|Above: The massive rift in the Larsen C ice shelf that resulted in an enormous iceberg breaking off in early July 2017. This image was photographed in November 2016 as part of NASA’s IceBridge mission. The fracture was more than 300 feet wide and about 1500 feet deep. Image credit: NASA.|
In a long-anticipated yet still-dramatic event, a chunk of ice nearly as big as Delaware has broken off from the Antarctic Peninsula. Satellite images from Wednesday, July 12, show that a slice of the Larsen C ice shelf covering about 1930 square miles has calved, or detached from the peninsula, over the past week (see Figure 1). The resulting iceberg is one of the 10 largest on record—not quite half the size of the world champion B-15 iceberg, which broke off from the Ross Ice Shelf in 2000.
A longstanding rift in the Larsen C ice grew in accelerating fashion over this decade and especially over the past two years, lengthening from about 60 miles long in October 2015 to more than 100 miles long by May 2017. (The Guardian has a nice graphic showing the rift's progress over time.) The fracture extended to the base of the ice shelf, making it all but inevitable that the iceberg would form.
|Figure 1. Progress of the Larsen C rift from July 6, 2017 (left) to July 12 (right). Data is gathered from the region every six days by the European Space Agency's Sentinel-1 mission of the European Space Agency. Image credit: Stef Lhermitte (Delft University of Technology), via Climate Central.|
Figure 2. If the iceberg that broke off from the Larsen C shelf this week were a sphere, this is how it would look positioned atop the San Franscisco Bay. Image credit: Climate Central.
The ABCs of Larsen ice shelf loss
First documented by Norwegian explorer Carl Anton Larsen in 1893, the four Larsen ice shelves extend for about 1000 miles along the east side of the skinny Antarctic Peninsula. Over the last 20-plus years, the shelves have experienced progressively larger calving events, working their way toward higher latitudes in alphabetic order.
|Figure 3. Map of ice shelves along the Antarctic Peninsula. Parts of the Wilkins Ice Shelf (lower left) have collapsed over the past decade. Image credit: A.J. Cook and D.G. Vaughan/Wikimedia Commons.|
- The Larsen A shelf disintegrated in January 1995, giving up about 580 square miles of ice to the sea.
- In a much larger series of calving events, roughly 1250 sq mi of ice (an area bigger than Rhode Island) broke off from the Larsen B shelf between January 31 and March 5, 2002.
- The ice released by the Larsen C calving of March 2017 was larger still, with a surface area more than 50% as large as the multi-month Larsen B calving released in one fell swoop. The Larsen C shelf lost more than 10% of its total area as a result of this week’s calving.
- The more-stable Larsen D shelf has not yet seen any major calving events of this scope in recent years. It is one of the few ice shelves in the region that has gained mass in recent decades, although the amount is small compared to the ice that’s been lost farther to the north.
A natural event within a warming climate
The Antarctic Peninsula is one of the fastest-warming regions on Earth, with average temperatures that have risen about 3°C (5°F) over the last half century. The increasing size and poleward shift of the Larsen calving events over the last 20-plus years would seem consistent with the warming effect of human-produced greenhouse gases. In fact, as noted by John Abraham, a 1978 paper by researcher John Mercer predicted this very thing: "One of the warning signs that a dangerous warming trend is under way in Antarctica will be the breakup of ice shelves on both coasts of the Antarctic Peninsula, starting with the northernmost and extending gradually southward."
At the same time, icebergs are a natural part of the hydrologic cycle through which snowfall is converted into ice and eventually pushed seaward. While scientists have found links to climate change in the Larsen B calving events, most researchers are not pointing to a direct role from climate change on the Larsen C calving, which at least one expert called "business as usual" for Antarctica. Martin O'Leary (Swansea University), who has studied the Larsen C evolution for years as part of the UK-based Project MIDAS, said in a statement: "Although this is a natural event, and we’re not aware of any link to human-induced climate change, this puts the ice shelf in a very vulnerable position. This is the furthest back that the ice front has been in recorded history. We’re going to be watching very carefully for signs that the rest of the shelf is becoming unstable.”
Ice shelves are over-water extensions of inland glaciers, which build up from the year-to-year accumulation of snowfall in frigid polar climates. As gravity pushes these glaciers toward the sea, their front edges can extend well into the ocean. The water beneath the shelves tends to be highly saline, which means it freezes at a colder temperature than freshwater.
A warming environment can compromise glaciers and their ice shelves through higher temperatures atop the ice, which leads to melt ponds that can percolate into crevasses and widen them. The ice shelves can also be destabilized from below, if changes in ocean circulation bring warmer water into contact with the underside of the ice shelf.
Warmer waters appear to be the main driver behind the retreat of hundreds of small glaciers on the west side of the peninsula, according to a 2016 study led by Alison Cook (Durham University). On the east side, where the Larsen ice shelves are located, air temperatures are rising, but there appears to be less influence from warmer waters.
|Figure 4. Schematic of an ice shelf (right) extending from a land-based glacier (left). The grounding line is the point where the ice, land, and sea intersect. Image credit: antarcticglaciers.org.|
Ice shelves and sea level
Fortunately, the ice released in ice shelf calvings does not itself raise global sea level, since the ice was already in the water. The real concern is losing the benefit of ice shelves as “stoppers” that help hold the upstream glaciers in check. When ice shelves are depleted, glaciers tend to move more quickly toward the sea. This pushes more land-based ice into the water, where the ice will melt and raise global sea levels.
The website antarcticglaciers.org outlines what we might expect in the wake of the Larsen C calving. Prior to this week’s calving, the front of the Larsen C ice shelf featured a regime where the so-called first tensile ice stress (or “pulling-apart” stress) was oriented perpendicular to the direction of ice flow. This tends to reduce the number of small, frequent calving events in favor of occasional large events, like the one that just occurred. Computer models suggest that the newly opened ice front at Larsen C may exhibit the opposite regime—one where the pulling-apart stress is aligned with the ice flow. This may lead to more rapid calving and a series of smaller icebergs, and it may raise the odds that the entire Larsen C shelf will collapse.
Even if all of the glacial ice behind Larsen C were to cascade into the sea, sea level would rise by no more than 4 inches. What concerns researchers much more is the prospect of ice shelf collapse on the west side of the much larger Antarctic continent, where the shelves are buttressing enormous volumes of ice. It appears that warm water is already thinning the ice shelves that extend into the Amundsen Sea from the West Antarctic Ice Sheet (WAIS). Because of a process called marine ice sheet instability, it is conceivable that large parts of the WAIS might cascade into the sea if these ice shelves were to collapse, pushing sea level upward by 12 feet or more. Recent research indicates that as much as three feet of rise could occur from this process alone before the end of this century.
The odds of such an outcome can be reduced if greenhouse emissions are controlled, according to Christina Hulbe (University of Otago), who examined the question of inevitability last month in Science. Despite the complexity of modeling ice sheet behavior, and the variety of results produced by various models, the take-home message is a simple one: “Whether the model ice sheets collapse depends on the level of warming."