New Crack in the PIG
November 3, 2011
PIG – the Pine Island Glacier, which has been called the “soft underbelly” of the West Antarctic ice sheet (WAIS), has just developed a large new crack, serendipitously observed by NASA scientists on a research flight. (above)
Recent studies have focused on the PIG as one of the critical areas to watch for evidence of how the West Antarctic Sheet will respond to changing climate. The British Antarctic Survey recently concluded:
It appears that PIG is not only out of balance but continues to move further out of balance. If there is a feedback process, where downstream thinning causes further ungrounding and acceleration, then this could result in a major retreat of the ice stream.
The new observations seem consistent with this conclusion.
Scientists on an aerial survey of Antarctica have come across an 18-mile-long break in an ice shelf — a sign that the sensitive area is giving birth to an iceberg that will be larger than New York City.
“We are actually now witnessing how it happens,” Michael Studinger, project scientist with NASA’s IceBridge survey, said in a statement Wednesday. “It’s part of a natural process but it’s pretty exciting to be here and actually observe it while it happens.”
The scientists were aboard a NASA jet on Oct. 14, making measurements of Pine Island Glacier and its ice shelf, when they came across the crack.
Glaciers naturally give birth to icebergs, but scientists are concerned that warming temperatures might be destabilizing those in Antarctica and Greenland by eroding the ice shelves floating on water that hold them back up against the mainland.
Without the ice shelves, those glaciers could flow much faster into the ocean, raising sea levels.
Scientists call Pine Island Glacier “the largest source of uncertainty in global sea level rise projections,” NASA noted in its statement.
“It is likely that once the iceberg floats away, the leading edge of the ice shelf will have receded farther than at any time since its location was first recorded in the 1940s,” NASA noted.
Grim Forecast from 1978
“…..from the same time period, was the 1978 publication by the late John Mercer, Ohio State U., who argued that a major deglaciation of the West Antarctic Ice Sheet (WAIS) may be in progress within 50 years. This conclusion was based on the fact that the WAIS margin was ringed with stabilizing ice shelves, and that much of the ice sheet is grounded below sea level. The loss of ice shelves — Mercer proposed — would allow the ice sheet to thin, grounding lines to retreat and the ice sheet to disintegrate via calving. This is a much faster means of losing mass than melting in place. Mercer further commented that the loss of ice shelves on the Antarctic Peninsula, as has since been observed, would be an indicator that this process of ice sheet loss due to global warming was underway.”
”Mercer’s ideas led Terry Hughes (1981) (my doctoral advisor at U. of Maine) to propose that the WAIS had a “weak underbelly” in Pine Island Bay. This bay in the Amundsen Sea is where the Pine Island Glacier (PIG) and Thwaites Glacier reach the sea. These are the only two significant outlet glaciers draining the north side of the WAIS. Together they drain 20% of the WAIS. Hughes called this area the “weak underbelly” because these glaciers lack the really huge ice shelves Ross Ice Shelf and the Ronne-Filchner Ice Shelf in which most other large WAIS outlet glaciers terminate. Both glaciers have a relatively rapid flow from the WAIS interior to the calving margin. Further the low surface slopes and smooth flow patterns of PIG suggested to Hughes that there was no indication of a landward rise in the elevation of the glacier bed; such a rise would help stabilize the glacier. Without a rise in the bed, glacier thinning and retreat could result in continual grounding line retreat. The grounding line is where the bottom of the glacier comes in contact with the ground below the ice sheet, in this case the sea bottom. The grounding line is an anchoring point for the outlet glaciers. The length of the glacier that is grounded is both slowed and stabilized by resulting basal friction. Beyond the grounding line toward the margin, the floating ice shelf is susceptible to rapid calving retreat and as the grounding line retreats, so would the calving front. Note in the image below that the situation is even less stable than Hughes speculated. The current grounding line is at a higher elevation than the bed of the glacier for the next 200 km inland of this grounding line. (Note, inland is to the left in the figure, below.) The deeper the basin, the thicker the ice must be to maintain grounding. This makes it tough to slow grounding line retreat once it begins in a deepening basin.”
A major Antarctic glacier may have passed its tipping point, according to a new modelling study. After losing increasing amounts of ice over the past decades, it is poised to collapse in a catastrophe that could raise global sea levels by 24 centimetres.
Pine Island glacier (PIG) is one of many at the fringes of the West Antarctic ice sheet. In 2004, satellite observations showed that it had started to thin, and that ice was flowing into the Amundsen Sea 25 per cent faster than it had 30 years before.
Now, the first study to model changes in an ice sheet in three dimensions shows that PIG has probably passed a critical “tipping point” and is irreversibly on track to lose 50 per cent of its ice in as little as 100 years, significantly raising global sea levels.
The team that carried out the study admits their model can represent only a simplified version of the physics that govern changes in glaciers, but say that if anything, the model is optimistic and PIG will disappear faster than it projects.
Richard Katz of the University of Oxford and colleagues developed the model to explore whether the retreat of the “grounding line” – the undersea junction at which a floating ice shelf becomes an ice sheet grounded on the sea bed – could cause ice sheets to collapse.