Scientists say satellite and airborne measurements show that ocean levels have risen over the last decade at the fastest rate in half a century. They point to melting glaciers and warming water as the cause. The impact could be devastating if the trend continues.

Glaciers have expanded and shrunk and sea levels risen and dropped many times in Earth's history as climate has warmed and cooled again and again. But today's rising sea levels will have a different impact than they did when the last ice age ended many thousands of years ago. "What is different today is many, many people are living in low-lying, vulnerable coastal regions," he said.

"It is estimated the 100 million people will be impacted by a three-foot [one-meter] rise in sea level. So naturally it's a significant issue over the long term that requires attention," Waleed Abdalati, an expert on the Earth's ice-covered regions at the U.S. space agency NASA.

So far the sea level rise in the past decade is measured in only millimeters, but it has been enough to worry scientists like Steve Nerem of the University of Colorado in Boulder. He says U.S. satellites that monitor ocean height have documented a 50 percent increase in the rate of sea level rise since 1992 compared to tide gauge measurements in previous decades.

"If we look at tide gauge data over the last 50 to 100 years, we see an average rate of sea level rise of 1.8 millimeters per year. Now, over the last 12 years, we have detected a change in the rate of sea level rise using NASA satellite data. There's a long term trend here and the average over these 12 years is about three millimeters per year," he said.

Mr. Nerem says data from ships show that about half of the sea level rise is from the natural expansion of the water as it heats, while the other half is water from melting sources of ice. NASA ice-sheet expert Eric Rignot notes that satellite data since the mid-1990s show glacier and ice-sheet shrinkage has been increasing faster than previously thought.

"A large part of the melting ice going into the ocean comes from the mountain glaciers right now. These are in Alaska, Patagonia, the Himalayas, and other glaciers," he said. "The glaciers in Patagonia and Alaska are shrinking two to three times faster than they did over the last century. In Greenland, we are seeing the ice sheets melting at the edges. In Antarctica, we've seen a rapid decay of ice in the Antarctic Peninsula, which is the part of Antarctica that sticks out toward South America."

Mr. Rignot points out that warming air is a major part of the problem.

For those who do not think an average of three millimeters a year is a significant increase in ocean levels, the man in charge of collecting such data at the U.S. oceans and atmosphere agency NOAA, Laury Miller, says a little extra water can do a lot of damage over time. "The problem at the coastline is more than inundation. It's more than just the vertical rise of the water. There are also the erosion effects, and the erosion effects extend much further than you would think just in terms of the vertical motion. The erosion effects of over a century could be as much as 50 to 100 feet [15 to 30 meters] of coastline eroded just with the numbers we're seeing today," he said.

At Pennsylvania State University, geoscientist Richard Alley says a loss of 15 percent of the Greenland ice sheet would provide enough new water in the ocean to flood south Florida and many other coastal regions of the world. "The world is warming now, and the ice sheets are doing sort of what people expected in a warming world. Many of us have been quite surprised at how rapidly that melting has occurred in places. Right now we don't really know enough to scare, but we don't really know enough to reassure either," he said.

Mr. Alley says more years of data collection are needed for predictions to be more precise.