The Big Ratchet: How Humanity Thrives in the Face of Natural Crisis by Ruth DeFries

have survived if they had to expend more energy than they consumed. For most of human history, people subsisted by expending less energy in collecting and hunting food than they obtained from eating it.
    With farming societies, besides clearing the land, there are other tasks to be done: hoeing, planting, weeding, warding off pests, harvesting, storing, cooking, and moving the food to where people eat it. All of these tasks use energy, if not from human labor, then from another source. When people settle in towns and do specialized, non-farming tasks, they need calories for energy to do the work. A farming family needs to produce not just enough to replenish the amount of calories that they burn themselves; they need to produce extra to feed the people in the towns. One solution to the conundrum of how to get the extra energy was to supplement human labor with the brawn of domestic animals. Animals can do the work from the energy they get eating grassthat people can’t digest. Later on, fossil fuels produced from the energy of the ancient sun took on the same role in the Big Ratchet’s story.
    People began to harness animals to pull plows in the early river civilizations around 6,000or 7,000 years ago. An ox pulling an ard, a rudimentary wooden plow, marked a pivotal transition. The pivot added animal power to humanity’s toolkit. A strong ox could equal the labor of six to eight adult men and plow a field faster than aperson with a hoe. Oxen were supplementing human labor, at a cost of breeding and feeding. So long as oxen could get enough food from grazing, the cost was well worth it. The animals sped up plowing, hauled goods, and later turned waterwheels to mill grains.
    Of course, whether people rely on oxen, mules, coal, or petroleum to supplement human labor, any form of energy traces back to the sun,the starting point for food and all civilization. The sun’s energy powers the entire web of plants, plant-eating animals, animal-eating animals, and on up the chain. It starts with photosynthesis, the amazing process billions of years old that converts the sun’s energy to sugars by breaking apart carbon dioxide from the air and water. Plants use the sugars to grow leaves, stems, and roots. Animals, in turn, get their energy from eating the plants. But much energy gets lost in the process. A common rule of thumb states that about nine out of ten calories that these animals eat are needed for energy to grow, stay warm, move, reproduce, and carry out other such maintenance chores. That leaves only one in ten calories stored in their flesh to pass up the chain. When carnivores eat the plant-eating animals, the process repeats, with another nine out of every ten calories getting lost. Ultimately, this means that only one in one hundred calories has transferred from the plants to the carnivores. That’s why there are typically only four or five layers in the pyramid of plants, herbivores that eat plants, carnivores that eat herbivores, and carnivores that eat carnivores. So much energy gets dissipated at each step that there’s not enough leftafter a few transfers. What’s more, unlike nitrogen, phosphorus, and water, usable energy does not recycle. It’s a one-way proposition. The energy lost when a carnivore eats an herbivore is gone forever.
    The problem for humanity is not a shortage of energy from sunlight. The problem is getting the sun’s energy into a form that people can eat. There are plenty of plants, but most are too woody or leafy to digest. It’s a lot of work to channel the sun’s energy to the plants and animals that we can eat. It takes a lot of energy. If animals can pull, haul, or otherwise do the hard work instead of farmers, there are even more calories produced for each calorie that the farmer burns. Animals can subsidize the farmer’s labor, enhancing the amount of calories each farmer could produce on the same amount of land with his labor alone. So long as the animals can get enough calories