University of Wisconsin-Madison scientists compiled the maps using satellite images and crop and livestock production data from countries around the world. The team presented their picture of global land use this week at a meeting of the American Geophysical Union in San Francisco.
"The satellite data tells us where cultivation is occurring with good spatial accuracy, while the census data is able to tell us what is being grown there," said Navin Ramankutty, a land-use researcher with Wisconsin-Madison's Center for Sustainability and the Global Environment (SAGE).
The maps suggest that an area roughly the size of South America is used for crop production, while even more land—7.9 to 8.9 billion acres (3.2 to 3.6 billion hectares)—is being used to raise livestock.
And with the world's population growing rapidly, the pressure is on farmers to find new land to cultivate, the study team says.
"How can we continue to produce food from the land while preventing negative environmental consequences, such as deforestation, water pollution, and soil erosion?" Ramankutty said.
The researchers also used past land-use data to create maps showing how agriculture has spread over the centuries. In 1700, for example, just 7 percent of the world's land was used for farming.
Figures from the Food and Agriculture Organization of the United Nations suggest that total farmland increased by 12.4 million acres (5 million hectares) annually between 1992 and 2002.
The SAGE scientists identified specific crops that help account for this growth.
In Brazil, for example, huge areas of rain forest have been replaced by soybeans, which aren't a traditional crop in South America. Production has been fueled by demand for soy from China.
Brazil's Mato Grosso state has seen the biggest expansion of soybean farming. A study by researchers at the University of Maryland found that 72 percent of land cleared for crops in that region between 2001 and 2003 was previously pasture for livestock.
"It is not clear how much of this expansion is replacing forests versus other land cover," Ramankutty said. "However, it is very likely that these pastures were formerly rain forest. So the transition may have been from forest to pasture to soybeans."
SAGE researcher Amato Evan said, "If current trends continue, we should expect to see increased agricultural production at the cost of increased tropical deforestation."
"And the production that is really driving the tropical cropland expansion are crops that are used as feed for cattle."
Countries with the least suitable agricultural lands are likely to be the ones hardest hit by increased food demand.
The team identified 16 such regions by comparing remaining potential arable land with projected population growth over the next 45 years. The regions include several parts of Asia, North Africa, and the Middle East.
"Most of the best lands are already cultivated, for sure," Ramankutty said. "But by some estimates, we can potentially double the amount of cultivated land by using the unrealized potential in Latin America and Africa."
He says, however, that these remaining unexploited areas are not necessarily best suited for agriculture, and that using them for farming would often mean clearing valuable natural ecosystems.
"I don't think we are in danger of running out of food," Ramankutty said. "The issue is about what we are going to do to the environment in the process of producing that food."
One potential solution could be "precision farming." The model uses new technology to improve productivity while reducing the use of water and the application of fertilizer and other potentially harmful chemicals.
According to Evan, heavy fertilizer use is a major problem in farming areas, such as Madison, where the runoff pollutes nearby lakes.
The precision system, currently being developed by NASA geoscientists, would work by using satellite data to help farmers decide how to use their resources with pinpoint accuracy based on the requirements of different areas of each field.
Meanwhile, the next phase of the SAGE project is to build an Internet-based database called the Earth Collaboratory. The resource would draw on the knowledge of scientists, local environmentalists, and the general public to help design localized plans for land use.
Jonathan Foley, director of SAGE, said the project "will truly be a brave new experiment that effectively bridges science, decision-making, and real-world environmental practice—collectively envisioning a new way to live sustainably."