Part 4—Find Glaciers on the Go
Step 1 Explore Greenland's Glacier Velocity Gates Layer
- In Visualize mode, turn the following layers on. Leave all other layers off.
- Greenland Glacier Velocity Gates
- Greenland Ice Sheet
- Use the Zoom In tool to zoom in on the line of dots representing Glacier Velocity Gates.
- Use the Measure tool
to check the distance between several successive pairs of "gates." To make a measurement, click on the Measure tool icon to select it (it will turn grey). Then move to the map where the crosshair cursor can be placed on any gate and click once to start measuring. Then move to an adjacent gate (a line will draw) and click twice. The distance, in meters, appears on the lower left information bar of the My World window.
- What is the average distance between these points? The average distance is 20 -30 kilometers.
- What is the average distance between these points?
- Note that the points are generally parallel to Greenland's coast, indicating that they are all roughly at the same elevation.
- How do you think this line of points might be used in measuring the downhill velocity of Greenland's ice? Describe your ideas to a lab partner.
- Once you've thought about it and described your idea, check the "Show me" link below to see if your ideas are similar to the ones scientists used.
The Glacier Velocity "Gates" provide a way to estimate how much ice is flowing downhill around the continent. The gates are actually points on a map chosen by scientists who work at the Ohio State University. They are approximately 30 km apart, surrounding the entire continent at an elevation of around 2000 m. The amount of ice that flows between every pair of gates is calculated by multiplying the average thickness of the ice between the gates by the downhill distance the ice moved. Ice thicknesses were derived from radar sounding. The downhill distance is measured every other year using NASA's Airborne Topographic Mapper laser system.
From scientist Dr. Gordon Hamilton, of the University of Maine:
The gates on the ice sheet are really only points on a map. They are physical in the sense that they describe a cross-section through which something (ice) flows. The gates for the Greenland project were spaced every 30 km along the 2000 m contour. So, the gate dimensions were 30 km x (1850) m, the value in parentheses being the "average" ice thickness along that 30 km stretch of the ice sheet. Ice thickness was derived from radar sounding.
The idea of gates is easier to understand if you think of an outlet glacier. Are you familiar with Jakobshavn Glacier (the world's fastest)? Anyway, it has a large catchment basin that extends all the way to Summit. Ice flows downslope towards the ocean and is eventually funneled out through Jakobshavn Glacier. The gate, in this case, is the glacier constrained by the fjord. In other words, the fjord geometry (i.e., its bedrock topography) defines the gate. So, why not use outlet glaciers as the gates? Well, usually we try to, because scientifically it is easier to define, but ....
Step 2 Check the Data from the Velocity Gates
- Activate the Greenland Glacier Velocity Gates layer by clicking its name in the Layer List, then click the table icon to open its data table.
- Each row of data (also known as a record) provides information for one of the 161 gate points. Sort the records by clicking the Elevation header to find the range of elevations for the gates.
Here's how Dr. Hamilton, one of the scientists that set up this experiment, answered this question:
... they were placed on the 2000 m contour primarily for logistic convenience. The trouble with outlet glaciers is that they move too fast! And that means lots of crevasses. The 2000 m contour in Greenland is generally far above the inland limit of crevassing, so it was safe (and possible) to land an aircraft (Twin Otter). Also, there is very little melting at this elevation, at least in late spring when the measurements were done. Any lower and we would have encountered wet snow, which makes it very difficult for the ski-equipped Otter to take-off.
Ideally, we want the gates to be as close as possible to the grounding line (or the ocean), but logistically this is rarely possible. The best way to do this is using satellite imagery to map velocities (that's what we're spending a lot of our time doing here in our lab at the moment). The disadvantage of placing the gates on the 2000 m contour is that the results only tell about the state of the ice sheet inside that ring. We know how much mass was added (from snowfall) and we know the flux of mass out through the gatessubtract one from the other to get mass balance. However, we have no way of knowing what is happening downstream of the 2000 m contour.
- Sort by the Velocity (m/year) column to find the fastest and slowest downhill velocities of the ice. Descriptions of the codes used in these tables is available in the mass balance (Acrobat (PDF) 66kB Nov22 07) (in PDF) file.
Step 3 Discover which Regions of the Ice Sheet are Moving most Rapidly
- Activate the Glacier Velocity Gates layer, then choose Layer > Edit Layer Appearance, to open the Legend Editor.
- In the Legend Editor, click the Color tab, then select the "Choose Color by" drop-down menu and select Velocity (m/year).
- Check the results on your map, noting any visible patterns that reveal areas of fast or slow movement of the ice.
- Turn off the Greenland Ice sheet layer in order to see the full range of colors of the gates.
- Experiment with other legend options to display the velocity gates data in new ways. For instance, you may want to show velocity using dots of different sizes, or use an arrow to indicate the bearing (direction) of the flow.
- Use Analyze mode to highlight gates where the average velocity of the ice is is greater than 80 m/yr.
- In Analyze mode, choose Select > By Value... to open the dialog box.
- Select records from Greenland Glacier Velocity Gates whose Velocity (m/year) is Greater than or equal to (>=) a value of 80.
- Click OK then accept the default name by clicking OK again.
- On your map, choose a highlight mode that will allow you to see your selection that shows the fastest moving ice.
- Is there a pattern to where the fastest moving glaciers are located? How might this be related to weather and climate patterns? They are on the western side of Greenland, which does get the heavier snows. The southern tip of Greenland is warmer because it is further south and gets more sunlight.
- If the glaciers in southern Greenland are melting, where does the melt water go? How might this fresh water impact the ocean?First the water goes into the crevasses and speeds up the melting of the ice sheet. It eventually makes its way into the ocean. Scientists are carefully monitoring the salinity of the ocean in this region.
- Finish this chapter by skimming through the article, and viewing the slide show, from National Geographic entitled The Big Thaw. For more information about the Greenland Ice Sheet, visit the links listed in the Going Further section of this chapter.