientists can’t yet predict what happens to ice sheets at given global temperatures. But he said the work confirms that “ice sheets are vulnerable to warming, and it doesn’t take very many degrees to really change the size of an ice sheet.”

On the Net:

Nature: www.nature.com/nature