Ice floating along the coast of Antarctica has been melting more quickly over the past two decades, according to a study published today. The findings provide a more comprehensive perspective on how warming oceans have reduced ice volume, though it's difficult to say how soon sea levels could rise as a result.

The study, published in the journal Science, measures the thickness of Antarctica's ice shelves — floating sheets of ice that extend from glaciers out into the ocean. Using satellite radar data collected from 1994 to 2012, the authors found that ice shelves melted steadily in West Antarctica over the 18-year period, with losses accelerating by 70 percent after 2003. That’s significant because ice shelves play an important role in limiting how fast Antarctica’s glaciers melt, thereby limiting the rate at which sea levels rise. "The ice shelf acts like a plug at the end of the glacier," says Laurie Padman, senior scientist at the Earth and Space Research institute and one of the study’s co-authors.

Melting ice shelves alone don’t significantly contribute to rising sea levels, since they’re already floating, but it’s the land ice behind them that pose the greatest danger. "If you thin those ice shelves or remove that ice completely, then the glaciers behind them can lose ice more quickly than they are now," Padman says.

Previous studies have shown that ice shelf volume in West Antarctica has declined in recent years, and that shrinking ice shelves can speed up glacier melting on the continent. But most cover large regions and relatively short time periods. Experts describe today’s study as the most comprehensive yet.

"We knew that things were accelerating in Antarctica," says Ted Scambos, lead scientist at the National Snow and Ice Data Center, who was not involved in the study. "What it’s done, thoroughly and accurately, is to merge all of the useful data together so we have a much better picture of the time scale that certain areas respond to changes in the ocean… It gives us a better picture, but it's not a brand new picture."

The study, co-authored by researchers from the Scripps Institution of Oceanography, also sheds light on important regional variations across Antarctica. For the first 10 years, West Antarctica’s losses were largely offset by gains in East Antarctica, which is less exposed to warm ocean waters and saw increased snowfall over that period. But the eastern region stopped gaining ice after 2003, speeding overall loss of Antarctic ice. Some ice shelves lost as much as 18 percent of their volume over the 18-year period, suggesting that they would completely disappear within this century, if current trends continue.

Rising sea levels would be the most immediate result of any rapid decline in ice shelf volume, though it's too soon to say when or how that would happen. Under certain scenarios, global sea levels could rise by several meters, but Scambos says further satellite data and ocean measurements are needed to get a fuller picture. "The real question is not whether sea level is going to rise," he says. "It’s what rate sea level is going to rise at."

Padman and his co-authors caution that their findings shouldn’t be extrapolated too far into the future. To do so would require reliable climate models to predict changes in the ocean and atmosphere around Antarctica, and a greater understanding of what’s driving the decline in ice shelf volume. They suggest that some of the most rapid ice loss is due to stronger winds that blow warm water into the western part of the continent, but it’s not clear why that’s happening. Without knowing the cause for these trends, forecasting the next 100 years remains a daunting task.

"It all really depends on how the ocean and the atmosphere around Antarctica changes during that time," Padman says. "So the last 20 years of performance is not a guarantee of future changes."