Posted by: JohnnyRook | January 12, 2009

How and Why Greenland Glaciers are Accelerating

An interesting phenomenon of Greenland Glaciers is that they have been speeding up at a rapid rate. At the same time scientists have noticed an increasing number of “glacial earthquakes” associated with glacial movement. The initial hypothesis proposed to explain this phenomenon was that global warming led to increased glacial melt at the surface of the glacier. This meltwater then flowed through moulins at the glacier’s surface down to the bottom of the glacier where the glacial ice rested on bedrock.

Water entering a moulin.

As the water accumulated, it “greased’ the bottom of the glacier eventually causing the glacier to “lurch forward” suddenly producing at the same time a seismic event reaching as high as 5.1 on the moment-magnitude scale which is similar to the Richter scale.

According to one of the scientists responsible for formulating the “lurching glacier” hypothesis:

“People often think of glaciers as inert and slow-moving, but in fact they can also move rather quickly,” says Göran Ekström, professor of geology and geophysics at Harvard who will be moving to Lamont-Doherty in the spring. “Some of Greenland’s glaciers, as large as Manhattan and as tall as the Empire State Building, can move 10 meters in less than a minute, a jolt that is sufficient to generate moderate seismic waves.”

New research published recently Geophysical Research Letters disproves the “lurching glacier” hypothesis and proposes an alternative. The paper: Step-wise changes in glacier flow speed coincide with calving and glacial earthquakes at Helheim Glacier, Greenland [PDF–subscription required] proposes that ice berg calving at the glacier’s front causes both the glacial earthquakes and the increases in the glaciers rate of movement.

January 22, 2009 UPDATE: To see a wonderful graphic from the January 2009 edition of National Geographic magazine and reprinted online at the Ohio State University School of Earth Sciences (SES) web site, which illustrates this process as it occurs on the Whillans Ice Stream as it flows into the Ross Ice Shelf in Antarctica, follow this link.

The research team, led by Meredith Nettles of Columbia University’s Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory, employed a network of GPS receivers to take measurements both of glacial calving and glacial movement on Greenland’s Helheim Glacier. As the authors explain:

We operated a network of continuously recording Global Positioning System (GPS) receivers on Helheim Glacier for a period of ~50 days in 2007, from July 4 to August 24. Twelve receivers were installed on the glacier, in a configuration including stations both on and offset from the centerline (Figure 1). The stations spanned an along-flow distance of about 20 km, with the downflow stations located just behind the calving front. Several stations installed within a few km of the calving front were removed and relocated to points slightly farther upglacier during a midcampaign field visit. One GPS receiver was operated at a rock site near the glacier throughout the campaign to help define a stable local reference frame; two additional rock stations operated for shorter durations.

Southern Greenland, with locations of glacial earthquakes; arrow marks Helheim Glacier (inset). Geometry of GPS network at Helheim Glacier during summer, 2007, overlain on a 2001 LANDSAT image. The position of the calving front at two times during the summer of 2007 is shown by the black dotted lines (easternmost line, July 4; westernmost line, August 15). Blue dots, locations of GPS stations on the glacier surface at the time of deployment; black ring shows station IS38 (Figure 3). Red dots, locations of rock-based reference stations. Yellow arrows show average station velocities determined over the duration of station deployment; white arrow shows scale

What the scientists learned over the course of their study, which took place during July and August, the peak calving periods, is that the glacial earthquakes were closely tied to calving events at the the mouth of the glacier. Major calving events usually preceded acceleration of glacier movement by anywhere from 15 minutes to an hour with a greater increase in glacier speed toward the front of the glacier and a lesser increase up-glacier. Interestingly the scientists did not observe a measurable coseismic displacement in association with Helheim’s glacial earthquakes. In other words the glacier maintained it’s its physical integrity although it began to move faster. Lending further support to the idea that it is the calving that causes both the earthquakes and the increase in glacial speed is the fact that the force of the glacial acceleration is insufficient to cause the seismic events as the earlier hypothesis had posited.

