Oil is obtained from the vast reservoirs of fossils (decayed organisms) which have been put under immense pressure and has released all the substances from the organism as it decayed...
This is what we burn for energy in industry and domestic uses!
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.
