[EnergyBulletin] The future will be just like today, only tougher. Oil depletion is basically just a matter of overpopulation — too many people and not enough resources.
The most serious consequence will be a lack of food. The problem of oil therefore leads, in an apparently mundane fashion, to the problem of farming.
What to Grow
The most practical diet would be largely vegetarian, for several reasons. In the first place, vegetable production requires far less land than animal production. Even the pasture land for a cow is about one hectare, and more land is needed to produce hay, grain, and other foods for that animal.
How Much Land?
The amount of land needed for farming with manual labor would depend on several factors: the type of soil, the climate, the kinds of crops to be grown. The highest-yielding varieties are not necessarily the most disease-resistant, or the most suitable for the climate or the soil, or the easiest to store. The weather also makes a big difference: too little rain can damage a crop, and too much rain can do the same. Unusually cold weather can damage some crops, and unusually hot weather can damage others. Without irrigation — relying solely on rain — the yield is less than if the crops were watered.