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.