It's clear there's a growing green transformation in cities throughout the world, with greenery poking out above concrete skyscrapers and apartment balconies. Urban gardens are no longer simply a sign of wealth, they have become a tool in both the fight against global warming and food shortage.

As part of the new green trend, many a city rooftop has been renovated into a garden oasis. These gardens aren't there just for looks; rooftops covered with a thin layer of grass or greenery provide insulation to buildings, which in turn lowers heating and cooling costs. This insulation is becoming ever more important in cities like Tokyo, whose concrete skyscrapers trap in additional heat. In fact, under Tokyo regulations enacted in 2001, all large office buildings must dedicate at least 20% of their rooftop space to greenery.

On the less legal side of the green revolution, are the Guerilla Gardeners, who plant and maintain gardens in neglected or abandoned properties in urban areas. Guerilla Gardeners took root in the early 1970's in New York City when artist Liz Christy and friends revitalized a deserted city lot, now known as Bowery Garden. The movement grew as thousands of land reform activists armed themselves with seed bombs under the cover of the night.

In Havana Cuba, the urban agriculture movement is not just a trend, it's a necessity. A combination of the Soviet collapse in 1989 and the U.S. trade embargo, primarily on petrol, caused a food crisis within the city. Residents of Havana responded by turning every available space -- empty lots, rooftops, and apartment balconies -- into produce gardens. For Cubans, it's not just urban gardening, it's urban farming - another new buzz word in the green revolution. Today, more than 50% of the produce in Havana is grown locally within the city.

Urban Garden
A workman mows the grass roof of a government building near the capital city of Torshavn on the Faroe Islands. Reuters
Urban Garden 2
A rooftop garden on a building across the street from the International Covention Centre where the United Nations Climate Change Conference (COP17) continues in Durban, South Africa. The garden is part of the Priority Zone project run by the city encouraging urban regeneration. Reuters
Urban Garden 3
A woman reads a book in a rooftop garden of an apartment building overlooking a residential area of Tokyo. Trapped by concrete and asphalt, heat from heavy traffic and millions of air-conditioning units have made summer in the cities hotter - a phenomenon known as "heat-island effect." By converting a bare roof top into a green oasis, it helps absorb heat and keeps temperatures inside the building lower. Reuters
Urban Garden 4
Margarets are in full bloom in an indoor garden set up in an office building in Tokyo. Various artificial lighting is provided in a number of rooms where herbs, vegetables and rice are being grown by Pasona Inc, with hopes to bring awareness about agriculture to city dwellers. Reuters
Urban Garden 5
A 'Guerrilla Gardener' unloads flowers from a car before planting them on a spot of urban wasteland in south London April 22, 2008. The 'Guerrilla Garderners' work under the cover of night, armed with seed bombs, chemical weapons and pitchforks. Their tactics are anarchistic, their attitude revolutionary. Reuters
Urban Garden 6
John Volk, executive secretary of the Friends Committee on National Legislation, stands atop the vegetated rooftop of the first "green" building on Capitol Hill in Washington. The landscaped roof controls runoff and helps control the temperature of the building. The FCNL Green Building is the office for the Quaker Lobby group in Washington. The building, which has been transformed from two historic Civil War era row houses, is being described as an example of practical ways to protect the environment by reducing energy consumption. Reuters
Urban Garden 7
Uighur workers build a pillar to hold flowers in front of a giant statue of the late chairman Mao Zedong in the People's Square in Kashgar, China. Reuters
Global warming New York Cuba New York