Veganuary” might be considered an attempt to cut out the middle creatures. Many vegans—even those who dabble only during the first month of the year—restrict their diet to reduce animal suffering. But many also do so for environmental reasons. Eliminating meat, fish, dairy and eggs would reduce emissions. Enjoying a prime steak or vintage cheese, for example, means feeding the animals that produce meat and dairy with plants, rather than consuming those plants directly. Beef farming produces 31 times more CO₂ emissions per calorie than tofu production does and generates only 5% of the calories that go into producing it.

That inefficiency means humans need to grow more plants than they would in a vegan world. For all the spread of veganism and the growing popularity of partial alternatives, such as veganuary, meat-eating is increasing globally. Its geography and composition is changing too. China’s appetite for its favourite meat, pork, appears to have peaked; beef is becoming more popular. India, which eats very little beef, is drinking more milk. Africa, with its fast-growing population, will demand more meat in future. Already, of all habitable land, half is used for agriculture, according to the UN Food and Agricultural Organisation. Of all food production around 80% is dedicated to pasture or crops for animal feed, according to Joseph Poore and Thomas Nemecek, the authors of an extensive study of global food systems.

The research comes with considerable uncertainty. Although it compiles data from 38,700 farms and 570 studies, mapping the environmental impact of food production is difficult and imprecise. Roughly half of the data are from before 2010, for instance. But the study gives a sense of what land use could look like, if diets changed radically.

If everyone were vegan, agriculture would need just a quarter of the land it uses today. Even a diet avoiding only meat from cattle and sheep would cut land use in half. What might that surplus space be used for? Quadrupling food production is not a viable option. Some current pastureland, for example in the Scottish highlands, could not be converted to high-yield cropland. But in most places where agriculture is currently expanding, such as the Brazilian Amazon, a shift from animal to plant production would mean more food per acre. Surplus farmland could be used for other purposes, such as forestry, or restored to rainforest.