Page 387 - Laboratory Manual in Physical Geology

A CTIVIT Y 13.6 The Changing Extent of Sea Ice

Name: ______________________________________ Course/Section: ______________________ Date: ___________

A. Refer to the satellite images of Arctic sea ice in FIGURE 13.16 on page 345 . These images were both taken in the month of

September, when sea ice is at its minimum thickness and extent. “Extent” refers to how far the ice extends in all directions: its
total area without regard for tiny ice free areas within the overall body of sea ice. It is easy to see that there was less sea ice in
September of 2012 than in September of 1979, but how much less? To find out, you need to measure the extent of the sea ice
in 1979 and 2012 by following these directions.
Step 1. Start by using a piece of thin string or thread about 30 cm (12 in.) long to measure the circumference of the
sea ice. Carefully lay the string along the edge of the body of sea ice so that the string totally surrounds it as perfectly
as possible. Then lay that length (segment) of string along the bar scale to determine the circumference of the sea ice in
kilometers.
Step 2. Assume that the circumference of sea ice that you just measured is like the circumference of a circle. Circumfer-
ence of a circle is equal to 2 times ∏ (3.14) times radius, so radius (r) equals circumference divided by 2 times ∏. So,
determine the radius of the ice sheet by dividing the circumference (Step 1) by 6.28.
Step 3. Area of a circle is equal to ∏ (3.14) times the square of its radius. So determine the area of the ice sheet by
multiplying the radius from step 2 by itself (to get radius squared), and then multiply that number by 3.14. Your
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answer will be in square kilometers (km ).
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1. Using the three steps above, what was the extent of Arctic sea ice in September of 1979, in millions of km ? Show your
work below.

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2. Using the three steps above, what was the extent of Arctic sea ice in September of 2012, in millions of km ? Show your

work below.

3. Based on your limited set consisting of just two years of data, what has been the rate of Arctic sea ice decline from 1979 to
2012 (in km per year)? Show your work.

B. Scientists at the National Snow and Ice Data Center (NSIDC) have measured the annual September extent of Arctic and

Antarctic sea ice in more exact ways. Arctic sea ice fills the Arctic Ocean, which is confined by land masses like Asia (Russia),
North America, and Greenland ( FIGURE 13.1 ). A table of the NSIDC data is provided on the next page.

1. Graph all of the data for extent of Arctic sea ice from 1979 to 2013, then use a ruler to draw a “best fit” line through the
points so that the number of, and distance to, points above and below the line is similar. Label the line as “Arctic.”
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2. What was the average annual extent of Arctic sea ice from 2000 to 2007, in millions of km ? Show your work.

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3. What was the average annual extent of Arctic sea ice from 2008 to 2013, in millions of km ? Show your work.

4. Based on your graph and calculations above, would you say that the annual amount of Arctic sea ice is decreasing,
increasing, or staying about the same? Explain.
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