On a cold and rainy Friday afternoon, Steven Dring is tending his baby carrots in a somewhat unusual setting. The green shoots are in a tray of volcanic glass crystals under LED lights – and the tray is in a tunnel 33 meters (108 feet) underneath a busy London street.

Dring is the co-founder of Zero Carbon Food, one of a clutch of projects trying to help feed the world's booming cities by farming in local spots – and often unexpected ones.

In India, social businesses are setting up small farms on the rooftops of crowded apartment blocks. China's government has built urban farming "showcases" to encourage city-dwellers to start projects at home.

In the coming decades, cities in rich and poor countries alike are set to swell and cause ever more pollution by transporting food from rural areas. Two-thirds of the world's population will live in cities by 2050, versus just over half now, according to UN forecasts.

"Thirty million meals are served a day in London. We've got to get all that stuff into the city, along with all the packaging needed to bring it in. So if you can bring any food production into the city, then that's good," said Dring.

Zero Carbon Food started farming in an abandoned World War II bomb shelter in Clapham North, an upper-middle-class neighborhood, in January 2014. The company has been growing salad leaves and root vegetables in a small test plot, using LED lights instead of sunshine and perlite crystals or thin fiber mats instead of soil.

The venture is now setting up its full site, which will fill the steel and concrete tunnels with vertically stacked trays that have 10,000 square meters (108,000 square feet) of growing space. It will start selling to restaurants and homes in the second quarter of 2015, Dring said, although it will only ever meet a minute fraction of the city's demand.

London's population is set to grow by roughly a fifth by 2030, when the number of residents will hit 10 million, according to the mayor's office. The projected growth rate is far above that of New York and around double the United Kingdom average – but below forecasts for many cities in the developing world.

"A city of course cannot grow all its produce, but it's about combining this with other farming," said Chungui Lu, a plant scientist and expert on urban agriculture at the University of Nottingham in the United Kingdom. "You can't grow wheat in a city, but you can grow a lot of high-value, fresh veg."

Chillies on the roof

In India, where the urban population is forecast to double to more than 800 million people by 2050, the idea of fitting farms into cities that already heave with people might seem impossible. But the lack of space on the ground has spurred an increase in rooftop farming, using the flat tops of apartment blocks.

Fresh and Local, a social business in Mumbai, has run an organic farm on top of a low-income building since 2012. The 2,000-square-foot farm grows items such as pomegranates, chillies, and bay leaves for the building's 50 families and 20 small businesses.

Fresh and Local is being hired to help set up two to three new urban farms in the city every six months, said founder Adrienne Thadani.

"There's a lot of misconceptions [such as] that there's no space, which isn't true," said Thadani. "For every building in Bombay, you have that square footage of flat roof."

Gardens of Abundance, a project in Hyderabad, a southern Indian city of 8.7 million people, has likewise set up 10 organic rooftop vegetable gardens since 2012. It has held workshops at 20 apartment blocks over the past year.

Garden plots can produce up to 20 kilograms (44 lbs.) of food per square meter per year, according to the United Nations. Many urban farming groups do not provide such figures for their production because yields vary with factors such as location and seed quality, they said.

Urban farming remains niche, partly because setting up a mid-sized plot requires time and money. London's high property prices drove Dring to an underground site that will cost £3 million ($4.7 million) to develop, using money partly raised on the equity crowdfunding website Crowdcube.

Thadani, meanwhile, has to get basic materials brought in from as far away as 150 kilometers (93 miles) outside Mumbai.

Governments sometimes step in to help. Some Indian states provide subsidized growing kits. China's government has part-funded a three-story farm in Beijing, said Lu, who is working on a research project at the site. "Showcases" for indoor farming have been set up in recent years in smaller cities such as Nanjing, China.

However, when it comes to agriculture, China's spending priority for now remains providing subsidies for poor rural farmers, said Lu. He points to Singapore as an Asian success story for urban farming.

'Chemical food'

Aside from cutting food miles – the distance it takes food to get from the grower to the plate – urban farming can have other benefits that are harder to measure, such as giving city-dwellers more knowledge about and control over their food.

Indian cities have a high demand for reliable supplies of safe vegetables. Poor infrastructure often causes food to rot en route from rural areas. A government study in 2014 found banned pesticides in crops, according to local media reports.

"There is a desire for a good quality food. When I started this, the women I worked with would say, 'I want to find a way to not have the 'chemical food,' " said Thadani.

In London, Dring's visitors often think his farm is unusually high-tech – but LED lights and trays are common in UK agriculture.

"There's such a huge disconnect between people and where their food comes from," he said. "Some kids in London probably think spaghetti grows on trees."

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(Reporting by Shyamantha Asokan; editing by Laurie Goering)

• This article originally appeared at Thomson Reuters Foundation, a source of news, information, and connections for action. It provides programs that trigger change, empower people, and offer concrete solutions.