By 2050, the world will host nine billion people—and that's if population growth slows in much of the developing world. Today, at least one billion people are chronically malnourished or starving. Simply to maintain that sad state of affairs would require the clearing (read: deforestation) of 900 million additional hectares of land, according to Pedro Sanchez, director of the Tropical Agriculture and Rural Environment Program at The Earth Institute at Columbia University.

The bad news beyond the impacts on people, plants and animals of that kind of deforestation: There isn't that much land available. At most, we might be able to add 100 million hectares to the 4.3 billion already under cultivation worldwide.

"Agriculture is the main driver of most ecological problems on the planet," said economist Jeffrey Sachs, Scientific American columnist and Earth Institute director. "We are literally eating away the other species on the planet."

Sachs made his remarks yesterday at a symposium hosted by the institute on how to improve agriculture to address the mounting challenge of feeding the world while combating climate change and stopping the wholesale loss of biodiversity, among other interrelated issues.

Agriculture—thanks to deforestation, nitrous oxide from fields, methane from cattle and rice paddies—is responsible for one third of global greenhouse gas emissions from human activity, making emissions from transporting food, known as "food miles," a "rounding error," said ecologist Jonathan Foley, director of the Institute on the Environment (IonE) at the University of Minnesota. Pasture has become the dominant ecosystem on the planet, he added, and humans directly employ some 40 percent of the surface of the planet. "Very little of that is urban."

In addition, agriculture accounts for at least 85 percent of human water consumption—a growing concern as aquifers diminish and hydrology changes in the face of climate change. And, by Sanchez's rough calculation, humans now use some 171 million tons of nitrogen as fertilizer every year, much of which ends up polluting lakes, rivers, streams and even the ocean. "Fifty-four percent of that is fertilizer—the Haber-Bosch process; 11 percent is atmospheric deposition—the plus side of pollution; 18 percent is in situ fixation," or nitrogen-fixing cover crops, like legumes, Sanchez said.

And it's not like so-called organic agriculture is helping with that: Nitrate leaching into waterways can come from manure, as in the Netherlands or overuse of fertilizer, as in Iowa. The result is the same: dead zones.

So how can agriculture be intensified to feed a growing population while addressing environmental concerns? Simply put, yields on existing lands must increase.

That's what Norman Borlaug and his colleagues achieved in the 1960s and 1970s with the Green Revolution that staved off famine for millions. Yet, "there can be no permanent progress in the battle against hunger until the agencies that fight for increased food production and those that fight for population control unite in a common effort," Borlaug said in his acceptance speech for the Nobel Peace Prize in 1970. "[Man] is using his powers for increasing the rate and amount of food production. But he is not yet using adequately his potential for decreasing the rate of human reproduction. The result is that the rate of population increase exceeds the rate of increase in food production in some areas."

That demographic contradiction is nowhere more true than in many countries of sub-Saharan Africa, where a population of 800 million must subsist on local yields of one ton per hectare—one third of yields in the rest of the developing world and one ninth those of the U.S., Europe, Australia and other parts of the developed world. Yet, "we already grow enough food to feed the world, we've been doing that for decades," noted ecologist Catherine Badgley of the University of Michigan at Ann Arbor (U.M.), who led a study assessing whether organic agriculture practices alone might adequately meet global nutritional requirements. "We need to address accessibility."

Global markets for food, however, spectacularly failed in 2008 as countries shut down exports in the face of rising grain prices. "International food markets are deeply wounded and faith in them has collapsed. Global institutions failed to keep food moving," Sachs said. Add to that the looming specter of growing crops for biofuel, which reduce available land for food, feed and fiber production, he said: "Biofuel is going to be an unmitigated disaster, that's as true in an African village as it is in Iowa." Norman Borlaug agreed in a warning he had issued in the 1980s to agricultural economist Mark E. Downing of Oak Ridge National Laboratory.

Genetically modified varieties—currently illegal in most of Africa, according to political scientist Robert Paarlberg of Wellesley College—might boost yields. Such biotechnology is "critical for achieving the ecological intensification required to meet human food demand on a global scale," argues agronomist Ken Cassman of the University of Nebraska–Lincoln. At the same time, genetic modification is not a panacea, despite claims for drought tolerance and the like from companies such as Monsanto. "Anything you do to reduce the water that plants transpire will reduce yield," he added.

Perhaps, fortunately, there is still a lot of room for improvement by more conventional means: the targeted application of fertilizer and the like. The Earth Institute's Millennium Village of Sauri in Kenya has tripled yields even in the face of a crippling drought gripping the region, and Malawi doubled yields through fertilizer subsidies in just four years. "If we want to increase production, it's better to have small to medium-size farms," argued U.M. ecologist Ivette Perfecto. "Precision agriculture is already done by [such] farmers."

At the same time, the collapse of agriculture in the "bread basket" of eastern Europe, such as Ukraine, leaves room to "triple food production in that region pretty easily," IonE's Foley said.

And, ultimately, a little change in diet might do a world of good. Global demand for beef is an inefficient way to get protein, possibly unhealthy, and a major driver of deforestation and greenhouse gas emissions. "Beef is costly per kilogram ingested of both mass and protein but also probably unhealthy," Sachs said. "We should not take dietary choices as a given but rather as something that needs to be evaluated," at least if we want a fighting chance to avoid the grim fate Thomas Malthus predicted.

"Sustainability is still an unsolved problem, it is the same problem Malthus identified about 200 years ago," Sachs added. "How we feed the planet, slow population growth, and thereby raise living standards is still an open question."

Editor's Note: We used Twitter to cover the conference live. Follow me @ dbiello or us @ sciam. And thanks to Jon Foley, whose presentation headline, "Another Inconvenient Truth," I have borrowed.