Food security: What crops will feed the world if we run out of farmland?
In The Future, Will We All Be Forced To Eat Yams?
The citizen’s guide to the future.
June 12 2012 6:30 AM

I Think, Therefore I Yam

When farmland is scarce, will we all eat roots and tubers?

A farmer holds up a bunch of cassava roots, dug up from his farm in Oshogbo, in Osun State on August 26, 2010.
A farmer holds up a bunch of cassava roots, dug up from his farm in Osogbo, in Osun State on Aug. 26, 2010.

Photograph by Pius Utomi Ekpei/AFP/Getty Images.

Since Thomas Malthus, alarmists have been pointing out that the world has a finite amount of arable land, whereas its human population keeps growing. Common sense would seem to dictate that eventually there won’t be enough farmland to feed everyone, and catastrophic famine will ensue.

Will Oremus Will Oremus

Will Oremus is Slate's senior technology writer. Email him at or follow him on Twitter.

The incredible pace of technological innovation has staved off that eventuality for hundreds of years, seemingly making fools of Malthus and intellectual successors like Paul R. Ehrlich, who in his 1968 book The Population Bomb predicted mass starvation in the 1970s and 1980s. Instead, the green revolution brought high-yielding crop varieties, fertilizers, and pesticides to hungry countries such as Mexico and India, leading to a doubling of food production between 1950 and 2010 with only a 10 percent increase in the amount of farmland. And in the past decade, global population growth has slowed, a deeply encouraging sign (and one that neither Malthus nor Ehrlich envisioned).

Yet the world’s food future may be shakier than ever. It’s not because of the absolute number of people or even the amount of available farmland, but because of what those people eat and how that farmland is used. In short, there’s enough land to feed the world—but not enough to feed the world Big Macs. Absent another productivity revolution, the 21st century might raise a new question for farmers: If beef provides the smallest amount of calories per acre of land required to raise it, what crops provide the most?


Today about 1 billion people “eat like Westerners,” in the words of University of California-Berkeley resource economist David Zilberman. That means, basically, that they wolf down historically unprecedented quantities of meat and dairy—getting up to half their calories from animals rather than plants. Meat consumption appears to be reaching a plateau in the United States and Europe, but it’s only now taking off in many poorer parts of the world. Zilberman believes that 40 years from now there may be 3 billion or 4 billion people who eat like Westerners. And that’s a problem, because every pound of edible beef takes some 20 pounds of grain to produce. Give a man a 12-ounce porterhouse and you feed him for a day; give him a pound of grain and you can feed a dozen other people for a day, too. (Raising meat also requires five to 10 times as much water as growing grains, using up a resource that may prove even scarcer than land.)  

In a 2009 study, the Food and Agriculture Organization estimated that production would have to rise 70 percent by 2050 to meet the world’s needs. But Zilberman and others believe that the continued rise of the global middle class could result in a number much higher than that. By his lights, even if we were to develop all of the world’s remaining arable land—much of it in Brazil and other Latin American countries—we’d have to wring twice as much productivity from every acre of land under cultivation. While that’s not impossible, it won’t be easy. Water is one obstacle, as is the increasing proportion of grain that’s being used for biofuels rather than food. A third problem, less talked about, is that intensive farming practices appear to be degrading the world’s topsoil, raising the possibility that usable farmland will actually become scarcer in future decades. Climate change could exacerbate the problem, as droughts turn what today is arable land into desert. (Taking off on the concept of “peak oil,” agriculture alarmists have labeled this specter “peak soil.”)

If that happens, people may have little choice but to eat less meat. What would they eat instead? Vegetables are crucial to nutrition, but they aren’t the most potent energy sources. Grains are better, but people already eat plenty of them, and they’re not at the top of the list in terms of the amount of nutrition they provide per unit of land. That honor goes to roots and tubers like garlic, sunchokes, and sweet potatoes.

A world of yam-eaters might seem far-fetched, but some food-security zealots are already preparing for the worst. One of them is John Jeavons, a Willits, Calif.-based advocate of what he calls “biointensive farming.” Back in the early 1970s, when people still feared the original population bomb, Jeavons began to explore how people could grow everything they needed on the smallest possible plot of ground. Building on the methods of organic-farming pioneer Alan Chadwick, Jeavons developed an eight-point gardening system that calls for close spacing of plants, vigorous composting and soil maintenance, and “calorie farming,” which means focusing on crops that produce the most nutrition in the least space.

According to the FAO, sweet potatoes top the list, yielding 70,000 calories per hectare per day, nearly twice as much as wheat—and far more than that if you use one of several fast-maturing varieties. Jeavons also recommends potatoes, leeks, and parsnips for those looking to maximize calories per acre. (Cassava, a crucial subsistence crop in many developing countries, is less efficient because it takes longer to grow.) In the ideal subsistence farm, Jeavons says, roots and tubers would account for 30 percent of all the land cultivated for food.

His techniques, spread through his nonprofit Ecology Action, have caught on with some farmers in countries like Kenya, where farmland is scarce and people are hungry. The question is, could that movement catch on globally?

Jeavons is optimistic that farmers, consumers, and world policy-makers will come around if and when the world’s impending food crisis becomes more apparent. But much of the world’s food is produced under contract with multinational processing and distribution companies whose interest is in maximizing profits, not calories. Factory farms tend to focus on one or two crops, a strategy that might damage the land over time but is highly efficient in the short term. Suggest to an agribusiness executive that he start devoting half his land to the cultivation of compost and another big chunk to salsify and Jerusalem artichoke, and he’ll probably laugh.

On the consumption side, new-wave nutrition gurus such as Michael Pollan have sold a lot of books calling for Americans to eat less fast food and more organic vegetables, and the nation’s meat consumption appears to have peaked around 2007. But organic farming actually yields less food per acre than today’s conventional methods, at least in the short term. And compared with fresh veggies, roots and tubers are probably a tougher sell for your average Whole Foods shopper. Even a spike in food prices might not be enough to make most Westerners change their diets substantially. “Things would have to get really bad to make people eat less meat and more cassava,” Zilberman says. (He thinks genetic engineering is a more realistic solution.)

Instead, it would hurt the people in poor countries who have trouble affording food as it is. In a world where there isn’t enough food to go around, one thing probably won’t change: Most starvation will still be a product of inequality rather than global supply shortage. It’s just that there will be a lot more of it there is now.

Also in the special issue on food: five “food frontiers,” including technologies to make diet food tastier and fight salmonella; smart packaging may help keep your produce from going bad; small-scale farmers decide whether to embrace automated agricultural equipment; and the case for bringing back home ec. This article arises from Future Tense, a joint partnership of Slate, the New America Foundation, and Arizona State University.

  News & Politics
Nov. 29 2015 9:02 PM Claimed When Castro took power, Americans fled Cuba and left a lot of property behind. Now the claims on these contested holdings—land, buildings, cars and more—are exciting speculators and could stand in the way of reconciliation.