The amount of land needed to grow crops world-wide is at a peak and an area more than twice the size of France can return to nature by 2060 due to rising yields and slower population growth, a group of experts said on December 17.
The report, conflicting with UN studies that say more cropland will be needed in coming decades to avert hunger and price spikes as the world population rises beyond 7 billion, said humanity had reached what it called "Peak Farmland".
More crops for use as biofuels and a shift towards more meat consumption in emerging economies such as China or India - demanding more cropland to feed livestock - would not offset a fall from the peak driven by improved yields, it calculated.
If correct, the land freed up from crop farming would be some 10 percent of what is currently in use - equivalent to 2.5 times the total area of France, Europe's biggest country bar Russia, or more than all the arable land now farmed in China.
"We believe that humanity has reached Peak Farmland, and that a large net global restoration of land to nature is ready to begin," said Jesse Ausubel, director of the Program for the Human Environment at the Rockefeller University in New York.
"Happily, the cause is not exhaustion of arable land, as many had feared, but rather moderation of population and tastes and ingenuity of farmers," he wrote in a speech about the study he led in the journal Population and Development Review.
The report, supplied to Reuters by Ausubel, projected that almost 150 million hectares (370 million acres) could be restored to natural conditions such as forest by 2060. That is also equivalent to 1.5 times the area of Egypt or 10 times Iowa.
It said the global arable land and permanent crop areas rose from 1.37 billion hectares (3.38 billion acres) in 1961 to 1.53 billion (3.78 billion acres) in 2009. It projected a fall to 1.38 billion hectares (3.41 billion acres) in 2060.
A June 2012 report by the UN's Food and Agricultural Organisation (FAO), however, said that a extra net 70 million hectares of land world-wide would have to be cultivated in 2050 compared to now: "Land and water resources are now much more stressed than in the past and are becoming scarcer," it said, referring to factors such as soil degradation and salinisation.
Ausubel's study admits to making many assumptions - rising crop yields, slowing population growth, a relatively slow rise in the use of crops to produce biofuels, moderate rises in meat consumption - that could all skew the outcome if wrong. It also does not factor in major disruptions from climate change that UN studies say could disrupt farm output with rising temperatures, less predictable rains, more floods, droughts, desertification and heatwaves. Still, it points out that both China and India have already spared vast tracts of land in recent decades.