Wilkins Ice Shelf
Break-up continues in April with loss of ice bridge, new ice bergs
Posted May 1, 2009
Going, going … but not quite gone.
The disintegration of an ice bridge linking the Wilkins Ice Shelf between two islands on the western side of the Antarctic Peninsula isn’t likely to cause the remainder of the floating chunk of ice to break up this year, according to the lead scientist at the National Snow and Ice Data Center (NSIDC) in Boulder, Colo.
Ted Scambos and colleagues at NSIDC have been watching the latest and perhaps final collapse of the Wilkins Ice Shelf for more than a year, when Scambos first sounded the alarm to the international science community in March 2008.
At that time, about 400 square kilometers of ice sloughed off the southwestern front of the ice shelf. The Antarctic winter offered no relief, as the shelf continued to fracture and pieces of ice continued to chip off, including another 160 square kilometers along the ice bridge.
The final shattering of the ice bridge in early April — at that point only about 500 meters wide and 40 kilometers long — garnered worldwide attention. U.S. Secretary of State Hillary Clinton even referenced the event as another example of climate change during the 50th anniversary meeting of the Antarctic Treaty member nations in Baltimore, Md. in April.
More recently, the European Space Agency (ESA) reported this week that icebergs have begun to calve off the new front of the ice shelf, with at least another 700 square kilometers lost to the ocean.
To a certain degree, Scambos explained, the importance of the bridge had diminished by the time it broke up. A line of fractures had already appeared south of the bridge, connecting various islands and ice rises, which are likely to become the new front of the ice shelf after the current break-up ends, he said. That’s where the bergs are now dropping into the ocean.
“When the ice bridge collapsed, these rifts re-activated,” he said prior to the new activity reported by ESA. “But it doesn't appear that there are new rifts forming even further into the shelf. I don't think we’ll see a complete break-up of the remaining southern two-thirds of the Wilkins this year, but we will continue to watch the evolution of these rifts — they’re likely to grow, and climate is likely to push the system towards more change in the years to come.”
The Wilkins Ice Shelf is located off the southwestern Antarctic Peninsula, arguably the fastest-warming region of the Earth. In the past 50 years, the Antarctic Peninsula has warmed by 2.5 degrees Celsius, according to the NSIDC.
In the early 1990s, when it first began to lose ice, the Wilkins Ice Shelf had a total area of 17,400 square kilometers. Between 1998 and 2008, it dropped in area to roughly 13,680 square kilometers, about the size of Connecticut.
Now, in just the last year, it’s lost another 3,600 square kilometers before the iceberg calving began. Scambos said he did not expect a total collapse just yet.
“This winter we'll get an idea of how much the loss of the bridge, and the new shape of the ice front, means to the stability of the remaining Wilkins. But I think it will be several years before the rest of the Wilkins breaks up,” he said. “I think sections of it nestled in southern bays will be around for decades, but it would be much smaller.”
The ice shelf floats on the ocean, so it won’t contribute to sea-level rise. The concern about the loss or thinning of ice shelves is that the glaciers that usually feed into them will increase speed, and that’s when sea level begins to creep up.
“But the Wilkins is rather odd,” Scambos noted. “It is mostly fed by snowfall directly upon it. There are a few glaciers in the far south, but really, I would not expect a dynamic, interesting signal from this.”
NSIDC scientists and others have previously noted that the collapse appears to be part of a pattern, and additional ice shelves in the region may be at risk. Several have retreated in the past 30 years, and at least six of them have collapsed completely — Prince Gustav Channel, Larsen Inlet, Larsen A, Wordie, Muller and the Jones ice shelves. The Larsen B has lost about three-quarters of its area, much of it in a spectacular disintegration in 2002, but a small southern patch still remains.
The collapse and disappearance of these ice shelves are occurring at a historic rate, Scambos said.
“They are not once-in-a-century events. They are once in several [centuries], or once in several millennia, events,” he said. The Wilkins was built over several centuries, perhaps as long as 1,500 years, he added, while the Larsen Ice Shelf at its southern extreme likely dates back well before the pyramids, more than 10,000 years.
“It is climate warming that is driving the disintegrations,” Scambos explained, “driven by changes in airflow and ocean currents that are completely consistent with anthropogenic increases in greenhouse gases.”
Some scientists believe the relative absence of sea ice — another effect of climate change in the region — made the ice shelf vulnerable to wave action from the ocean. Normally, sea ice buffers the ocean waves, which can pummel the unprotected ice shelf, causing it to flex, particularly if melt ponds have weakened it. Melt ponds are pools of open water on the ice surface that absorb rather than reflect heat.
Scambos said sea ice in the region was phenomenally low in the region of the Wilkins when the ice bridge shattered, but the latest break-up began in the center of the bridge plate and not the edges.
“I’d expect that if waves were the trigger, the event would have begun at the edges,” he explained. “That is not to say that waves are not a factor in other break-up events. The shelves, when nearing their climate limit, are like racks of standing dominos — any significant stimulus, like waves, or winds, or stresses, could start a toppling runaway.”
Another well-known Antarctic researcher believes it may not have been simple wave action but ocean swells from a major storm that first caused the Wilkins to break up last year.
Doug MacAyeal, with the University of Chicago, has done previous research on icebergs in Antarctica. His work studying the break up of a berg called B-15a earlier this decade suggested a storm off the Alaskan coast finally shattered the iceberg. The long-period waves — 30 to 100 seconds between crests — are more likely to affect the thick ice than localized wave action, he explained.
Distant storm waves may have been the initial trigger last year that first sent the Wilkins to pieces. “The real big waves that the surfers like, they are these long waves — these 30- to 100-second waves — that have huge wave lengths, meaning the crests will be more than a kilometer apart. Those are the waves that surfers like and the waves that ice shelves respond to the most.”
Localized wave action is then responsible for the rapid break up that occurs as the ice shelf crumbles, as icebergs flip, drop and pop, creating a storm of tsunami waves that becomes self-perpetuating. As more icebergs crash about, more waves form, which causes more destruction to the ice shelf in a positive feedback loop.
“The ocean became extremely rough. It was like a mosh pit of waves,” said MacAyeal, who has been studying the process of rapid disintegration for the last 10 years.
What ice shelf might fall next?
Scambos said that outside of the Antarctic Peninsula and the north coast of West Antarctica, the rest of the continent is responding more slowly to climate change. The South Pole is even experiencing a slight cooling trend.
“So regions outside the Peninsula and Pine Island Bay are not under immediate threat,” he said. “It will continue to be the Peninsula and Pine Island [Glacier] ‘in the headlines’ for climate change for a while.”
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