Humans have a knack for reproduction. By 2050, the world population is expected to reach 9 billion people, nearly doubling global demands on food and livestock feed. But there's one major problem: Farms can't keep up.

Until now, humanity's rapid population growth has been countered by the "green revolution"--advances in pesticides, fertilizers, and the genetic modification of crops. But no matter how much we maximize crop yield per acre, the limiting factor for food production is land. And farms use a lot of it.

But if we can't build out, why not build up? Vertical farms, like the one pictured here, are a proposed innovation that might allow us to do just that. While they're still in the planning stages, these "skyfarms" could revolutionize our agricultural system.

Vertical farms are the brainchild of Columbia University professor Dickson Despommier, who stumbled onto the idea almost by accident in 1999.

As a class project, Despommier challenged his medical ecology students to feed 50,000 people using thirteen acres of rooftop gardens. The students were unsuccessful, but when Despommier suggested they instead try growing plants vertically on every floor, the project changed course.

The idea has since drawn the interest of investors and architects from across the globe. The "Living Tower" design seen here is complete with solar panels, a rainwater purification system, and two large wind turbines on top for energy production.

Vertical farms use hydroponics and aeroponics, soil-free growing techniques once researched by NASA to grow plants in space. One major problem with conventional "horizontal" farms is loss of water to runoff. But plant cultivation in either a water-based nutrient solution, shown here, or a nutrient-rich mist enables water to be almost completely conserved.

Some hypothetical vertical farms have even been designed to recycle processed sewage (pdf), turning waste into water that meets drinking water standards.

Natural phenomena like droughts, floods, and insects are the bane of every farmer. Some even estimate that climate change could cause the U.S. production of certain crops to fall by a whopping 80 percent in the next century. By moving production indoors, vertical farms eliminate weather-related crop failures and protect plants from harmful pests.

The "Pyramid Farm" was designed to take advantage of this concept, and to function as a self-sufficient ecosystem. Using heating and pressurization systems that separate sewage into water and carbon, it would also fuel its own machinery and lights using plasma arc gasification.

A fringe benefit of indoor farming? The death of the farmer's tan.

What's the use of pesticides if there aren't any pests? An indoor environment could allow for virtually chemical-free growing, producing organic crops in a totally controlled setting.

And while a significant investment of time and research (as well as millions of dollars) would be needed to put these ideas into practice, experts say that one acre of multi-layered, indoor farming could theoretically produce the equivalent of four to thirty acres from conventional farms, depending on the crop.

This "Living Skyscraper" was designed with residential apartments at its core and a fringe of edible plant life spiraling across its outer surface.

Also called "farmscrapers," vertical farms could make cities self-sustainable. A 30-story vertical farm the size of one Manhattan city block could theoretically provide food year-round to 50,000 people.

By mid-century, 80 percent of the world's population is expected to reside in urban centers. Properly situated, vertical farms could eliminate the need for long-distance crop transport and refrigeration, reducing fossil fuel use and greenhouse gas emissions. They'd even scrub CO2 from city skies, giving us a much-needed breath of fresh air.

Not everyone agrees that vertical farms will be economically viable. The biggest concern is energy. As with all indoor grow operations, artificial lighting like fluorescents or the LED's shown here must be supplied for any plant life that isn't exposed to sunlight.

Proponents insist that the reduction of farm equipment needed for harvest and transport would make up the difference by cutting back on fossil fuels. Some even argue that vertical farms could save energy by recycling wastewater and composting non-edible plant material to generate methane energy.

But most scientists agree that further studies are needed before we can know for sure whether these farms are feasible.

At least on a small scale, vertical farms are no longer just a theory.

On September 20, 2009, England's Paignton Zoo launched a program called Verticrop(TM) that implemented vertical growing techniques to raise organic plants for animal feed. Using hydroponics, indoor lighting and rotating planters, the zoo has successfully reduced water and nutrient consumption by 95 percent relative to conventional systems.

Large-scale vertical farms, like the "Type O2" design seen here, are still only theoretical, but certain cities have expressed a serious interest in making them a reality. Incheon, South Korea; Abu Dhabi, United Arab Emirates; and Dongtan, China seem the most likely candidates, but it will be at least another five to ten years before we see them get off the ground.

In the meantime, dazzling designs continue to emerge, like the scifi-esque structure proposed for Dubai that would use seawater as a coolant, which made DISCOVER's list of zany ideas for the green cities of the future.

For more on vertical farming, visit Dickson Despommier's website or wait for his new book on the subject: The Vertical Farm: The World Grows Up, due out in October of 2010.