Ice shelf retreat on the Antarctic Peninsula

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Ice shelf retreat on the
Antarctic Peninsula
An investigation of the collapse of ice
shelves in relation to climatic variables
Introduction
• The example uses ArcMap to investigate the retreat of ice
shelves on the Antarctic Peninsula in relation to climate
variables. Ice shelves are the floating extensions of a
grounded ice sheet where the glacier flow transports ice from
the interior of Antarctica to the sea.
• They may be up to 200m high above the sea and sometimes
are a kilometer thick, with a considerable amount of ice below
sea level.
• In the last few years ice shelves on the Antarctic Peninsula
have broken up linked to climate change (pdf).
• In this exercise we will investigate this phenomenon.
Step 1: Adding polygon data of the land and ice
shelves
• Open a new project in ArcMap
• Using the Add layers icon along the top row, navigate to your
data and add the “Land” file. This will add data to your
ArcMap project.
• This layer shows all the land, both rock and ice, that is above
sea-level and on solid ground.
• Next use the same button to add one of the ice-shelf layers
(see next slide)
Default view of the land and ice shelf layers.
Land is shown as blue.
Step 2: Investigating the attributes of the data
• First let us look at the attributes of the ice shelves.
• Attributes are information associated with the geographic data
in a GIS.
• Our ice shelves have two attributes, their name and the year
the ice shelf was recorded in its location.
• To access the attributes right click on the layer you want to
look at in the table of contents (the pane on the right hand
side).
• This brings up a drop-down box with several alternatives;
choose the “open attribute table” option (third from the top).
• This brings up a tabular view of the data like this:
Each line on this table represents
a separate polygon on the map,
and associated with each polygon
is the name of the particular ice
shelf and the year it was like that.
the attribute table of the
“ice shelf” layer
Selecting from the attribute table. Left click on one of the
lines – it will highlight in blue and the corresponding polygon
on the map will show in the same colour.
All of the data for the Larsen B ice shelf.
Some of the data overlaps. Hold down the shift button and
highlight all of the line of a particular ice-shelf to look at this.
Step 3: Visualizing the ice shelf data to show how
the shelves have retreated with time
• Go into the symbology menu by right clicking the layer you
wish to symbolize in the table of contents; in this case right
click on the “ice shelf” layer.
• From the drop-down menu chose “properties” (at the
bottom) and make sure the symbology tab is clicked.
• An example is shown on the next slide
The symbology menu
In the left hand pane of this box choose “Categories”.
Make sure the Category field is set to “Year”, then click
“Add all values” at the bottom of the box.
Organise the colour scheme to make the map more
understandable by clicking on the “color ramp” and choose a
suitable colour scheme.
The data symbolized by year
• This symbology shows
dark blue where the ice
shelves were present
recently and other
colours where they have
retreated in previous
years.
• The warmer the colour
the longer ago the
retreat occurred.
Step 4: Excluding some of the data from the
visualization to see which ice shelves are intact
• Display the most up to date data; that from 2010 by using
the visualization tools.
• In the symbology tools choose the left hand pane, change
from “Quantities” to “Categories”.
• Click on “ Add values...” .
• Now scroll down until you find 2010, select it and click OK.
• One last thing before you click the final OK – un-tick the box
that says “all other values...“, then click OK to get back to the
main screen.
Step 4: Excluding some of the data from the
visualization to see which ice shelves are intact
• See if you can now re-set the symbology. Go back into the
symbology menu and tick the box that says “all other
values“ on rather than off. Now you can see where the ice
shelves have disappeared.
Step 5: Investigating the ice shelf retreat pattern
• The retreat pattern can be investigated in conjunction with
an environmental variable.
• Isotherms, lines of equal mean temperature are used to see
if the pattern of ice shelf retreat is linked to temperature.
The label dialogue box
• Using the Add layers
icon along the top row,
navigate to your data
and add the “Isotherm”
file.
• Next we need to label
the isotherm data so that
we can read it.
• Right click on the layer
and click on the
properties option.
• Change the tab from
symbology to the “label”
tab. This should open a
dialogue box like the one
to the left.
Choose the attribute to
label:
The label dialogue box
In the next slide can you see any
pattern between the temperature
and where ice shelves have been
lost? Why do you think that there
are no ice shelves on the northwest side of the Peninsula?
•In the box called Text
string Label field choose
the temp field.
•The rest of the defaults
should be fine, but make
sure you click the tick box
in the top left of the menu to
turn the labels on, then click
OK.
•This will label your lines
with the mean annual
temperature.
Isotherms in conjunction with present ice shelves (in green) and lost ice shelves (in yellow).
The link between temperature and ice shelf loss.
• Scientists believe that one of the controls on where ice
shelves are located is mean annual temperature.
• They suggest that ice shelves in areas where the mean
temperature is higher than -9°C are vulnerable to collapse.
• On the Antarctic Peninsula, no ice shelves exist in the
warmest areas to the north-west (top left of the map).
• Over the last 50 years the region has warmed significantly
by around 3 – 4°C. this means that areas which were colder
than -9°C are now -5vC or -6°C; too warm to support ice
shelves.
The link between temperature and ice shelf loss.
• The first shelves to collapse were in the warmer areas in the
north and west of the region, first the Wordie and Prince
Gustav Ice Shelf, then the Larsen A, and more recently the
Larsen B and part of the Wilkins ice shelves.
• It is expected that as the area warms further and the -9°C
isotherm retreats southward more ice shelves will collapse.
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