In An Inconvenient Truth (AIT), Al Gore, recalling the sudden break up of Larson-B ice shelf in the Antarctic Peninsula in 2002, implied that the vastly larger Greenland Ice Sheet might suffer the same fate during the 21st century:
These pools [of meltwater on the top of the Greenland glacier] have always been known to occur, but the difference now is that there are many more of them covering a far larger area of the ice…they are exactly the same kind of meltwater pools that…scientists observed on top of the Larsen-B ice shelf in the period before its sudden and shocking disappearance…this meltwater is now believed to keep sinking all the way down to the bottom, cutting deep crevasses and vertical tunnels that scientists call “moulins.” When the water reaches the bottom of the ice, it lubricates the surface of the bedrock and destabilizes the ice mass, raising fears that the ice mass will slide more quickly toward the ocean. [AIT, p. 192]
To illustrate how moulins might destroy the Greenland Ice Sheet, Gore presented a photograph and a diagram from a study of “moulins” by Jay Zwally of NASA and colleagues, published in the journal Science in 2002. Gore’s film computer-animates the diagram so that the Greenland Ice Sheet literally begins to split in half along the dashed line. However, the Zwally et al. study did not hypothesize any such crack up. It found that moulins do accelerate glacial movement in Greenland in the summertime, but only by a few percent. For example, glacial flow in 1998 increased from 31.3 cm/day in winter to 40.1cm/day in July, falling back to 29.8 cm/day in August, increasing the annual movement of one ice stream by 4.7 meters. For perspective, the Greenland Ice Sheet extends 2,530 kilometers (1,570 miles) north-south, and has a maximum width of 1,094 kilometers (680 miles).This week’s edition of Science magazine contains a new study (subscription required) by the University of Washington’s Jan Joughin and colleagues that updates the Zwally group’s research. The Joughin team actually found somewhat more glacial acceleration associated with moulins than did the earlier study. However, the study’s bottom-line conclusion is pointedly non-apocalyptic:
Surface-melt-enhanced basal lubrication has been invoked previously as a feedback that would hasten the Greenland Ice Sheet’s demise in a warming climate. Our results show that several fast-flowing outlet glaciers, including Jakobshavn Isbrae, are relatively insensitive to this process. . . . Our results thus far suggest that surface-melt enhanced lubrication will have a substantive but not catastrophic effect on the Greenland Ice Sheet’s future evolution.
In a companion article, Science magazine columnist Richard Kerr quotes Pennsylvania State University glaciologist Richard Alley on moulin-induced ice-sheet lubrication. ”Is it run for the hills the ice sheet is falling into the ocean?” asks Alley. “No, it matters but it’s not huge.” Kerr notes that an entire 4-kilometer-long, 8-meter-deep meltwater lake recently disappeared down a moulin in about 1.4 hours “at an average rate of about 8700 cubic meters per second, exceeding the average flow over Niagara Falls.” Sounds terrifying, doesn’t it? But, continues Kerr, “For all the lake’s water dumped under the ice that day and all the water drained into new moulins in the following weeks, the ice sheet moved only an extra half-meter near the drained lake.” To repeat, the Greenland Ice Sheet is measured in thousands of kilometers. Apocaplyse Not!