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Vertical farming promises a future in which our food is grown in pockets of spaces in our cities and beneath our feet. But how far can it really go? W When the Pasona Urban Farm opened in the nine-storey office of a Japanese recruitment company in 2010, it promised a future in which food was grown within feet of the people who would eat it. Tomatoes hung down from meeting-room light fittings, a rice paddy filled a large conference space, and mushrooms grew in drawers hidden discreetly under benches. The office looked more like a museum of farming than a place of work. While the modern concept of vertical farming – growing food in trays or pipes stacked on top of one another like a giant plant lasagne – dates from around the 1990s, it could be argued that farmers have sought ways to grow more in less space and with less soil for centuries. Step-over fruit trees – usually apple and pear trees – are grown as low as 1ft (30cm) off the ground and spread laterally to fill thin strips of space in allotments and orchards. The step-over technique is based on espalier training, which might date back to Ancient Roman grape cultivation. If we did find a way to overcome the problems of cost and energy use, what would a world where all our food is grown in such farms look like? But vertical farming in the modern sense is now spreading rapidly. One example is the vertically farmed strawberry brand Oishii, based in New Jersey. In 2021, a punnet of its coveted Japanese Omakase strawberries retailed for $50 (£44) in a high-end New York supermarket. For some, this was evidence that with time vertical farming could rival and ultimately exceed traditional farming for quality. For others, the outlandish price highlighted the huge challenge to make vertical farms commercially viable. While the technology shows much promise, the costs in both money and energy are still high. This means leafy salads, smaller vegetables and fruits such as tomatoes and strawberries – high value crops which grow quickly – are about the limit of what is currently available commercially from vertical farms. But where exactly is the limit of what we can actually grow in a vertical farm? And if we did find a way to overcome the problems of cost and energy use, what would a world where all our food is grown in such farms look like? Could we ever move to completely soil-free farming? Pasona Group's urban farm in Tokyo, Japan, allowed office workers to harvest their own food at work (Credit: Andia/Getty Images) Pasona Group's urban farm in Tokyo, Japan, allowed office workers to harvest their own food at work (Credit: Andia/Getty Images) Farming upwards There is no strict definition of what a vertical farm is, but they typically consist of shallow trays stacked within a building, lit with LED lighting at each level. Many vertical farms have no windows and some are even built underground. These kinds of farms must supply everything – water, nutrients, sunlight and possibly pollinators and pest control too. Others might be built in massive greenhouses – making the most of the Sun's light and heat, but still controlling other inputs like water. What If...? This article is part of "What If…", a series which uses real-world evidence to explore hypothetical questions about would-be worlds. You might also like: What if all roads went underground? What would a flying-free world look like? What if polluters footed the climate bill? While sometimes soil is used, increasingly vertical farms use hydroponic or aeroponic systems, where water (for hydroponics) or water vapour (for aeroponics) infused with nutrients is circulated directly around the roots of the plant. "The water efficiency and nutrient efficiency is really quite high in hydroponics and aeroponics because the roots are able to get those nutrients and water in a lot quicker," says Laura Vickers, a plant biologist and head of the Urban Farming Group at Harper Adams University in the UK. "There's no organic matter, there's nothing else for the plant to compete with or to extract the water from." This means using vertical farms can considerably reduce both the water and fertilisers needed to grow food. Meanwhile, the closed-off, controlled environment of vertical farms can help to stop pests getting in – in turn possibly reducing the need for pesticides. The technology can also allow crops to grow in locations where conventional farming is not possible. Astronauts on the International Space Station, for example, are growing their own food in soilless systems under LED lights – with cabbage, mizuna mustard, lettuce and kale among the crops on the menu. More generally, though, the small amount of land needed for vertical farms means food can be grown near to cities, says Natalia Falagan, an engineer from Cranfield University in the UK, which could give huge benefits. Shorter supply chains could improve both food security and the quality of food, since the faster food reaches us, the less its nutrients deteriorate and the less unseen toxins, such as mycotoxins, develop. It would also result in far fewer food miles than many imported alternatives and help to guarantee the provenance of crops, she says, while also reducing the burden on natural resources like soil, water and the atmosphere. What's more, with advanced plant science, the crops could be engineered to be healthier and tastier, she adds. The urban Vertical Harvest in Jackson, Wyoming grows lettuce, micro greens and tomatoes in a large vertical hydroponic farm (Credit: George Rose/Getty Images) The urban Vertical Harvest in Jackson, Wyoming grows lettuce, micro greens and tomatoes in a large vertical hydroponic