There’s a sexy new idea in the world of sustainability: vertical farms. In the past few years, articles have started popping up in mainstream and green publications, touting the idea of turning urban skyscrapers into produce-producing behemoths.

But what does that mean, really? Never fear. LIME.com’s got your back. Here’s your cheat sheet to this new idea.

What is a vertical farm?

At its most basic level, a vertical farm is city-based building devoted to food production. Architects and engineers have envisioned a number of different ways of making this happen. One suggestion is to create a NASA-like hydroponic system where vegetables are grown in water. Another idea is based on greenhouses. Some include places to raise chickens, pigs, and cattle. And others include aquaponic ponds for harvesting fish.

How did the idea come about?

A Columbia University professor of public health came up with the idea in 1999, during a graduate class he was teaching on medical ecology, specifically looking at the interaction of environment and human health. The professor, Dickson Despommier, has since become a vigorous advocate of the idea of vertical farms, arguing that they are a powerful solution to the coming shortage of farm land.

Incidentally, Despommier’s name, in French, means “of the apple tree" — a fitting moniker for a man who might one day be called a modern-day Johnny Appleseed.

What are their benefits?

The United Nations has said the world will need 60 percent more farmland in the next 30 years in order to accommodate projected population increases. Advocates of vertical farms argue that producing food inside cities will be one way to meet that demand. Despommier’s model predicts that a 30-story farm on a single city block could feed 50,000 people — or the same amount of food currently produced on 588 acres of flat land.

Urban-based farms solve another problem close to environmentalists’ hearts: The massive amount of energy required to transport food from farms to cities.

And depending on how the farms are actually built, they could become the forerunners in green building. Many designs envision self-contained systems, where graywater is recycled for crop use, waste turns into fuel for power, and wind turbines and geothermal systems provide additional energy.

Why do some people think they won’t work?

Some critics have argued that the lighting and heating required to make vertical farms work — especially to provide light to crops that don't receive natural sunlight — will make the projects cost-prohibitive.

Others have questioned whether vertical farms will be able to afford the cost of urban real estate and, if not, whether it makes sense to sacrifice city blocks to vertical farms, when other uses for the land — for office buildings or residences — could generate more revenue.

When will we start seeing them?

The idea is still in the exploratory stage, so it’s not clear when ground will be broken on the first vertical farm. But there is interest in making these a reality. A zoo in Devon, England, is running a pilot project with a small-scale farm. And a New York City official has expressed interest in bringing the idea to the Big Apple.

In the meantime, most of the ideas out there are the products of architects’ imaginations and design competitions. Some of the most dramatic include the Toronto SkyFarm (above), the Harvest Green Tower (below), and the New York Dragonfly (below). Many other examples are available on the The Vertical Farm Project website.

Harvest Green Tower (Image courtesy of Romses Architects)

New York Dragonfly (Image courtesy of Vincent Callebaut Architectures)

Image above courtesy of The Vertical Farm Project.