Balconies, walls, even containers - all kinds of urban spaces can be turned into mini farms. Around 10 billion people will be living on our planet by 2050 and two out of three of those will be in cities. Providing nutrition for those billions in an age of changing climate and rapidly-draining resources is one the biggest challenges of the 21st century. Which is where urban farming can help.

City dwellers yearning for nature are increasingly turning to urban gardening. The reasons for this revival are many. One is to increase quality of life and create social bonds within the community. Another is that urban farming boosts local economies by creating jobs and encouraging the development of more marketable crafts, providing greater food security.

Urban farming helps limit the effects of a warming climate by cooling down cities, and increases biodiversity within our urban environments. The HK Farm in Hong Kong, created in March 2012, is a network of rooftop gardens around Yau Ma Tei, one of Hong Kong's oldest neighborhoods and one that hasn't seen much agricultural activity for the better part of a century.

Brooklyn Grange operates the world's largest rooftop soil farms on two roofs in New York City. They grow over 22,000 kilograms (48,500 pounds) of organic produce each year. It also maintains over 30 honey beehives on roofs across the city. It began in 2010 with the aim of creating a sustainable model for urban farming, producing vegetables for the local community while benefiting the ecosystem.

Prinzessinnengärten was launched as a pilot project in 2009 in Berlin's Kreuzberg district at a site which had been a wasteland for over half a century. Rubbish was cleared away and transportable organic vegetable plots built. And it is now a space for locals to discover more about climate protection. The team also temporarily transforms unused spaces such as building sites into urban farmland.

Atop a roof garden at the Roppongi Hills business and shopping complex in Tokyo, Japan, people plant rice seedlings in a paddy field. Elsewhere in Tokyo, City Farm grows melons, tomatoes, soybeans, and rice using traditional semi-aquatic conditions. Community members can participate in threshing events, cooking projects, and sake-making courses.

Elliniko community garden, on the outskirts of Athens, is one of the "guerrilla gardens" that have popped up all over Greece, as people struggle to feed themselves under tough economic conditions. It is located on an old airport abandoned in 2001. After clearing the land, volunteers planted fruit and vegetables to help the growing number of Athenians looking for ways to feed their families.