farm (Credit: George Rose/Getty Images) Having vertical farms in the midst of cities could also help to make us more aware of how our food is produced, says Falagan. Pasona Urban Farm, for example, wasn't just for show – it allowed office workers to harvest their own food at work, which the company hoped would improve both their mental and physical health. Falagan says that having a closer connection with where our food comes from might encourage consumers to be more careful with food waste, too. "If people know how much goes into producing our food," she says, "then maybe they will think twice about throwing it out." Most vertical farms currently concentrate on low-biomass crops – things like herbs, salads and small fruits and vegetables which weigh less per portion – because there is a greater yield for the amount of heat, light and water that needs to be supplied. They also have short life cycles, so make maximum use of space, says Vickers. A leafy green will grow quickly and be ready to harvest in days or weeks, whereas a pumpkin would take months to grow and needs to cure in sunlight. Salads and herbs do not need to flower and be fertilised, which gives them another advantage over other crops when it comes to vertical farming A meta-analysis of urban farming in a variety of settings across 53 countries found lettuces, kale and broccoli were particularly suited to vertical farms. Crops like spinach can be grown from seed to harvest in 30 days, meaning a vertical farmer could have 12 harvests from the same tray each year. What's more, if the farmer were to stagger planting their crop from one row of trays to the next, they could have an almost continuous supply of food all year round from their farm – something a traditional farmer can only dream of. Lost bees Salads and herbs do not need to flower and be fertilised, which gives them another advantage over other crops when it comes to vertical farming. Most fruits and vegetables do require pollination, and where it is needed for vertical farms it would have to be done artificially at a cost. (Read more about how humans are farming a wider variety of tiny insect livestock than ever before.) Farmers already often rely on commercial bees to encourage pollination and maximise fruiting for both indoor and outdoor farming. For example, the almond industry, of which 85% is based in California's Central Valley, depends on billions of honey bees that are shipped tens of thousands of miles to and from Florida to pollinate the almond blossoms each year. The hives are brought in on trucks and the bees are released into each grove before being moved on to the next farm. In the UK, strawberries grown in polytunnels are also pollinated with commercial honey bees – though, curiously, pollination by wild bees results in larger fruits. Plant petals reflect UV light to attract bees, as illustrated in this ultraviolet-induced visible fluorescence image of Gazania flowers (Credit: Calvin Jennings/Getty Images) Plant petals reflect UV light to attract bees, as illustrated in this ultraviolet-induced visible fluorescence image of Gazania flowers (Credit: Calvin Jennings/Getty Images) Commercial bee pollination has already been applied to indoor farming, says Vickers. But using bees in an indoor environment poses a problem – the artificial lights can make it difficult for bees to navigate. To humans, indoor farms often appear illuminated in neon pinks and purples. This is because plants grow best when exposed to mostly blue and red wavelength light, which to us looks pink or purple from a distance (although other wavelengths such as green light are also used depending on the crop). But bee vision is very different from ours. They cannot see the colour red, but can see in the ultraviolet (UV) spectrum, which humans cannot. Outdoors, plant petals reflect UV to help bees navigate to them. This puts bees at a distinct disadvantage in vertical farms, where UV is currently not used. Adding it could make indoor farms even more expensive, especially considering this same light is completely free for outdoor farmers. "We think about vertical farming from the plant's perspective," says Vickers. "And we don't think about what the bee sees and how it finds the plants. The bees struggle to navigate under the current LEDs." Vertical grains Other crops, such as cereals, are pollinated by wind, which could be easily replicated by creating a draught within the vertical farm, says Vickers. But while there are some examples of vertical farms experimenting with growing cereals, it's still rare. "Barley or wheat, which takes many months to grow, are not currently cost effective," says Vickers. The reason for this high cost is the biggest barrier to the widespread use of vertical farms – they require huge amounts of energy and infrastructure. While being able to grow crops year-round increases the efficiency of a vertical farm, the challenge these farms face is to keep energy use down when the alternative – growing outside – comes with free sunlight and rainwater. This reliance on energy can also make vertical farms highly vulnerable to volatile energy prices. In many places, the electricity used to power vertical farms also still comes from fossil fuels, meaning overall greenhouse gas emissions can be far higher than for open-field farming. Switching to renewable energy is the obvious way to reduce this carbon footprint, although, since most renewables themselves require land, this can reduce the land-saving benefits of indoor farming somewhat. Farmers tend to illuminated vegetable crops inside the Sentral Farm building, in West Java, Indonesia (Credit: Dasril Roszandi/Getty Images) Farmers tend to illuminated