Peak oil

[Tarrel]

One of the biggest effects of a prolonged oil shortage, is the impact on the industrial agriculture system. Oil is used to make pesticides, run tractors, move the end-product around, move the farm employees to their place of work, etc. In addition, nitrogen-based fertilisers are made from natural gas, which will become in higher demand for fuel and power-generation when oil becomes less available.

An agriculture system (of the kind we have now) starved of oil will see its yields reduced dramatically. Expect food shortages, higher prices generally, short-term price spikes and civil unrest in less developed nations leading to instability in those nations, together with attempted mass migration.

So, in prepping terms, there are direct effects (food shortages, crises) and indirect effects (exacerbating all the other threats we prepare for (unrest, civil breakdown, conflict, etc).