Around the vast, jagged edges of the Antarctic continent, floating ice shelves act as plugs that hold back the giant glaciers that extend well inland. It is the thinning of these ice shelves that is allowing many Antarctic glaciers to flow faster into the sea, raising global sea levels.

Scientists have been racing to understand just how quickly these dynamics are playing out, and what it might mean for coastal areas around the world. The news of late has not been reassuring.

On Thursday, new research was published in the journal Science, which found that ice shelves are shedding volume even faster than previously thought.

To put it another way, the plugs holding back a ticking time bomb of sea level rise are steadily being weakened.

The glaciers of West Antarctica are already responsible for the majority of the Antarctic continent's contribution to global sea level rise, and if these glaciers were to completely collapse, sea levels could rise by at least 4 feet, potentially inundating coastal megacities around the world.

Even greater sea level rise of possibly up to 15 feet would result if other parts of the West Antarctic Ice Sheet were to destabilize as well.

Movement of the Pine Island Glacier between March 3-15, 2015, seen from an ESA satellite. Purple hues show the fastest movement. Image: ESA

The ice shelves themselves are already floating in water, so thinning of this ice does not alter sea levels. However, what happens with these shelves should be viewed as a precursor to what will happen to the glaciers behind them, scientists say.

The mechanism that's thought to be thinning the coastal ice shelves is increasing ocean temperatures, which are helping to undermine the ice shelves from below, and in some cases even penetrate inland, lubricating parts of the land-based glaciers themselves.

For the new study, researchers from the Scripps Institution of Oceanography in San Diego as well as a nonprofit, private research firm in Oregon, compiled 18 years of continuous satellite measurements from three different satellite missions to show how the thickness of Antarctic ice shelves has been changing from one decade to the next.

They found that the average ice shelf volume around Antarctica accelerated from showing "negligible loss" to "rapid loss" between 1994 and 2012. The period of rapid loss began in about 2003, the study found.

The study is an advancement from previous research in that it provides a longer record of relatively precise measurements of ice shelf thinning, whereas many other studies have examined several years' worth of data from individual satellite missions that tend to last about five to 10 years.

Floating ice shelves have been eroded the most in West Antarctica and the Antarctic Peninsula, which juts into the Southern Ocean on the northwest side of the continent. West Antarctic losses have increased by 70% in the last decade alone, the study finds.

In two particular regions, some ice shelves have lost up to 18% of their thickness in less than 20 years, the study found.

“Eighteen percent over the course of 18 years is really a substantial change,” said Fernando Paolo, a graduate student at Scripps, in a press release. “Overall, we show not only the total ice shelf volume is decreasing, but we see an acceleration in the last decade.”

If current rates of thinning were to continue, the researchers calculate that the ice shelves would lose half their volume — and therefore hold back far less land-based ice — within the next two centuries.

Ian Joughin, a scientist at the Polar Science Center of the University of Washington, told Mashable that the study provides valuable new data, and is important for its affirmation of other work.

"It agrees well with other work in West Antarctica and shows steady thinning over approximately the last two decades," he said. "This is a big, unambiguous signal."

Joughin was a coauthor of one of two studies published last year that concluded that the collapse of West Antarctic glaciers was "unstoppable," and could occur as soon as 200 years from now, or possibly earlier. Joughin's study found that, based on recent ice loss rates and the movement of the Thwaites Glacier in West Antarctica, as well as computer model projections, "early-stage collapse has begun" with that particular glacier.

Joughin was not involved in the new research.

The Brunt Ice Shelf seen from a NASA plane in 2011. Image: Michael Studinger/NASA

Richard Alley, a climate scientist at Penn State University who also was not involved in the new study, says that it does not take much warming to melt ice shelves enough to free up much of the inland ice that it had been holding back. "Our emerging understanding is that especially for Antarctica, the temperature of ocean water where the ice starts to float is the most important control on the ice sheet," he said in an email to Mashable.

Once glaciers begin moving faster toward the sea, it can be difficult, if not impossible, to stop them. A satellite operated by the European Space Agency (ESA) recently clocked the Pine Island Glacier, one of the fastest ice streams in Antarctica, moving 328 feet toward the sea between March 3 and March 15.

This glacier empties about a tenth of the West Antarctic ice sheet into the ocean, and averages a movement of about 2.5 miles a year, according to the ESA.

Pine Island is the largest glacier in the West Antarctic Ice Sheet and one of the fastest ice streams on the continent, with an average of over 4 km per year. About a tenth of the ice sheet drains out to the sea by way of this glacier.

Eric Rignot, a climate scientist at NASA's Jet Propulsion Laboratory In Pasadena, California, was another author of the unstoppable melt papers published last year. He says the new study, which he did not participate in, confirms a pessimistic view of the future for many of Antarctica's ice shelves and the glaciers they help regulate.

"...A lot of ice shelves are changing around Antarctica and ice shelf changes are precursors to what the glaciers will do, so it is important to watch these canaries in the coal mine; a lot of them are getting sick right now, some of them are even terminally sick," Rignot said via email.

"Sorry to be morbid, but the high thinning of some of them truly means they will eventually fall apart in the coming decades," Rignot said of the ice shelves.