Our combined seismological and geodetic observations suggest two plausible scenarios for glacier speedup. In the first, a large calving event leads to the loss of resisting forces at the calving front, resulting in glacier acceleration [e.g., Howat et al., 2005], and produces one or more glacial earthquakes, perhaps through one of the mechanisms of Tsai et al. [2008]. The apparent small difference in the timing of acceleration and calving results from the finite duration of the calving process, the uncertainties in our estimate of the time of glacier acceleration, or both. In this scenario, the seismic precursors to the glacial earthquakes are associated with disintegration of the calving front in preparation for a major calving event. In the second scenario, the glacier accelerates as a result of a process other than calving, such as the passage of a meltwater pulse under the glacier, and this process leads to calving and associated glacial earthquakes. The speedup is sustained and perhaps enhanced by a calving-related loss of resisting forces at the calving front. In either scenario, changes in tidewater-glacier speed are closely tied to the behavior of the glacier terminus.

The second scenario still has meltwater playing a role as a contributing factor to iceberg calving and earthquakes, which in turn lead to faster glacial movement. But the calving and the earthquakes are seen now as the proximate cause of the increase in the rate of glacial movement rather than as an effect.

The researchers conclude:

Our results demonstrate that large outlet glaciers can accelerate in a near-instantaneous, step-like fashion, and show a clear link between such acceleration and large calving events. In addition, our observations invalidate the lurching-glacier model [Ekström et al., 2003; Tsai and Ekström, 2007] for Greenland’s glacial earthquakes, and tie the earthquake source closely to processes at the calving front. The glacial earthquakes and the rapid accelerations we document emphasize the importance of short-time-scale processes occurring at Greenland’s outlet glaciers, and highlight the need to understand the role such processes play in controlling longer-term, seasonal and interannual, variability in glacier behavior.

Here are some images of the Helheim glacier from NASA’s Earth Observatory:

These images from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite show the Helheim glacier in June 2005 (top), July 2003 (middle), and May 2001 (bottom). The glacier occupies the left part of the images, while large and small icebergs pack the narrow fjord in the right part of the images. Bare ground appears brown or tan, while vegetation appears in shades of red.

From the 1970s until about 2001, the position of the glacier’s margin changed little. But between 2001 and 2005, the margin retreated landward about 7.5 kilometers (4.7 miles), and its speed increased from 8 to 11 kilometers per year. Between 2001 and 2003, the glacier also thinned by up to 40 meters (about 131 feet). Scientists believe the retreat of the ice margin plays a big role in the glacier’s acceleration. As the margin of the glacier retreats back toward land, the mass of grounded ice that once acted like a brake on the glacier’s speed is released, allowing the glacier to speed up.

Note: Here then you have another example of real science, in which one hypothesis is proposed and then discarded in favor of another as the evidence changes. (Note that Dr. Nettles was one of the researches on the team that came up with the original “lurching glacier” hypothesis, which this study duscredits. This is true “skepticism” in action where one honestly weighs all the data and revises one’s opinions based on the credibility of the evidence free from any political agenda. The denialists/delayers who like to style themselves as “skeptics” have nothing to do with this process of scientific inquiry. Instead, their goal, when they are not outright lying, is to cherry pick the data in search only of those studies which might possibly be construed to support their positions.

Related post:

Ocean Cooling: A Science Lesson for Denialists/Delayers

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Responses

  1. Sea level rise is thought to accelerate the breakup of ice shelves – found mostly in Antarctica. The lifting ice, floating on a higher ocean, breaks off the shelf by uplift.

    But if glacier calving into a sea level mouth of the glacier, then with rising sea levels then shouldn’t glacier movement also increase? Or maybe the rising ocean will penetrate further up under the glacier.

    So does increased rate of change work to increase the rate of change?

  2. So, a hand basket dropped into hell does accelerate the same as anything else (32ft/sec squared)…


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