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.

The challenge of feeding a planet that’s set to have 3 billion more people on it by 2050 - made even more acute by climate change as some parts of the planet become wetter, while others drier - means the pressure is on to find ways to feed the planet. So farming has to become more productive – and new areas to grow, especially in dry climates, must be found. One potential solution: hydroponics.

Farming with little space and producing higher crop yields: hydroponics fits the bill. Though it may sound like something out of Star Trek, it's actually been around since the Aztecs – they built floating farms around the city of Tenochtitlan. Hydroponics essentially means growing plants without soil, and instead using a nutrient-rich solution to supply them with water and minerals.

With hydroponics, plants – usually supported by soil – are propped up artificially instead, and a nutrient solution is applied to the suspended roots using a number of different methods, including spraying them with a solution mist. Together with artificial lights, heaters and other equipment, the nutrient solutions help plants develop faster, produce larger yields and grow all year round.

Hydroponics can recycle water, meaning it could use as little as 10 percent of the water a conventional farm uses – making it an option in arid environments. And the closed system means nutrients don't escape, cutting fertilizer down to as much as a quarter of what a conventional farm would use. Also, almost no pesticide is needed, since soil pests aren't an issue for plants grown without soil.

When growing sideways isn't an option, try going upwards: hydroponic growing trays can be piled on top of one another, and plants can be grown more closely next to each than in the soil, making it very efficient in terms of space. As for what kind of space they can grow in, the sky is the limit: with no need for scarce farmland, one possibility could be to have hydroponic farms in skyscrapers.

Running a hydroponic farm can be complex, energy-intensive and expensive. Plants require many essential nutrients, and the farm needs a large amount of equipment. Heat and light, supplied for free by the sun in conventional farms, have to be provided artifically and paid for. And power failures could mean whole crops are destroyed if they go too long without water and light.

Hydroponics can theoretically be used to grow any crop, although the technique lends itself best to plants such as cucumbers, salad greens, tomatoes, peppers and herbs. Given its long history, hydroponics still isn't widely used. But that looks set to change: the global hydroponic farming industry was estimated to be worth $21.2 billion in 2016. That's forecast to grow by 7 per cent each year.