The Big Ratchet is a decent, if breezy environmental history of agriculture. DeFries discusses human history in terms of energy, and especially available calories, in terms of a pattern of 'ratchet, hatchet, and pivot'. Some innovation increases the available food supply, a limit is reached, a new innovation moves around that limitation. The titular big ratchet, of course, is the combination of intensified agricultural techniques developed in the 20th century that increased the global population from 1.6 billion to over 7 billion.

Most animals live at the margins of survival. Agriculture allows the accumulation of surplus, and the support of a non-farming political, cultural, technological, and military elite. Agriculture advanced slowly in the centuries since the Mesopotamian breadbasket, limited by nutrients, pests, and imprecise breeding. Malthus, writing at the end of the 18th century, was correct from his local perspective. The fertility of the people always seemed to outstrip the fertility of the land. Hunger is mankind's constant companion.

The 20th century Green Revolution banished hunger, at least for a time, through artifical nitrogen fertilizer from the Haber-Bosch process, mineral phosphorus, mechanized farm equipment, new breeds, and chemical best control. It also had numerous side effects: maritime dead zones from fertilizer run-off, ecocide from pesticide use, disruptions to ancient ways of living.

In DeFries's telling, we're reaching another hatchet, as the limits of Green Revolution techniques become clearer. What comes next is unclear. This book is more focused and optimistic than Harari's Sapiens. Civilization is about agriculture, and even as problems arise, they are dealt with, if not to complete satisfaction. Yet I wish DeFries had written less of a general history, and perhaps brought in some of her specialized knowledge on land use change, deforestation, and the coexistence of heavy agriculture and natural cycles.