November 14, 2008

Slowdown in Greenland

Filed under: Arctic, Polar, Sea Level Rise

No self-respecting global warming presenter would ever miss the chance to warn the audience that higher temperatures could melt ice in places like Greenland, the melting water could lubricate the interface between ice and rock, and watch-out … the ice could increase its velocity, fall or move quickly into the sea, and cause a rapid rise in sea level. If you happen to be Al Gore, you might show us melting ice, water pouring into some moulin (Figure 1), and then cap it off with an image of water drowning out the World Trade Center Memorial. This story in its near infinite varieties appears on literally thousands of websites dealing with the global warming issue.


Figure 1. A moulin is typically a narrow tubular chute through which water enters a glacier from the surface. Moulins can go all the way to the bottom of the glacier, and the water from moulins may help lubricate the base of the glacier, and according to Gore and others, this could accelerate the movement of the ice.

A recent article in the prestigious Science magazine has an alarming title that should thrill the greenhouse crusade: “Large and Rapid Melt-Induced Velocity Changes in the Ablation Zone of the Greenland Ice Sheet.” Holy cow – it’s just like Gore warned us. Throw the words ‘large,’ ‘rapid,’ ‘melt-induced velocity changes’ together in a title of an article in Science and expect to get some high-level media attention. However, once we dove into this article, we found an amazing twist given the threatening title, and we could not wait to feature the piece on World Climate Report.

To begin, the research was conducted by a large team with the Institute for Marine and Atmospheric Research at Utrecht University, Netherlands; the authors state that “This work was supported by several grants from the Netherlands Organization of Scientific Research and the Netherlands Polar Programme.”

van de Wal et al. focused their attention on measurements that are being made on the ice along the west coast of Greenland just north of the Arctic Circle (Figure 2). For the past 17 years, annual measurements have been made along the “K-transect” to measure movements of the ice sheet. However, they state “we started more detailed position measurements in 2005 by taking advantage of technological developments of GPS equipment and data processing. The new instruments record hourly position of stakes, which are drilled into the ice. The GPS (single-frequency) units need to be serviced only once in a year and deliver an ice velocity record with a temporal resolution of 1 day or better.” To say the least, geospatial technologies are showing up everywhere in our lives from the family car to the golf course and now to our favorite transects in Greenland.


Figure 2. The K-transect in west Greenland at 67°N. The background NASA–Modis/Terra image is dated
26 August 2003. K is Kangerlussuaq, whereas 4, 5, SHR, 6, 7, 8, and 9 are surface mass balance
sites. ELA, Equilibrium Line Altitude. The equilibrium line (indicated by the black line) is at about
1500 m above sea level. The image clearly shows zones, from right to left, of snow (Site 10), wet
snow (Site 9), dark ice (Site 8), and clear ice (Sites 4, 5, and SHR) (from van de Wal et al., 2008).

Probably the largest surprise in the article can be seen in the Figure 3 in which we can see the velocity changes at many sites over the 17-year period. The authors note that “The overall picture obtained by averaging all stake measurements at all sites for individual years indicates a small but significant (r=0.79, P < 0.05) decrease of 10% in the annual average velocity over 17 years”. Despite all the talk about moulins, melting, rapid acceleration of ice, van der Wal et al. reveal that the ice movement in western Greenland over the past 17 years has … slowed significantly!


Figure 3. Variations in annual velocity along the K-transect over 17 years; sites with a significant
decrease over time are depicted as thick lines (from van de Wal et al., 2008).

In discussing their results we find some very interesting language, to say the least. At one place they write “it has been suggested that the interaction between meltwater production and ice velocity provides a positive feedback, leading to a more rapid and stronger response of the ice sheet to climate warming than hitherto assumed. Our results are not quite in line with this view.” At World Climate Report, we are far less polite, and instead of saying “our results are not quite in line with your view”, we tend to say “our results suggest you are wrong”!

van der Wal et al. further write “Longer observational records with high temporal resolution in other ablation areas of the ice sheet are necessary to test the importance of the positive-feedback mechanism between melt rates and ice velocities. At present, we cannot conclude that this feedback is important.” Again, we tend to say this moulin link to drowning the World Trade Center Memorial is nonsense, and the empirical evidence is overwhelmingly in our favor.

So how did this article ever get titled “Large and Rapid Melt-Induced Velocity Changes in the Ablation Zone of the Greenland Ice Sheet”? Well, as seen in Figure 4, the Garmin’s (or some other product line) showed an unusually large increase in velocity from one site a week in August in 2006. No one says Mother Nature is not capable of surprises, and the research team was a bit taken back by the sudden movement. But when we examine this article, we are most impressed with the results over the 17-year period and the lack of support for the notion that somehow the velocity of ice is increasing during a time of greenhouse gas build-up!


Figure 4. Variations in velocity at various sites in August 2006 (from van de Wal et al., 2008).

Reference:

van de Wal, R.S.W., et al., 2008. Large and Rapid Melt-Induced Velocity Changes in the Ablation Zone of the Greenland Ice Sheet. Science, 321, 111-113.




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