Unit 4 Reading: Reflections on Greenland Ice

Scientists use a variety of methods to investigate ice sheet changes. In this unit, you will look at graph and map data to think about how, if at all, the Greenland ice sheet is changing. Greenland is the world's largest island and lies east of Canada at the northern margin of the Atlantic Ocean and the southern edge of the Arctic Ocean. Much of the island (~81%) is covered in the second largest ice sheet in the world. The highest elevations on Greenland are more than 3,700 meters (12,139 feet) above sea level and are underlain by a thick pile of ice more than 3 kilometers (1.9 miles) thick. It is estimated that global sea levels would rise by 7 meters (23 feet) if all of Greenland's ice were to melt. While this is unlikely to occur in the near future, recent investigations have revealed an increased rate of melting for Greenland's glaciers. You are going to analyze some data from the past decade from the Greenland ice sheet, including areas from Greenland's interior and marine-terminating outlet glaciers, to decide for yourself about the potential fate of Greenland's ice.

One of the most important concepts that you will need to consider in thinking about Greenland's ice is albedo, the measure of a surface's reflectivity. Light-colored materials have high reflectivity, that is, they reflect most of the incoming sunlight that strikes any surface composed of reflective materials with a high albedo value. An example of a material with a high albedo (high reflectivity) is fresh snow, whose albedo is approximately 0.84. In other words, fresh snow reflects approximately 84% of the incoming sunlight that strikes it. In contrast, glacial ice that is not covered with fresh snow exhibits an albedo range of 0.2 to 0.6 (20–60%). Fresh snow has a relatively high albedo, but once that snow melts, the underlying ice has lower albedo values due to a variety of factors, including:

• The presence of pollutants or other impurities (e.g., dust, soot from wildfires) in the ice.
• The rounding of the edges of the ice crystals that make up snowflakes as a result of warming diminishes reflection.
• Melting of ice creates pools of meltwater with a lower albedo.

Scientists use satellite data to measure the albedo on the Greenland ice sheet throughout the year and produce a series of graphical plots of these data that illustrate annual trends. Looking at albedo variations over the course of the year allows scientists to compare how the albedo varies at different elevations within the ice sheet. Specifically, scientists notice that the albedo of the ice sheet is high in winter when much of Greenland is covered by fresh snow. During spring, the albedo declines as the snow begins to melt, reaching a minimum in summer and rising again as fall transitions back to winter. Scientists can analyze data from several years to identify trends that may relate to changes in global climate patterns.

Would you expect the entire Greenland ice sheet to exhibit the same albedo values and seasonal/annual/decadal variations? Part of your assignment for this unit will involved assessing annual albedo data for different elevations within the ice sheet to determine if trends are consistent across Greenland.

Here are some suggestions for reading an albedo plot. These plots allow you to determine how albedo has varied seasonally in a particular year, as well as how albedo has varied over the past 12 years.

• First, determine what is being depicted on the x-axis and the y-axis. In the case of the albedo plots, the x-axis represents months of the year from the beginning of March through the end of October. The Y-axis illustrates the percent reflectivity of the surface; in this case, the albedo ranges from 72% to just over 90%.
• The title of the plot is important as well because it tells you from which elevation the albedo data were taken. In the case of the example plot, we are looking at albedo data obtained from places on the Greenland ice sheet between 2000 and 2500 m elevation.
  
• Each color represents albedo values for a different year between 2000 and 2012. Notice that the black line, which represents 2012 data, abruptly ends in early July. If you look to the left of the y-axis, you will see that the data were only available through 13 July 2012.

You may also encounter an albedo anomaly map. You learned about pressure, sea surface temperature, wind, and precipitation anomalies in Unit 2. Similarly, an albedo anomaly (or reflectivity anomaly) is the difference between the long-term average albedo and the albedo at a given time. The figure on the right illustrates the albedo anomaly for June 2012 vs. June 2000–2011, meaning that:

albedo anomaly = 2012 albedo in June - long-term average albedo in June from 2000–2011

• An albedo anomaly of 0 means that in a particular place, the albedo in June 2012 was the same as the average June 2000-2011 albedo and is shown using pale pink on the map.
  
• A positive albedo anomaly, illustrated using pink, red, orange, and yellow, means that for the area in question, the albedo in June 2012 was higher (in other words, more reflective) than the average June 2000–2011 albedo.
• A negative albedo anomaly, illustrated using blue and purple, indicates that the albedo in the area in question was lower (in other words, less reflective) in June 2012 than the average June 2000–2011 albedo.
  
  

Examining area change plots is also useful in thinking about changes in the Greenland ice sheet. The plot on the right shows the area change for a set of Greenland's marine-terminating outlet glaciers from 2000 to 2009. Time is on the x-axis, and area (in km²) is on the y-axis. To calculate an average rate of change of the ice area, you will first need to select the window of time for which you'd like to make the calculation and subtract one year from the other. For example, if you wanted to calculate the average rate of change from 2000 to 2009, you would subtract 2000 from 2009 and wind up with 9 years. Next, you would need to determine the area (from the y-axis) in 2000 and subtract that value from the 2009 area. Now you have the change in area over 9 years, which allows you to calculate the average rate of change.