Expansion and Contraction of the Sahara Desert 

These images, constructed using multispectral data from
Advanced Very High Resolution Radiometers (AVHRR) on NOAA
satellites, show the changes in the extent and area of the
Sahara Desert from 1984 to 1990. The desert is shown in
shades of brown, and full vegetative color is shown in
purple. Colors in between brown and purple indicate
intermediate amounts of vegetation. 

Between 1980 and 1984 the desert grew
steadily larger. Data gathered during this four-year period
showed the southern boundary of the Sahara creeping
southward as much as 240 kilometers. 

In fact, satellite observations between 1980 and 1990 showed
a fluctuation between expansion and contraction of the
Sahara Desert. The overall 10-year trend, however, is an
expansion of the desert.

(Summary and images were extracted from the EOS Edu-Pack Home Page, http://spso.gsfc.nasa.gov/eos_edu.pack/p17.html)

Questions:
a. If you were to look only at these two images, would you think
that desertification was a problem?  (That is, would you think
the Sahara Desert was growing larger?)

b. What do you think are some possible causes of year to year
fluctuations in the location of the desert's boundary?  [Hint:
think about climate variability (wet and dry years), think about
farming and migration practices of humans - would you keep trying
to farm an area that was experiencing drought?]

c. Why do you think policy-makers in the Sahel regions are
interested in this kind of information?