For most of human history, farming has meant working the land: ploughing fields, waiting for rain and hoping the season would be kind. Over the past two decades, however, a very different kind of agriculture has emerged. In vertical farms, crops are grown indoors in stacked layers, sometimes reaching ten or more levels high, under carefully controlled artificial conditions. Many of these farms use no soil at all. The concept was popularised in 1999 by Dickson Despommier, a professor at Columbia University in New York, who set his students the task of working out how much food could be produced on the rooftops of Manhattan. When the answer proved disappointing, the group turned its attention to the inside of buildings instead, and the idea of the 'farmscraper' was born.

Vertical farms rely on several growing techniques. The most common is hydroponics, in which plant roots sit in water enriched with the minerals they need. In aeroponic systems, by contrast, the roots hang freely in the air and are sprayed at regular intervals with a fine, nutrient-rich mist. A third method, aquaponics, links plant production with fish farming: the waste produced by fish in tanks is broken down by bacteria into a form that plants can absorb, and the plants in turn help to clean the water, which is then returned to the tanks. Whatever the method, light is usually supplied by LEDs. These are not simply ordinary lamps; growers adjust them to emit mainly red and blue light, the parts of the spectrum that plants use most efficiently for photosynthesis.

Supporters of vertical farming point to a number of advantages. Because temperature, humidity and light are all controlled, harvests do not depend on the weather, and crops can be produced throughout the year. Farms can be located close to the cities where their produce will be eaten, which shortens supply chains and means that vegetables reach shops fresher. The sealed growing rooms keep out most insects, so pesticides are rarely necessary. Water use is another major benefit: since the water in hydroponic systems is collected and reused rather than lost to the ground, some farms report using as little as a tenth of the water needed to grow the same crop in a field. And because the plants are stacked, a single building can produce far more food per square metre of land than a conventional farm.

Yet the range of crops grown in this way remains narrow. The great majority of vertical farms concentrate on lettuce and other leafy greens, herbs such as basil, and microgreens – the young shoots of vegetables, harvested only a week or two after sowing. These plants are small, grow quickly and have a high market value, which makes them well suited to tightly packed shelves. Staple crops are a different matter. Wheat and rice, for example, require large amounts of light over a long growing period, and the grain they produce sells for relatively little. Researchers have calculated that growing wheat indoors would cost many times more than buying it, and for the foreseeable future such crops are expected to stay in the fields.

The biggest challenge, however, is energy. Replacing the sun with electric light is expensive, and lighting, together with the heating, cooling and ventilation that indoor growing demands, makes up the largest share of a vertical farm's running costs. The environmental picture is also more complicated than it first appears. A farm's carbon footprint depends heavily on how its electricity is generated: one powered by coal may be responsible for more emissions than the field farm and the lorries it was meant to replace. When energy prices rose sharply in the early 2020s, a number of well-funded vertical farming companies in Europe and North America were forced to close or to scale back their operations.

The industry has responded in several ways. Some operators have moved to sites where renewable electricity is cheap and plentiful, while others have designed hybrid buildings that combine artificial lighting with natural sunlight. Labour costs, the second-largest expense, are being reduced as well: in the most advanced facilities, robots now handle tasks such as seeding, moving trays between levels and harvesting. Location matters too. Vertical farming makes the strongest economic sense in places where conventional agriculture is difficult and imported food is costly, such as desert countries or densely populated islands. Singapore, which imports the large majority of its food, has set itself the goal of producing 30 per cent of its nutritional needs locally by 2030, and indoor farming forms part of that plan.

Few experts now believe that vertical farms will replace traditional agriculture. The fields will continue to supply the world's grain, and sunlight remains far cheaper than electricity. But as cities grow and climates become less predictable, farming upwards is likely to become a valuable addition to the global food system, especially for fresh, perishable produce.