How does desertification happen?

Overgrazing of drylands is a big one. An immediate reason is the removal of vegetation. This is caused by a few things like drought, climate change, tillage, deforestation, and overgrazing. 

Desertation is the accumulated results of ill-adapted land use and the effects  Dersertification is the accumulated results of ill-adapted land use of a harsh climate. For human and nature represent the most immediate causes:
over cultivation
overgrazing
exhaust the soil
deforestation
poor drained irrigation
land degradtion.

Did you know it's illegal to own only one guinea pig in switzerland?   : )

Answer carbon dioxide ice;