WASHINGTON – Melting ice and warming waters have raised average sea levels worldwide by more than an inch since 1995, new data from space satellites and robotic submarines have revealed.

That’s twice as fast as the rate the oceans rose during the previous 50 years, ocean experts said Thursday. If the current rate continues or accelerates, as they say is likely, the world’s seas will rise at least a foot by the end of this century, causing widespread flooding and erosion of islands and low-lying coastal areas.

“Even a small change will matter to a whole lot of coastal people,” said Richard Alley, a geoscientist at Pennsylvania State University in State College. “If 15 percent of Greenland ice sheet were to melt, much of South Florida would be under water.”

More than half the sea rise was caused by a recent speed-up in the melting of glaciers and ice sheets, especially in Greenland and Antarctica, according to Laury Miller, the chief of the Laboratory for Satellite Altimetry at the National Oceanic and Atmospheric Administration.

“Many of us have been quite surprised at how rapidly that melting has occurred,” Alley said.

“Right now we don’t really know enough to scare, but we don’t really know enough to reassure, either,” he said.

Until recently, for example, the Jakobshavn glacier, Greenland’s largest, was shrinking 3 miles per year. But since 2002, it’s been shedding up to 6 miles of ice per year, said Eric Rignot, an ice-sheet expert at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Glaciers in Alaska and South America “are shrinking faster now than 10 years ago, and two to three times faster than they did over the last century,” Rignot said. Mountain glaciers in Europe, Asia and Africa also are contracting rapidly.

Even more dramatic melting is going on in the much larger glaciers and ice sheets of Antarctica. Three years ago, a chunk of ice as large as Rhode Island collapsed off the coast of the frozen continent.

Particularly worrisome is an area called Pine Island Bay. “There’s enough ice in that sector alone to raise the sea level by 1 full meter,” or 39 inches, Rignot said.

The remaining increase in sea level is mostly the result of higher water temperatures, because water naturally expands as it gets warmer.

The sea rise “amounts to about a half of an inch in the past decade due just to ocean warming,” Miller said.

Ocean warming is a slow process, and its effects are just beginning to be felt. “It takes a long time for that heat to kick in,” Miller said. “We haven’t seen all of that yet.”

Faster ice melting and the rise in ocean temperatures are natural consequences of global warming, the experts said. Most scientists agree that the Earth’s water and atmosphere are heating up, although there are disagreements about the cause and what can be done about it.

A sea-level rise of an eighth of an inch per year – about a 1.2-inch rise every 10 years – may not sound like much, Miller said. But by the end of the century, that adds up to a foot, bringing not only higher seas but also more widespread erosion.

“The problem at the coastline is more than just the vertical rise of the water,” Miller said. “The erosion effect over a century could be as much as 50 to 100 feet of coastline eroded, just with the numbers that we’re seeing today.”

The new evidence of rising seas comes from NASA satellites launched in the last two or three years, which measure the thickness of ice and the movement of water from land to the ocean.

NOAA also has acquired two years of precise ocean-temperature data from a worldwide fleet of 1,700 small robotic submarines that can dive as deep as 2,000 feet.

“Now the challenge is to develop an even deeper understanding of what is responsible for sea-level rise and to monitor for possible future changes,” Miller said.