1.0 Observing ice loss in Antarctica

Modified on Thu, 19 Jan 2023 at 03:14 PM

We will use Earth Blox to observe the loss of a part of Antarctica's Brunt Ice Shelf in 2021. The workflow uses Sentinel-1 radar images over the period January-April 2021 and creates a time series so that you can see how the crack in the ice expands just before the iceberg completely detaches from the main ice shelf.

Since 2012 large cracks had started to appear in the ice shelf, some going all the way through the ice to the sea below. It finally broke free at the end of February and then drifted out into the ocean in the first week of March 2021.

Instructions to how to observe the ice loss as a time series

Step 1: Load up the area

  • Clear the map using the X button. This ensures there are no other areas. 
  • Attached to this article is a file called Brunt-Ice-Shelf.geojson. Download it.
  • This is a file that describes a rectangular area that covers the Brunt Ice Shelf. 
  • Use the Upload Area button on the right hand side of the map to upload this file into Earth Blox, which will automatically give it an Area Number (which will be Area 1, unless you have another area already on the map).
  • It should look like this:

Step 2: Set up the radar data block

  • Select the radar block, and choose dates from 2021-01-31 to 2021-04-06.
  • Leave the default selection of Sentinel 1 as the source of the imagery. 
  • Choose Both for the orbit.  
  • Choose HH for all of the RGB options. (See below for a description of what these other options mean.)
  • And finally, select a layer name. 

Step 3: The time series block

  • From the ANALYSIS toolbox, bring in the TIME SERIES block into the workspace. 
  • Now drag out the visualisation block in the existing workflow and click it into the time series block. 
  • Then click the time series and visualisation combined block into the radar data block. Remember, when you drag in the time series block make sure it “snaps” in place
  • Select Weekly on the time series block.
  • It should look like this:

Step 4: Run the workflow

  • When you run this workflow it will generate a time series, with each image a weekly average of the Sentinel 1 data.
  • Press the play button to run the images in sequence as an animation, or click on the timeline bar to choose a particular month.
  • Your images should look like this:

Step 5: Interpret the image

  • In a radar image, the water is black and the ice is light grey.
  • When you play the animation, you will see the large piece of ice break off at the end of February. 
  • Because there are sometimes two images in one week, and the ice is moving between each image, you may notice some "ghosting" of the image of the ice sheet. You can experiment by adjusting the start date to see what impact that has.
  • You can use the Map Measurement Tool in the map window to measure around the iceberg and calculate its area and perimeter (remember to make sure the last point matches the first point so that it closes the shape, otherwise it will give the length of the line, but no area). 
  • The advantage of using radar here is that it can see through the clouds, so an image is guaranteed every week. Try changing the Area 1 Box to explore other parts of the coastline of Antarctica, or the coast of Greenland.

Note that icebergs often break off ice sheets – in itself, this is not evidence of global warming. Through repeated monitoring satellite data can help quantify the total changes in ice volume in Antarctica by carefully measuring the changes in ice loss over time. The important thing is to monitor whether the rate of loss of ice is changing over time. 

Final Note: What do the other values refer to in the radar block?

  • The option to choose between dB and DN refers to the way the image is scaled. DN=digital number, which is the linear scaling of the radar data.  dB=deciBels, which is a logarithmic scaling of the data and often makes it easier to see the features in the image. 
  • The VV and V options in the RGB output refer to the polarisation channel.  Radar signals can be transmitted or received as either horizontal (H) or vertical (V) waves.  The VV channel is both transmitting and receiving in V, whereas the VH channel is transmitting V and receiving in H. (the "cross-polarisation"). 
  • The Composite option allows you to combine multiple dates together to create a new image. In the above example, you could choose, say, 2007 to 2011 in one block, and using the Mean option create an image layer that is the average of those five years.  The other options would give you the minimum or maximum value, or the median. 
  •  Since radar can see in the dark, you get images on both the daytime and nighttime sides of the Earth as the satellites orbits. Since the orbit goes up and over the poles, one side is going Northwards (ascending) and the other is traveling southwards (descending).  Sometimes there are good reasons to choose only one or the other, but in this instance we want as many images as possible, so we choose Both


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