â€œHumanity now stands at Peak Farmland, and the 21st century will see release of vast areas of land, hundreds of millions of hectares, more than twice the area of France for nature,â€ declared Jesse Ausubel, the director of the Program for the Human Environment at Rockefeller University, in a December lecture. Ausubel was outlining the findings in a new study he and his collaborators had done in the Population and Development Review. Unlike other alleged resource â€œpeaks,â€ peak farmland reflects not the exhaustion of resources but the fruits of human intelligence and growing affluence.

The trend toward reducing farmingâ€™s impact on nature took off with the Green Revolution of the 1960s. That leap in agricultural productivity was sparked by plant breeder Norman Borlaug and his colleagues, who created new high-yield varieties of wheat and rice, an effort so successful that Borlaug received the Nobel Peace Prize in 1970.

While Borlaug was working to avert famines, others were declaring them inevitable. â€œThe battle to feed all of humanity is over. In the 1970s the world will undergo faminesâ€"hundreds of millions of people are going to starve to death in spite of any crash programs embarked upon now,â€ the Stanford biologist Paul Ehrlich declared in his 1968 dystopian screed, The Population Bomb. The epicenter of Ehrlichâ€™s alarm was impoverished India.

In 1960 Indiaâ€™s population was 450 million, and the average Indian subsisted on a near-starvation diet of just more than 2,000 calories per day. Indian farmers wrested those meager calories from 161 million hectares (400 million acres) of farmland, an area a bit more than twice the size of Texas. By 2010, Indian population rose by more than two and half times, national income rose 15-fold, and the average Indian ate a sixth more calories. The amount of land devoted to crops rose about 5 percent to 170 million hectares. Had wheat productivity remained the same that it was in 1960, Ausubel and his colleagues calculate that Indian farmers would have had to plow up an additional 65 million hectares of land. Instead, as people left the land for cities, Indian forests expanded by 15 million hectaresâ€"bigger than the area of Iowa.

The trajectory of rising agricultural productivity was similar in post-Mao China. Chinaâ€™s population doubled, and its GDP rose 45-fold. While the amount of land harvested for corn in China also doubled, each acre produced 4.5 times more than it did in 1960. Ausubel and his colleagues calculate that rising Chinese corn productivity spared 120 million hectares (an area more than twice the size of Texas) that would otherwise have been plowed up. The United Nationsâ€™ Food and Agriculture Organization reports that Chinese forests expanded 30 percent between 1990 and 2010.

In the United States, corn production grew 17-fold between 1860 and 2010, yet more land was planted in corn in 1925 than in 2010. (The area planted in corn has started increasing again, thanks to the federal governmentâ€™s biofuels mandates and subsidies.) Today U.S. forests cover about 72 percent of the area that was forested in 1630. Forest area stabilized in the early 20th century, and the extent of U.S. forests began increasing in the second half of the 20th century.

If global crop yields had remained stuck at 1960 levels, Ausubel noted in his lecture, farmers around the world â€œwould have needed about 3 billion more hectares, about the sum of the USA, Canada, and China or almost twice South America.â€ Plowing down this amount of the worldâ€™s remaining forests and grasslands would have produced what Ausubel calls â€œSkinhead Earth.â€

What about the future? The researchers offer a 50-year forecast via their ImPACT equation, which calculates how much land will be used for crops (Im) by multiplying population trends (P), affluence (A) as GDP per capita, consumption (C) as calories per GDP, and technology (T). The United Nations expects population growth to continue to slow, global affluence to increase at around 1.5 percent per year, people to spend relatively less on food as their incomes rise (Engelâ€™s law), and the amount of crop per each hectare to rise by 2 percent annually. (In aggregate, farmers today can produce nearly three times the food they did in 1960 on the same amount of land.) The authors also take into account the growing global desire for meat, which means growing more grains to feed animals, and the diversion of crops into other non-food products such as biofuels.

Currently, American corn farmers average about 180 bushels per acre, and the world average is around 82 bushels per acre. The authors assume a modest 1.7 percent per year increase in corn yields between 2010 and 2060, which implies that â€œthe average global yield in 2060 would resemble the average U.S. yield in 2010.â€

One concern is that farmers may be approaching the biological limits of photosynthesis, which would constrain crop yields. But the authors note that the winners of the annual National Corn Yield Contest currently produce non-irrigated yields of around 300 bushels per acre, nearly double average U.S. yields. Ausubel suggests that the difference between the global average of 82 bushels and contest-winning 300 bushels per acre yields means that â€œmuch headroom remains for farmers to lift yields.â€

Cranking various population, economic growth, and yield trends through the ImPACT equation, the authors conservatively conclude that in 2060 â€œsome 146 million hectares could be restored to Nature, an area equal to one and half times the size of Egypt, two and half times France, or ten times Iowa.â€ Under a slightly more optimistic scenarioâ€"one where population growth slows a bit more, people choose to eat somewhat less meat, agricultural productivity is modestly higher, and thereâ€™s less demand for biofuelsâ€"would spare an additional 256 million hectares from the plow. That would mean nearly 400 million hectares restored to nature but 2060, an area nearly double the size of the United States east of the Mississippi River.

As Ausubel notes, sparing land usually also means sparing water, which would lessen pressure on the worldâ€™s fresh water supplies. Crops need nitrogen to grow, but excess nitrogen fertilizer running off fields pollutes streams and is responsible for algal blooms that produce low-oxygen dead zones in many coastal regions. Researchers are hard at work on producing biotech varieties that need far less nitrogen.

Efforts to dramatically boost the photosynthetic efficiency of staple grain crops are moving forward, prompting optimistic conjectures that â€œfarmers could grow wheat and rice in hotter, dryer environments with less fertilizer, while possibly increasing yields by half.â€ Currently about 40 percent of the worldâ€™s grain is fed to livestock to produce meat. In the ultimate move toward what Ausubel calls â€œlandless agriculture,â€ the biotech company Modern Meadows hopes to use tissue engineering and 3D printers to make meat. Obviously, such breakthroughs would free up even more land.

â€œNow we are confident,â€ the authors conclude, â€œthat we stand on the peak of cropland use, gazing at a wide expanse of land that will be spared for nature.â€ Now thatâ€™s a real Green Revolution!