When I last diaried on the breakup of the Wilkins ice shelf there were a great many questions about glacier movement and sea-level rise so I thought that while we are waiting for the Wilkins ice shelf to collapse completely, it would be worthwhile to take the time to take a look at what causes ice shelves to break up and what the consequences of such a breakup are.
We can do this by examining the collapse of the Larsen B ice sheet, which took place in 2002.
Before we talk about ice shelves let me mention the wonderful overlay that the the British Antarctic Survey has released for Google Earth showing the breakup of Antarctic ice sheets. You can use it to locate and learn more about the ice sheets we’re going to look at in this diary. (The site also has a second interesting overlay from the UK Met Office’s Hadley Office with projections of regional temperature increases over this century.) To use the overlays you must first have Google Earth installed. If you don’t already have Google Earth you can get it here.
What is an ice shelf? Let’s begin with a definition from Wikipedia (you can also find a list of the world’s ice shelves at the same link):
An ice shelf is a thick, floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are found in Antarctica, Greenland and Canada only.
This is different from an ice sheet which instead of floating on the sea rests on land and covers an area of at least 50,000 square kilometers.
Ice shelf collapse is a process that has taken place many times as the earth’s climate moved from glacial stages to interglacial ones. See: Catastrophic ice shelf breakup as the source of Heinrich event icebergs (subscription required).
As Climaticide has worsened scientists have observed an increase in the rate of ice shelf collapse.
In the last several decades, glaciologists have observed consistent decreases in ice shelf extent through melt, calving, and complete disintegration of some shelves.
The Ellesmere ice shelf reduced by 90 percent in the twentieth century, leaving the separate Alfred Ernest, Ayles, Milne, Ward Hunt, and Markham Ice Shelves. A 1986 survey of Canadian ice shelves found that 48 km². (3.3 cubic kilometers) of ice calved from the Milne and Ayles ice shelves between 1959 and 1974. The Ayles Ice Shelf calved entirely on August 13, 2005. The Ward Hunt Ice Shelf, the largest remaining section of thick (>10 m) landfast sea ice along the northern coastline of Ellesmere Island, lost 600 square km of ice in a massive calving in 1961-1962. It further decreased by 27% in thickness (13 m) between 1967 and 1999. In summer 2002, the Ward Ice Shelf experienced another major breakup. 
Two sections of Antarctica’s Larsen Ice Shelf broke apart into hundreds of unusually small fragments (100’s of meters wide or less) in 1995 and 2002.
The most studied of the recent ice shelf collapses is that of the Larsen B ice shelf which was exceptionally large:
The disintegration of the huge Larsen B ice shelf in Antarctica was an unprecedented event in the past 10,000 years of geological history, a study has found.
In March 2002, scientists announced the Larsen B ice shelf on the Antarctic Peninsula had entered a phase of rapid break-up with more than 50 billion tons of ice spilling into the Weddell Sea to form thousands of massive icebergs. It had been known for many years that the ice shelf was thinning and in retreat but the speed of its final collapse astonished scientists. It took just 35 days for the Larsen B ice shelf to fall away completely after a Nasa satellite detected the first ruptures in the 1,255 square miles of ice at the end of January 2002.
Below is an image of the Larsen B ice shelf in January 2002 before it’s collapse:
Here is another image of the same ice shelf in March of the same year after the breakup had occurred:
And here is an animation of the collapse:
N.F. Glasser and T.A. Scambos (PDF) in their paper A structural geological analysis of the 2002 Larsen B ice shelf collapse give 5 reasons for the importance of Antarctic ice shelves:
Ice shelves fringe ~45% of the Antarctic continent. They are
important in global Earth-system processes for five main reasons:
(1) ice shelves play a significant role in the global ice-volume/sea-level system because the calving of icebergs from their termini accounts for ~90% of Antarctic ice loss (Vaughan and Doake, 1996; MacAyeal and others, 2003);
(2) ice shelves influence the dynamics, and therefore the system response time, of upstream inland Antarctic ice (Rott and others, 2002; De Angelis and Skvarca, 2003; Scambos and others, 2004);
(3) rapid heat exchange in sub-ice-shelf cavities has a significant impact on the global ocean heat
budget (Williams and others, 2001; Joughin and Padman, 2003);
(4) ice shelves are capable of entraining, transporting and depositing large quantities of glacigenic material (Evans and Pudsey, 2002; Evans and O´ Cofaigh, 2003; Glasser and others, 2006); and
(5) catastrophic iceberg-calving events from ice shelves have been proposed as a cause of major Late Quaternary climatic perturbations (Hulbe and others, 2004).
