John Wahr of the University of Colorado in Boulder and colleagues, in a study published on Thursday, found that thinning glaciers and icecaps were pushing up sea levels by 1.5 millimeters (0.06 inches) a year, in line with a 1.2 to 1.8 mm range from other studies, some of which forecast sea levels could rise as much as 2 meters (2.2 yards) by 2100.

Sea levels have already risen on average about 18 centimeters since 1900 and rapid global warming will accelerate the pace of the increase, scientists say, threatening coastlines from Vietnam to Florida and forcing low-lying megacities to build costly sea defenses.

To get a better picture of the pace of the melting, Wahr and colleagues used a satellite that measures variations in gravity fields to study changes in the mass of large ice-covered areas. The data covered 2003-2010.

The glaciers and ice caps included those in the Arctic, South America, Asia as well as Greenland and Antarctica.

Globally, the rate of sea level rise has accelerated in recent decades to reach about 3.5 millimeters a year, with more than half coming from thermal expansion of the oceans.

Water expands as it gets warmer.

While the creeping annual increase might seem small, the rate of sea level rise is expected to grow. Yet scientists have struggled to refine estimates given the uncertainty about the future pace of global warming, growth trajectory of greenhouse gas emissions and the rate at which ice caps will melt.

Using satellite data instead of more limited and time-consuming data from ground measurements was crucial, Wahr said in an email to Reuters.

The team found that loss ice from Greenland and Antarctica was pushing up sea levels by just over one millimeter a year, comprising most of the 1.5 mm annual rise.

Glaciers and mountain ice caps elsewhere comprised the rest, at 0.4 mm/yr between 2003-10.

"That's a large number, and represents a lot of melting ice," said Wahr. "But it's at least 30 percent smaller than previous global estimates, none of which have used GRACE," he said, referring to the name of the satellite.

FASTER MELTING

The United Nations' Climate Panel estimates sea global sea level rise of 18 to 59 centimeters from 1990 to the 2090s. But those numbers do not include melting from polar regions where the vast majority of the world's freshwater is locked away.

Some climate scientists say the rise is more likely to be between and 1 and 2 meters. They point to accelerating melting of the Greenland and West Antarctic icesheets over the past two decades. Both contain enough water to raise global sea levels by about 60 meters.

Other glaciers and mountain icecaps contain enough water to raise sea levels by nearly a meter.

GRACE measured the changes to ice mass over regions greater than 100 square kilometers. The data showed ice-covered areas in Asia, including the Himalayan and Karakoram ranges, was much less than other estimates, meaning the region contributed very little to sea level rise, in part because many glaciers were at freezing high elevations.

Wahr said the study gave a much clearer picture of what was happening to large ice-covered areas globally, particularly in remote parts of the Himalayas.

"There are simply too many glaciers, and most of them too remote to access, to be able to monitor all of them from the ground. There are more than 200,000 glaciers world-wide," he said, adding only a few hundred have been monitored over time spans of several years or more.

"With GRACE, though, we're able to directly monitor the sum total of all ice loss in an entire glacier system or ice cap."

Ongoing monitoring by the satellite should help scientists get a better handle on the pace of ice melting and sea level rise as the planet heats up.

According to the National Oceanic and Atmospheric Administration, all 11 years in the 21st century so far, including 2011, rank among the 13 warmest in the 132-year temperature record.

(Editing by Ed Lane)

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