Cities have long been considered the antithesis of nature. They're filled with paved streets and sidewalks that cover over the soil and obliterate vegetation. And tall buildings block sunlight and fresh air and absorb heat. Even city parks can't alleviate the artificial ambiance in dense urban neighborhoods.

But now, there's a growing worldwide movement to remedy that situation, by making flora a part of city rooftops and even the walls of urban buildings. And a few visionary architects are even pushing the edgy concept of vertical forests — literally, skyscrapers with growing space for trees built into their facades.

"We call it living architecture," explained Steven Peck, president and founder of Green Roofs for Healthy Cities, a Toronto-based organization that promotes the incorporation of living things into architecture.

The most obvious manifestation of the trend is the rise in the number of green roofs — that is, building tops that have been transformed into grass-covered parks, gardens and even small farms.

RELATED: Green Roofs Shown to Offset Warming

Living architecture isn't a new idea. The first documented green roofs may have been the Hanging Gardens of ancient Babylon, and in the 19th century, pioneers on the American Great Plains sometimes built sod houses that utilized plants and soil as sturdy building materials. In the early 1990s, architects such as Frank Lloyd Wright and Le Corbusier incorporated rooftop gardens and green terraces into some of their projects, and numerous public buildings erected in the U.S. in the 1930s had greenery on top of them, according to Graeme Hopkins' and Christine Goodwin's book "Living Architecture: Green Roofs and Walls." Green roof building was held back in part by the technical difficulty and expense of supporting plants and soil and supplying them with water. But in the 1970s and 1980s, German architects began developing lightweight systems that were easier to maintain.

In the past decade, the green roof concept has taken off, thanks to research that showed how green roofs could help reduce the amount of storm water runoff, a cause of flooding, by 65 percent, and reduce the transfer of heat from a roof to the rest of the building by 72 percent, reducing the cost of air-conditioning dramatically. Reducing use of air conditioning offers relief to cities plagued by the urban heat island effect, in which city areas can be many degrees hotter than surrounding areas. According to a University of Michigan study, a 21,000-square-foot green roof can save its owners $200,000 in over its lifetime, two-thirds of that from energy costs.

Peck said that the green space on roofs has been increasing steadily, and now amounts to about 20 to 25 million square feet in North America alone.

RELATED: Plants Take Bus South for Winter

But some architects haven't been content just to use rooftops for greenery. In Milan, architect Stefano Boeri's "vertical forest," a pair of residential towers — one close to 370 feet tall — with terraces designed to nurture 900 trees and over 2,000 other plants, was completed in 2014 by Boeri's firm. The skyscraper grove is designed to create a "micro climate" that filters dust particles out of city air, in addition to absorbing carbon dioxide and softening acoustic pollution.

Boeri's firm and other architects have designed additional vertical forest projects for other cities, including a residential skyscaper covered with vegetation that's been proposed in Sao Paulo, Brazil, according to Dezeen. In Paris, the firm of architect Edouard François has proposed the Tower of Biodiversity, a high-rise covered with plants that "allows the wind to spread class one purebred seeds into the urban environment."

Nancy Somerville, chief executive of the American Society of Landscape Architects, said that it's unclear whether such skyscraper-scale green wall projects ultimately will turn out to be sustainable, given the expense of providing irrigation and growing conditions to keep trees and plants growing. "We'll need to see more," she said. But she's convinced that living architecture is a growing trend.

Photo: Milan's "vertical forest" is an example of so-called living architecture. Credit: Christos Barbalis via Wikimedia Commons

WATCH VIDEO: The Shocking Truth About Biodegradable Plastics