Sometime between 10 and 12 July, Antarctica lost 1 trillion metric tons of ice, when a section of the Antarctic Peninsula’s Larsen C ice shelf broke free.
Scientists have been watching the Delaware-sized slab of ice in West Antarctica for more than a decade, after the Larsen A ice shelf collapsed in 1995 and Larsen B collapsed in 2002. This January, May, and June, scientists reported rapid advances of the rift. By July, the shelf appeared to be hanging on by a thread.
But now a 5800-square-kilometer section is free, drifting in the Weddell Sea. “We have been anticipating this event for months, and have been surprised how long it took for the rift to break through the final few kilometers of ice,” wrote Adrian Luckman, a glaciologist at Swansea University in the United Kingdom, on a research blog. Luckman is the lead investigator of Project MIDAS (Impact of Melt on Ice Shelf Dynamics and Stability), a U.K.-based Antarctic research project that has been watching the widening crack in Larsen C. “We will continue to monitor both the impact of this calving event on the Larsen C Ice Shelf, and the fate of this huge iceberg.”
— Project MIDAS (@MIDASOnIce) July 12, 2017
But questions still remain, including what this breakage means for the rest of Antarctica, parts of which are under threat of total collapse. Here are six points of perspective, pulled from Eos and AGU blogs, that put this latest event into context:
1. Now That It’s Free, Where Will Larsen C’s Iceberg Go?
If the iceberg stays intact, it may drift along the Antarctic Peninsula’s coast for the next year or so, say scientists who study the movement of icebergs. The researchers use actual positions of icebergs along with computer models to investigate how these chunks of ice move once they break off.
Smaller icebergs less than 2 kilometers long, driven by wind, end up out in the open ocean and break up fairly quickly, the researchers found. But Larsen C is massive—wind won’t affect its movements as much as its own weight will, the researchers predict. What may affect its movements is whether it breaks up.
If the iceberg doesn’t break up, “chances are good that it will first drift for about a year through the Weddell Sea, along the coast of the Antarctic Peninsula. Then it will most likely follow a northeasterly course, heading roughly for South Georgia and the South Sandwich Islands,” they said.
2. Ice Loss Beyond Calving—Warm Water Eating Ice from Below
As demonstrated by the now free-floating ice mass that just broke off Larsen C, ice shelves can lose mass through huge calving events. But other mechanisms are also at play.
The Larsen C ice shelf floats on the sea, which means that the underlying ocean is also continually melting it from below. This process is extremely difficult to observe, so it’s difficult to quantify how much ice is lost through basal melting.
Traditional methods to estimate this quantity use satellites to track elevation changes at fixed coordinates on an ice shelf. But recently, a group of researchers tried a new technique to track how much an ice shelf melts from below. Instead of using fixed geographical coordinates, they pinpointed the same spot on the moving ice shelf for each measurement. This technique, they suspect, removes any effect from the motion of the shelf’s rough surface.
Using this technique, researchers found that the Ross ice shelf lost about 50 gigatons of ice per year from basal melting between 2003 and 2009; Filchner-Ronne lost about 125 gigatons per year over the same interval. However, snowfall and ice dynamics compensated for both losses, the researchers note.
3. Warm Waters Triggered Similar Ice Loss 2000 Years Ago
Underlying warm seawater may have caused an ice shelf collapse deep in the past as well. At the 2016 American Geophysical Union Fall Meeting in San Francisco, scientists presented research indicating that warm water helped fuel ice loss in Antarctica’s Amundsen Sea 2000 years ago.
Those researchers looked at sediment cores near the current Cosgrove ice shelf and found fossils of tiny marine critters, evidence of an influx of warm seawater. The presence of tiny fossils with other evidence points to a warm-water contribution to the ice shelf’s melting, the researchers noted. That warm water could have come from the Circumpolar Deep Water, a mass of warm water that circles Antarctica along with the Antarctic Circumpolar Current.
If Larsen C’s huge iceberg becomes the first of many events that lead to the ice shelf’s collapse, more worrisome events may follow. “Once you lose the ice shelf, you lose the continental ice,” noted Rebecca Minzoni, a paleoclimatologist from the University of Alabama in Tuscaloosa and the lead researcher on this study.
4. What’s Next for Larsen C?
Although iceberg calving generally stokes worries of sea level rise, scientists note that the Larsen C iceberg will not contribute to sea level rise because it was already floating on the ocean.
However, scientists worry that it could speed up the collapse of the remaining ice shelf. And if the ice shelf goes, continental ice is at risk of flowing into the ocean and directly influencing sea level rise.
Collapse of an ice shelf is directly related to its grounding line, the line where grounded ice transitions to a floating ice shelf. “As an ice shelf grows in area and thickness, it can buttress the inland ice, stemming its outflow. If this happens, the grounding line can stabilize or advance, thereby slowing sea level rise. But if the grounding line retreats, sending inland ice farther afloat, the shelf may begin a runaway collapse,” scientists involved in predicting sea level rise wrote in Eos.
“As the grounded surface area of the outflowing glacier decreases, so does its friction against the bedrock, allowing it to flow with greater ease. It may also shorten as icebergs begin to calve off, possibly leading to complete shelf disintegration. This allows the inland ice to accelerate its flow into the ocean—and accelerate sea level rise,” they continued. “Indeed, when Antarctica’s Larsen B Ice Shelf collapsed in the early 2000s, the inland ice flow sped up nearly tenfold, proving that a mechanical link exists between ice shelf and inland ice sheet.”
So the question now is, how will the recent calving event influence changes to the shelf’s grounding line?
5. Record-Breaking Temperature Reported near Larsen C
Several hundred kilometers away from Larsen C is the site of the highest recorded temperatures on the Antarctic continent. On 24 March 2015, temperatures at Esperanza station at the tip of the Antarctic Peninsula soared to a rather pleasant 17.5°C (that’s 63.5°F).
“Temperature extremes in the Antarctic are important to evaluate and document in the face of changing regional and global climate,” scientists involved with verifying this temperature extreme wrote in Eos.
Although this record temperature isn’t directly related to the giant iceberg that just broke off Larsen C and although calving events are part of the normal life cycle of ice shelves, extreme dynamics on Antarctica provide context for overall trends. They highlight “the need to continually monitor all of the Antarctic region to ensure that we have the best possible data for climate change analysis at both the regional and global scales,” researchers noted.
6. How Icebergs Calve
Larsen C began to crack in 2006. As the crack propagated, its growth accelerated, arcing from the southeast to the north. Rifts such as this one “form at the margins of an ice shelf, where the ice is thin and subject to shearing that rips it apart,” explained Ian Howat, a glaciologist at Ohio State University.
This is just one way an ice shelf tears apart, Howat and colleagues discovered. Another way involves ice at the center of the shelf weakening to form crevasses that later punch through the ice. These internal rifts propagate to the edges, and the shelf breaks apart from the inside out.
Howat’s team observed such behavior within the Pine Island Glacier, part of the ice shelf that surrounds the West Antarctic ice sheet. “Something weakened the center of the ice shelf, with the most likely explanation being [that] a crevasse melted out at the bedrock level by a warming ocean,” he explained.
So although the massive chunk of ice newly floating in the Weddell Sea is a significant event in its own right, it’s just one manifestation of one mechanism for ice loss in West Antarctica.
Wendel, J.,Kumar, M. (2017), Six points of perspective on Larsen C’s huge new iceberg, Eos, 98, https://doi.org/10.1029/2017EO077735. Published on 12 July 2017.
Text © 2017. The authors. CC BY-NC-ND 3.0
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