They go on to note that Antarctic ice shelves have retreated in two stage:
1. a climatically driven gradual retreat that lasted for years or decades
2. a sudden collapse phase
Warming of the atmosphere or ocean currents can be a major factor in ice shelf collapse.
Because mean summer temperatures have risen to near-melting and the length of the melt season has doubled during the last two decades,
meltwater ponds have appeared on the surface of many Antarctic Peninsula ice shelves during the melt season (Van den Broeke, 2005). It has been suggested that this meltwater acts as a mechanical force in the crevasses, causing breaks in the ice shelf and thus accelerating ice-shelf disintegration (MacAyeal and others, 2003; Scambos and others, 2003; Van der Veen, 2007).
Photo right: Meltwater stream flowing into a large moulin in the ablation zone (area below the equilibrium line) of the Greenland ice sheet. (Image courtesy Roger J. Braithwaite, The University of Manchester, UK via GISS)
NOTE: The image above is of a melt stream and moulin on an ice sheet. If the water reaches the bedrock the ice sheet rests upon it can lubricate the ice sheet bottom and speed up it’s movement. Since an ice shelf floats in the water, if melt water works to the bottom of the sheet, it may contribute to fracturing it but does not make it move faster.
If you look at the first image of the Larsen B ice shelf above you will note striated features on the part of the shelf nearest the sea. Behind the ice sheet one can see the glaciers that flow into it. The ice from the various glaciers come together as they enter the ice sheet, however they are not all moving at the same rate. The striations, then, are crevasses and rifts resulting from the lateral sheer as faster moving ice moves past slower moving ice.
According to Glasser’s and Scambos’s analysis:
Prior to collapse, large open-rift systems (with floating brash ice) were present offshore of Foyn Point and Cape Disappointment. In the years just preceding break-up, these rifts became more pronounced and ice blocks in the rifts rotated because of the strong lateral shear in the zone separating active and less-active flow units. Velocity mapping of the suture regions indicates that the major rifts are a recent feature of the ice shelf, and werenot present over ~20 years ago.
The collapse of the Larsen B ice shelf was the result of atmospheric and oceanic conditions in combination with structural weaknesses within the ice shelf. Glasser and Scambos summarize the forces at work in the followong chart:
So, what were the consequences for sea-level rise of the collapse of this huge ice shelf? From the ice sheet itself directly, nothing. As the ice was already floating in the water it had no effect on sea levels. However it did have an indirect effect within a short time.
Two studies published in 2004 after the collapse of the Larsen B ice shelf, one by Scambos et. al. at the National Ice, Snow and Data Center, Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica and the other by E. Rignot et. al. at the Jet Propulsion Laboratory in Pasadena, Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf (both available through the American Geophysical Union-subscription required)reached similar conclusions: after the collapse of the ice shelf, which had served as giant buttresses for the glaciers behind them, the rate of glacier flow increased between 3 and 8 times, depending on the glacier and the study.
According to Rignot:
The mass loss associated with the flow acceleration exceeds 27 km3 per year, and ice is thinning at rates of tens of meters per year. We attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf. The magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution ot sea level change.
Temperature map is from the NASA Earth Observatory
It should be observed here that the Wilkins ice sheet whose collapse prompted this discussion is an anomaly. For one thing, glacier flow contributes only minimally to the ice shelf. Hence its collapse will not have the same sort of impact that the collapse of the Larsen B ice shelf did, the effect of which was a 0.1 to 0.16 mm sea level rise.
According to NASA’s Earth Observatory:
The Larsen Ice Shelf is “typical” in that it is primarily fed by a land-based glacier. The Wilkins Ice Shelf is somewhat unusual in that only the southern end of the shelf appears to be fed by land-based ice; the rest of the shelf may have formed from accumulation of sea ice that held fast to the coastline through many seasons, as well as snow cover. Glaciologists estimate that the part of the Wilkins Ice Shelf that formed from sea ice may be 300 to 400 years old, and the part that is fed by glacier flow is older, perhaps up to 1,500 years old. Because the Wilkins Ice Shelf is only marginally fed by glacier flow, however, its collapse was not expected to have the same impact on sea level rise as the collapse of the Larsen B potentially could.
Crossposted at Daily Kos