Antarctic glaciers thinning so fast, it's like a switch was flipped

A new study has recorded a sudden and rapid thinning of once-stable glaciers along the southern Antarctic Peninsula, demonstrating that significant changes in glacier mass can occur surprisingly quickly as ocean and air temperatures rise.

The findings support what researchers have been seeing in other parts of Antarctica, with scientists warning last year that four key glaciers on the West Antarctic Ice Sheet appear to be on the verge of wholesale retreat with nothing to stop them.

The new study points to a common cause among the glaciers it studied: Warm water is melting away the underside of the glaciers where they meet the sea floor, weakening the ice shelves that slow the glaciers’ slide the ocean. The researchers "observe a relatively strong [common] response across multiple glacier systems that clearly points to changing ocean condition as the main culprit," says Alex Gardner, a glaciologist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., who was not a part of the study, in an e-mail.

The study is unique, he says, because of how effectively it pinpointed the major driver of the changes. The study, which appears in this week’s issue of the journal Science, also points to the speed with which these changes are occurring.

"It's like a switch was flipped for a pretty extensive region of the peninsula," adds Jonathan Bamber, a glaciologist at the University of Bristol in Britain and a member of the team conducting the study. "That isn't something that you would necessarily expect based on the modeling studies that people have done."

With other studies showing warmer air temperatures weakening Antarctic ice shelves, too, the results represent a one-two punch that suggest a shaky future for the continent's ice, researchers say.

As Antarctica’s ice shelves collapse, the glaciers they buttress will contribute to sea level rise. Currently, the glaciers in the study, which lie along 500 miles of the southern Antarctic Peninsula coast, are losing some 56 billion tons of ice a year to the ocean, according to the new study.

The losses began suddenly in 2009 and come in addition to losses from the West Antarctic Ice Sheet, which is shedding 80 billion to 110 billion tons of ice a year, according to the study.

Some losses from nearby ice shelves have been underway for decades. But the seemingly abrupt onset of significant ice losses along the southern coast of the Antarctic Peninsula is an eye-opener, suggests Dr. Gardner of JPL.

Recent studies have shown that Antarctica's two continental ice sheets are more sensitive to changes in ocean and air temperatures than previously thought, he notes. But as relatively warm water from deep reaches of the Southern Ocean moved onto the continental shelf, the thinning sped up, melting the ice shelves from underneath, the researchers of the new study concluded.

The results come on the heels of a study published last week involving the breakup of an ice shelf known as Larsen B. Two-thirds of Larsen B suddenly broke away in 2002; what's left is expected to vanish within the next decade, essentially removing the brake on glaciers feeding it.

The 2002 event was a defining moment for glaciologists studying Antarctica's glaciers and ice shelves, says Ala Khazendar, another glaciologist at JPL and the lead author of last week's Larsen B study. It helped resolve a debate over whether glaciers would “feel anything” when the ice shelves were removed.

When this segment of Larsen B broke away, "the glaciers feeding that part accelerated tremendously," he explains in an interview. The glaciers feeding what was left of Larsen B didn't respond.  

"That ended the debate," he says. It became clear that ice shelves were vital to controlling the flow of glacial ice to the sea.

It's the final segment of Larsen B that Dr. Khazendar and colleagues expect to vanish soon. The much larger Larsen C, about twice the size of Massachusetts, is expected to eventually follow its smaller relatives.

Averaged over the continent, Antarctica is losing ice, and at a increasing pace, although the losses and speeds vary by region. The West Antarctic Ice Sheet is experiencing the largest ice losses, followed by the Antarctic Peninsula, and the Wilkes Land region of eastern Antarctica, according to an analysis published in February in the journal Earth and Planetary Science Letters. The exception is Dronning Maud Land, which has seen an increase in mass due to increased snowfall.

The new results came about as part of a larger study by a team led by Ben Wouters, also at the University of Bristol. The team found a “quite remarkable signal” that the glaciers the Antarctic Peninsula’s southern coast were losing height over time, Dr. Bamber says. Other researchers had noticed the changes as well, he adds. But they in essence wrote off the changes to processes on the glaciers’ surfaces, such as continued settling of partially compacted snow.

The team tested that idea and found that such surface processes couldn't account for the height losses. That suggested that the flow of the glaciers was speeding up, which would tend to thin them vertically.

The thinning coincided with record high water temperatures measured along the bed of the Bellingshausen Sea, which laps at this region of the Antarctic coast.

Taken together, the evidence pointed to this warm water melting away the glaciers where their undersides meet the sea floor – the so-called grounding line. The melting forces the grounding line to retreat toward the coast, weakening the shelves and reducing their ability to slow the glaciers' advance to the sea.

The losses of Larsen A and B have been attributed at least in part to warmer temperatures along the peninsula, which melted the ice sheets' surfaces. The water then enters crevasses where it freezes, expands, and acts as a wedge, widening the crevasses until the ice calves into mammoth icebergs. 

Recent modeling work implies that as global warming leads to warmer air temperatures, melting from below and above likely would work in tandem, accelerating ice losses beyond the rates researchers have previously estimated.

Between 3 million and 5 million years ago, during the Pliocene epoch, global average temperatures were about 2 to 3 degrees Celsius (3.6 to 5.4 degrees Fahrenheit) higher than today's and atmospheric carbon dioxide concentrations were at about the same level as today's. At that time, sea levels were more than 60 feet higher than they are today.

Using that period – minus the sea level increase – as an analogue for today, researchers at Pennsylvania State University in University Park and the University of Massachusetts at Amherst modeled the impact of that climate on Antarctica's ice.

Melting ice from underneath was insufficient to generate ice losses that led to that period's sea-level rise, the team found. So they added hydraulic fracturing as melt water flowed into crevasses and froze.

This led to more-rapid disintegration of the shelves and the formation of ice cliffs on the leading edge of the now shelf-less marine glaciers. These cliffs were unstable, prompting additional ice to shear from the cliff face.

Taken together, these processes led to ice losses that more closely matched those that occurred during the Pliocene. But while losses from the East Antarctic Ice Sheet took several thousand years to occur, losses from the West Antarctic Ice Sheet took mere decades – largely owing to a seabed shape that favors destabilization of the glaciers.

The team is now exploring the impact the two melt mechanisms working in tandem could have under conditions projected for human-triggered global warming, according to Robert DeConto, a climatologist at UMass-Amherst and a member of the modeling team.