“There were firm ceilings on the amount of energy that the first two sources, foraging and farming, could provide. For foraging, the ceiling was a little under 10,000 kilocalories per person per day, and for farming, a little over 30,000 (these energy budgets had to cover not just food, by the way, but also fuel, shelter, clothing, and everything else that people do). Contemporary Americans consume on average about 230,000 kilocalories per day, most of it turned into electricity to power our machines. We do not know — yet — if there is also a hard ceiling over what is possible in fossil fuel societies, but the implication of history seems to be that there is.”