How did the sahara desert change over time

From lakes and grasslands with hippos and giraffes to a vast desert, North Africa's sudden geographical transformation 5,000 years ago was one of the planet's most dramatic climate shifts.

The transformation took place nearly simultaneously across the continent's northern half, a new study finds. The results will appear in an upcoming issue of the journal Earth and Planetary Science Letters.

http://www.livescience.com/28493-when-sahara-desert-formed.html

an expansion is one of two basic business cycle phases; the other is contraction. the transition from expansion to contraction is a peak, and the changeover from contraction to expansion is a trough.