Part 3—Observe Ice Sheet Melt Extents

Step 1 Investigate Melt Extents from 1992 to 2003

Satellite views of a portion of the Greenland Ice Sheet in 2001, 2002, and 2003. The zone where surface melting has occurred is gray rather than white. Click the image for a larger view. MODIS images from NASA.

The extent of melting on an ice sheet is the area where some of the solid ice at the surface has turned to liquid water. This surface melting doesn't mean that the entire thickness of the ice sheet disappears, only that liquid water has formed at the surface of the solid ice.

A melt pond on the surface of sea ice with a Coast Guard ice breaker in the background.
Because the presence of liquid water changes how ice reflects light, instruments on satellites can detect areas of the ice sheet that experience melting. The total area that experiences surface melting within a year is known as the Melting Extent for that year.

Surface melting may also result in the formation of melt ponds. A melt pond is a pond of water that forms on the surface of an ice sheet or sea ice. Melt ponds are often shallow, but can become very deep if the liquid water melts its way down through the ice.

  1. If you haven't already done so, click the checkboxes to turn off the Greenland AWS Stations and Greenland Ice Sheet layers.
  2. Turn the Greenland Melting Extents by Year layer on, then click the name of the layer to activate it. The background of the name will turn white when the layer is activated.
  3. On the Layer name, click the pull-down menu and select the 1992 Melt Extent. Each area highlighted on the map shows surface melting during the summer season of 1992.
  4. In this image, the pull-down tab is the little triangle (circled).

  5. Repeat this action to view the Melt Extents for the years 1992 - 2003. What trends do you notice in the data?
    The Melt Extents are increasing each year.

Step 2 Compare Melting in 1992 and 2002

  1. Click the Analyze tab to switch to Analyze mode
  2. Create a selection of records to show where melting was present in 1992, by choosing Select > By Value....
  3. In the dialog box, use the pull-down menu to select records from Greenland Melting Extents by year Whose 1992 (value) is Greater than or Equal to (>=) a value of 1. Note: A value of "1" in the data indicates an area that has experienced melting.
  4. Click OK, and accept the default name given to the selection by My World.
  5. Click OK again. You will be returned to your map (in Visualize mode) where you should now see a radio button below Greenland Melt Extents in the layer menu.
  6. Repeat this procedure for 2002.
  7. You should now have two radio button radio buttons under the Greenland Melting Extents by year layer.

  8. Click the 1992 radio button to activate the selection.
  9. Click the Show Table of Selection button above the layer list. Near the top of the table, read the number of records (areas) of the total of 22,900 records that experienced melting in 1992.
  10. Close the table for 1992 and then repeat the process to see the number of areas that experienced melting in 2002. Use the same techniques to answer the following questions.
    • How many more areas showed melt in 2002 than in 1992?
    • Repeat the procedure for some of the intervening years. Optional: Create a graph to see if the increase in melting is part of a trend, or just an anomalous year.
    • Each record indicates an area that measures 25 km by 25 km (or 625 km2). Use this information to calculate the area that experience surface melting during 1992 and 2002.
      In 1992, 868 areas recorded melting, or 542,500 sq km. In 2002, 3075 areas recorded melting or 1,921,875 sq km.

Step 3 Investigate the Number of AWS Locations within the Melt Extent in 1992 and 2002

  1. Click the Visualize tab to return to Visualize mode, and turn on the Greenland AWS locations layer again. Be sure it is still above the Greenland Melting Extents by year layer in the layer list.
  2. Switch back to Analyze mode. You'll make a spatial selection to find out the number of weather stations within the melt extent in 1992 and in 2002.
  3. In Analyze mode, choose Select > by Spatial Relationship... >
  4. Set the dialog to Select Records from: the Greenland AWS stations which Cross records in 1992 >=1.
  5. Click OK and enter a name for the result.
  6. Click OK again to return to the map.
  7. Double-click your result in the layer list to display the table and find out how many stations were selected.
  8. Repeat the procedure to select AWS locations within the melt extent in 2002.
  9. Answer the following questions about the relationship between the melting ice sheet and working conditions.
    • How many weather stations were within the melt extent in 1992? In 2002?
      In 1992 there was 1 station in the Melt Extent and in 2002 it had increased to 8 stations in the Melt Extent.
    • Think of the scientists' working conditions you saw in photographs in Part 1. Now imagine the conditions at those AWS locations that are within the melt extent in 2002. What additional challenges do you think scientists faced with surface melting occurring at the sites?
      The snow and ice become softer when covered in liquid, and it can be difficult and dangerous moving around on a snowmobile.

Step 4 What does Surface Melting do to an Ice Sheet?

The percent of sunlight that reflects back into space off of Earth's surface is known as albedo. Snow-covered surfaces are especially reflectivejust think for a minute how bright snow looks on a sunny day. Liquid water is much less reflective than ice and the water absorbs much of the sunlight's energy that reaches it.

One of the results of surface melting is that the ice surface becomes less reflective. As melting begins, the surface starts reflecting less energy and absorbing more, resulting in further melting. This reduces the albedo even further, which causes more melting, and so on... This is known as a "feedback loop."

  1. Use the checkbox to turn off the Greenland AWS Locations and Greenland Melting Extent by Year layers.
  2. Turn off all layers except Continents.
  3. View the image below Greenland Melt extent 1979-2002 and answer the following questions:
    • How many years of data are on the chart?
    • How much has the melt area increased since 1992?
    • How many square kilometers was the melt in 2002?
    • What other trends in the data did you notice?

    Time series of Greenland melt extent derived from passive microwave remote sensing from 2010 (red), 2007 (blue) and the 1979-2007 average (green), after Mote (2007).

    To learn more about this graphic and the Greenland Ice Sheet in 2010, visit Arctic Theme Page and click on the links to the Report Card. This graph is in the 2010 report card.

    Another related article, from November 2011, is available on the NASA Earth Observatory Site Greenland Melt Season.
  4. Based upon what you have learned so far in this chapter, consider the following questions:
    • What months of the year would you predict to be the peak of the melting season in Greenland?
    • What do you think scientists look for in order to detect melting on the ice sheet?
    • How can scientists monitor melting on an ice sheet as large as Greenland?
    • How might the observed increase in melting extent change the downhill flow of the ice sheet?
  5. Continue on to Part 4 to learn the answers to these questions.

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