Antarctic Ice Shelves Have Been Thinning for 300 Years, Making Collapse More Likely as World Warms

Hundreds of years of thinning may have created the perfect conditions for ice shelves to collapse in parts of the Antarctic Peninsula—a stretch of land that forms the northernmost part of the icy continent—as climate change takes hold. That's according to new research.

The Antarctica Peninsula is currently experiencing dramatic ice loss, but identifying when this process began and what the main drivers were further back in time is made difficult by the fact that scientists only have data from very recently.

But now, for a study published in the journal Scientific Reports, a team of scientists led by William Dickens from the British Antarctic Survey, reconstructed a 6,250-year record of glacier meltwater in the Antarctic Peninsula to shed light on past drivers of ice loss and their present-day implications.

To do this they analyzed variants of oxygen (isotopes) in single-celled algae, which were found preserved in marine sediments from the Peninsula's northeastern tip.

These algae serve as a kind of historical record of glacier meltwater discharge—the amount of water released from melting ice shelves and icebergs. Essentially, lower isotope values indicate a higher amount of meltwater, thus providing clues to how much ice shelves are thinning.

Using this analysis, the scientists noticed that there was an increasing trend in the amount of glacial melt after the year 1400 A.D in the eastern Antarctic Peninsula.

And by 1706, the discharge had reached levels unprecedented in the new 6,250-year record, according to the researchers. Furthermore, after 1912, there was another significant acceleration in the rate of glacial discharge.

The data shows that these increases came after a long period where glacial discharge remained relatively stable, lasting for the first five thousand years or so of the new ice record.

The scientists propose that the acceleration of ice-shelf thinning in the eastern Antarctic Peninsula after 1400 may have something to do with changes in a weather phenomenon known as the Southern Annular Mode (SAM)—also known as the Antarctic Oscillation (AAO.)

These terms refer to a belt of westerly wind that encircles Antarctica and moves north or south, dominating the middle to higher latitudes of the Southern Hemisphere, according to the Australian Bureau of Meteorology.

The authors say that shifts in the SAM drove stronger westerly winds, while also increasing atmospheric warming and ice shelf-melting in the eastern Antarctic Peninsula. It also brought warm water into the Weddell Gyre—a system of rotating currents off the coast of Antarctica—which could have led to an increase in melting on the undersides of ice shelves in the study area.

The latest findings are significant when it comes to our understanding of how glaciers will be impacted by man-made (anthropogenic) global warming, the authors say.

"A possible implication of our data is that ice shelves in this region have been thinning for at least around 300 years, potentially predisposing them to collapse under intensified anthropogenic warming," the authors wrote in the study.

"While the rates and drivers of modern warming are now well documented, the interplay between SAM and its impacts on ice sheet mass loss on longer, centennial to millennial timescales is poorly understood," the authors wrote.

As the world warms, Antarctica is losing mass at an accelerating rate, contributing to rising global sea levels. The Antarctic Peninsula is one of the worst-affected regions, with up to 87 percent of its glaciers retreating and several ice shelves already collapsing. According to the scientists, while global warming is a significant driver, at least some of the increased melting here is related to shifts in the SAM.

"This has resulted in a strengthening of warmer westerly winds and increased surface melting on the eastern side of the Antarctic Peninsula, which has been linked to the collapse of the Larsen B ice shelf," the authors wrote.

Recently, scientists have detected more frequent changes in the SAM which could be the result of increased greenhouse gas levels or depletion of the ozone layer in the Southern Hemisphere. Combined with the impacts of climate change, these frequent changes to the SAM could to lead further accelerated ice loss in the future.

Antarctic ice shelf
An Antarctic ice shelf. James Smith

"The most worrying thing now is that this accelerated melting has already led to a decay of large ice shelves at the eastern, colder side of the Antarctic Peninsula, and this will be even more accelerated by recent and future climate warming. It looks like there is no, or not much, buffering capacity for climate cooling left in this part of Antarctica," Gerhard Kuhn, an author of the study from the Alfred Wegener Institute for Polar and Marine Research in Germany, told Newsweek.

Richard Aronson, a marine scientist at the Florida Institute of Technology who was not involved in the study, said that most changes we see in the natural world happen for more than one reason.

"In science-speak we call it multiple causality," he told Newsweek. "Our mission as scientists studying environmental change is to figure out the relative contributions of the different causes—and the interaction of those causes—to the patterns we see. We know that the Antarctic Ice Sheet is thinning at an increasing rate, posing a great threat in the form of ever-rising sea levels."

"The authors [of the study] conclude that natural oceanic–atmospheric cycles drove the particularly sharp increase in ice discharge starting in the 1700s. Now greenhouse gas emissions from human activities are warming the planet, jacking up those cycles, accelerating ice loss, and precipitating the breakup of ice shelves attached to the Peninsula," he said.

"As the ice shelves break up, the marine communities beneath them are being irrevocably changed. Would this have happened without the greenhouse effect? It's hard to tell, but it certainly would not be happening at the breakneck speed it's happening now, giving us very little time to figure out how to adapt to rising sea levels."

Ted Scambos, a Senior Research Scientist from the University of Colorado, who was also not involved in the study, described the latest publication as a "good" paper but noted that it does not change the general understanding of climate change in the Antarctic Peninsula.

"Climate has always changed, it's true, and the ice sheet has always been chasing the moving climate, aiming for equilibrium," he told Newsweek. "But the pace of the last several decades, and the strong forcing induced by man-made gases in the atmosphere, have interrupted the natural cycle and pushed an already-warm climate much warmer, rapidly. This study has teased out the earlier trend, but in no way minimizes the dramatic events of the past 50 years."