I’ve already mentioned that we humans take 40 percent of the globe’s primary productivity every year. You might have assumed we and our livestock eat our way through that volume, but this is not the case. Part of that total—almost a third of it—is the potential plant mass lost when forests are cleared for farming or when tropical rain forests are cut for grazing or when plows destroy the deep mat of prairie roots that held the whole business together, triggering erosion. The Dust Bowl was no accident of nature. A functioning grassland prairie produces more biomass each year than does even the most technologically advanced wheat field. The problem is, it’s mostly a form of grass and grass roots that humans can’t eat.
So we replace the prairie with our own preferred grass, wheat. Never mind that we feed most of our grain to livestock, and that livestock is perfectly content to eat native grass. And never mind that there likely were more bison produced naturally on the Great Plains before farming than all of beef farming raises in the same area today. Our ancestors found it preferable to pluck the energy from the ground and when it ran out, move on.
Today we do the same, only now when the vault is empty we fill it again with new energy in the form of oil-rich fertilizers. Oil is annual primary productivity stored as hydrocarbons, a trust fund of sorts, built up over many thousands of years. On average, it takes 5.5 gallons of fossil energy to restore a year’s worth of lost fertility to an acre of eroded land—in 1997 we burned through more than 400 years’ worth of ancient fossilized productivity, most of it from someplace else.
Society is going to change drastically, perhaps fundamentally, when the oil finally runs out. Even now we expend 6 or 7 calories worth of transport energy (in the form of gasoline) to make and sell each single calorie of food. This is unsustainable in the long run if oil is a finite, non-renewable resource - which it is.