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Tuesday, June 28, 2011

Joseph Romm: Ocean Currents Speed Melting of Antarctic Ice, as “Seawater Appear[s] to Boil on the Surface Like a Kettle on the Stove” [Pine Island Glacier accelerates, has greater exposure to above freezing ocean water]

Ocean Currents Speed Melting of Antarctic Ice, as “Seawater Appear[s] to Boil on the Surface Like a Kettle on the Stove”

by Joe Romm, Climate Progress, June 27, 2011
Upwelling seawater along parts of Pine Island Glacier Ice Shelf has carved out caves in the ice and drawn wildlife like this whale. Credit: Maria Stenzel, all rights reserved.
Upwelling seawater along parts of Pine Island Glacier Ice Shelf has carved out caves in the ice.
The news release by Columbia University’s Earth Institute explains:
Stronger ocean currents beneath West Antarctica’s Pine Island Glacier Ice Shelf are eroding the ice from below, speeding the melting of the glacier as a whole, according to a new study in Nature Geoscience. A growing cavity beneath the ice shelf has allowed more warm water to melt the ice, the researchers say—a process that feeds back into the ongoing rise in global sea levels. The glacier is currently sliding into the sea at a clip of four kilometers (2.5 miles) a year, while its ice shelf is melting at about 80 cubic kilometers a year – 50% faster than it was in the early 1990s – the paper estimates.
We knew that Pine Island Glacier (PIG) is disintegrating much faster than almost anybody imagined — see “Nothing in the natural world is lost at an accelerating exponential rate like this glacier” (8/09).  And we knew deep ocean heat is rapidly melting Antarctic ice (12/10), which noted: “Global warming is sneaky. For more than a century it has been hiding large amounts of excess heat in the world’s deep seas. Now that heat is coming to the surface again in one of the worst possible places: Antarctica.”
This new study, “Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf” (subs. req’d), gives us a better understanding of just how PIG is being undermined from underneath: “We conclude that the basal melting has exceeded the increase in ice inflow, leading to the formation and enlargement of an inner cavity under the ice shelf within which sea water nearly 4 ◦C above freezing can now more readily access the grounding zone.
Here is a particularly remarkable observation the scientific team made one day:
One day, near the southern edge of Pine Island Glacier Ice Shelf, the researchers directly observed the strength of the melting process as they watched frigid,  seawater appear to boil on the surface like a kettle on the stove. To Jacobs, it suggested that deep water, buoyed by added fresh glacial melt, was rising to the surface in a process called upwelling. Jacobs had never witnessed upwelling first hand, but colleagues had described something similar in the fjords of Greenland, where summer runoff and melting glacier fronts can also drive buoyant plumes to the sea surface.
In 2001, the IPCC “consensus” said neither Greenland nor Antarctica would lose significant mass by 2100. They both already are.  As Penn State climatologist Richard Alley said in March 2006, the ice sheets appear to be shrinking “100 years ahead of schedule.”  And the ice loss is accelerating (see “JPL bombshell: Polar ice sheet mass loss is speeding up, on pace for 1 foot sea level rise by 2050“).
Accelerated disintegration of  the West Antarctic ice sheet (WAIS) is most worrisome (at least for this century) because it’s going to disintegrate long before the East Antarctic Ice Sheet does.  Not only is the WAIS melting from underneath, it is, as I wrote in the “high water” part of my book, inherently less stable:
Perhaps the most important, and worrisome, fact about the WAIS is that it is fundamentally far less stable than the Greenland ice sheet because most of it is grounded far below sea level. The WAIS rests on bedrock as deep as two kilometers underwater. One 2004 NASA-led study found that most of the glaciers they were studying “flow into floating ice shelves over bedrock up to hundreds of meters deeper than previous estimates, providing exit routes for ice from further inland if ice-sheet collapse is under way.” A 2002 study in Science examined the underwater grounding lines -- the points where the ice starts floating. Using satellites, the researchers determined that “bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed.” And that melt rate is positively correlated with ocean temperature.
The warmer it gets, the more unstable WAIS outlet glaciers will become. Since so much of the ice sheet is grounded underwater, rising sea levels may have the effect of lifting the sheets, allowing more-and increasingly warmer-water underneath it, leading to further bottom melting, more ice shelf disintegration, accelerated glacial flow, and further sea level rise, and so on and on, another vicious cycle. The combination of global warming and accelerating sea level rise from Greenland could be the trigger for catastrophic collapse in the WAIS (see, for instance, here).
The time to act was a while ago, but now is far better than later.
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