The Sheldon Glacier with Mount Barre in the background, is seen from Ryder Bay near Rothera Research Station, Adelaide Island, Antarctica, in this NASA handout photo. (NASA/REUTERS)

The giant polar ice sheets in Greenland and Antarctica are losing three times as much ice as 20 years ago, adding to the rising sea level that threatens low-lying coastal areas, according to a study hailed by scientists as the most accurate assessment of polar ice melt to date.

In one startling finding, Greenland’s melt was five times higher than it was in the mid-1990s. It represented more than two-thirds of the ice loss, according to the study, published online Thursday in the journal Science; Antarctica’s slower thaw accounted for the rest.

The study by an international group of 47 experts who study satellite mapping data is the first to pull together 50 different ice-sheet loss estimates over two decades and reconcile the research methods and findings into a single report. The group was led by Erik Ivins, a scientist at NASA’s Jet Propulsion Laboratory, and Andrew Shepherd, a professor of Earth observation at the University of Leeds

The findings “are now two to three times more reliable” than ice-melt and sea-level rise estimates in studies used by the Intergovernmental Panel on Climate Change to compile its most recent report in 2007, Shepherd said.

The new findings concerning Greenland’s and Antarctica’s ice- sheet loss fall within the range of the IPCC report, but the intergovernmental panel’s estimates — based on widely ranging information — were so broad that the IPCC failed to make a crucial assessment: whether Antarctica was growing or shrinking. Melting in Greenland, which last July had its biggest thaw since 1973, was well known.

“Without these efforts, we would not be in a position to tell people with confidence how the Earth’s ice sheets have changed and to end the uncertainty that has existed for many years,” Shepherd said. “This will give the wider climate science community greater confidence in ice losses and lead to improved mode predictions of future sea-level rise.”

As Hurricane Sandy showed, an understanding of sea-level rise is critical, said Julie Brigham Grette, a professor of glacial geology at the University of Massachusetts Amherst. And that under­standing hinges on the fates of the Greenland and Antarctica glaciers, the main reservoirs capable of affecting sea-level rise, according to the IPCC.

Although ice-sheet loss contributes to sea-level rise, the main culprit is global warming. A rise in temperature because of greenhouse gases is transferred to the oceans, causing them to expand. Sea level is expected to creep up by about 40 inches in the next 90 years, Grette said.

“It’s not like it’s going to happen in the future, it’s happening now,” she said. “People don’t understand why we’re talking about a few millimeters. A half-foot of rise on the Eastern Seaboard makes it easier for a storm coming up the East Coast to cause flooding.”

It was the concern over sea-
level rise that led to the idea of combining the findings of previous studies to get a clearer view of ice-sheet loss. After the 2007 IPCC report, scientists realized that disagreements in the estimates based on different satellite measuring techniques were muddying the picture. Three years later, at an IPCC sponsored workshop in Kuala Lumpur, Malaysia, “there was a specific recommendation that this ought to be tackled,” Shepherd said.

The study brought together 47 experts from laboratories around the world, many of whom authored the studies that led to the confusion. Space agencies contributing money for travel to cities such as Boulder, Colo., and Oxford, England, but the experts’ time was volunteered for the yearlong study, which ended in July, Shepherd said.

Richard Alley, a climate scientist at Penn State University, called the project “a spectacular achievement” that will lead to a better understanding of sea-level change and how humans impact it.

The researchers took a wealth of data collected over a long span of time and came up with a conservative estimate that people can embrace, Grette said.

“It’s not wild. It’s not alarmist. It’s probably closer to the truth, and it’s very important because of the sea-level issue,” she said. “This provides a really important baseline that we didn’t have before.”