vegetable crops inside the Sentral Farm building, in West Java, Indonesia (Credit: Dasril Roszandi/Getty Images) Botanically speaking, barley or wheat are quite possible within a vertical farming system due to their short root structure, says Vickers, which is important for growing them in trays. And while it's unlikely we'll ever grow all our grains in vertical farms, it could still prove to be useful in some situations. In 2020, agricultural scientists from several US universities published an article arguing vertical farming might one day play a role in hedging against future unexpected supply chain disruptions. As the ongoing war in Ukraine has highlighted the world's dependence on cereals from eastern Europe and sparked a search for alternative sources, it may be a proposal worth considering. Indoor orchards But Vickers says that it is actually woody crops, such as fruit trees, which would really test the limits of what is possible to grow in a vertical farm, not least because they would require significant support without soil. Botanically, however, there is no reason that indoor orchards are not possible, she says. "A tree already is a kind of vertical farm," says Mark Horler, chair of the UK Urban AgriTech, which represents the country's indoor agriculture community. "Trees take water with nutrients in it, and distribute it across a series of layers that then grow leaves and fruits. As ever, nature was there first." Along with Kerstin Schreiber, a geographer at McGill University, Horler is co-founder of the Soya Project, an initiative set up to promote the benefits of vertical farming. He is currently investigating the potential of growing willow saplings indoors in vertical farms to speed up their growth before being planted outside, and so to hopefully increase their survival rate. "The rate of failure in reforestation and afforestation projects is phenomenal," he says, citing how vulnerable saplings are to the elements when they are very small (in one study, 83% of replanted trees died within 10 years). "Only a really small proportion make it to the final grade." But if given the head start, Horler hopes the saplings will stand a better chance. And he's not the only one looking to vertical farming to give trees a boost. Zoe M Harris, an environmental scientist from the University of Surrey in the UK, is leading a similar project which has recently been awarded £4m ($4.8m) in UK government funding, while a project propagating apple tree saplings in aeroponic systems started in England last year. Early trials of a vertical farm project in Scotland managed to grow tree seedlings six times faster than is typical in traditional outdoor sowing. Vertical farms – such as the Altius farm in Denver, Colorado pictured here – is now spreading rapidly (Credit: Kevin Mohatt/Alamy) Vertical farms – such as the Altius farm in Denver, Colorado pictured here – is now spreading rapidly (Credit: Kevin Mohatt/Alamy) Horler says he sees no reason why trees couldn't be grown permanently indoors for food. A vertical orchard might look a bit different to the ones we are used to seeing, however. "What you'd want is a series of thin saplings that fruit really often, rather than waiting for one tree which produces all its fruit all at once," says Horler. Most apple trees are already dwarf varieties of wild species, he says, so dwarfing them further isn't out of the question. In Asia, dwarf mango trees are being trialled because the smaller variety helps harvesters pick their fruits. And in 2019, a collective of researchers were able to produce a tomato plant that, in Horler's words, "almost looks like a bunch of grapes" – the tiny plant was covered in ripe tomatoes. Plant scientists would need to solve some long-standing problems before all our food could be grown vertically, says Laura Vickers Dwarfing crops makes them more suitable for vertical farming, agrees Vickers. But the variety that performs best outdoors won't necessarily be the variety that performs best within a controlled environment, she adds, so dwarfing an established variety might not always work. For example, certain varieties might perform best outdoors because they are hardy to temperature variations or diseases, which might not be such important factors indoors. Vertically farmed trees and crops that take longer to mature might also be tricky to keep disease-free, says Vickers. Between harvests of herbs and salads, a vertical farmer can sterilise trays before the next crop is planted. However, mango trees, for example, can take years to mature, in which time mould and bacteria can thrive. "The disease has the time of its life because – just like the plant – it has the perfect environment, perfect temperature, perfect amount of oxygen, perfect nutrients and access to all these lovely plants as well," says Vickers. If conditions are ideal, however, Horler says a vertically farmed tree might be able to actually invest more in its fruits. "A plant will devote its resources to what it thinks it can do best," he says. "If it's being tested, then it will put more effort into protecting itself from harder conditions…[But] if it's in ideal conditions and having an easy life, then it will grow lots of leaves and flowers and fruit, it won't grow these massive metres-long root systems." From the Swiss Alps to the Rocky Mountains, ski resorts are grappling with the impacts of climate change. How are they adapting in a warming world? A Anzère is often hailed as Europe's greenest ski resort. But the Swiss village had a difficult start to its 2023 winter season. Like many other Alpine ski resorts, low-lying Anzère was forced to close some of its pistes due to lack of snowfall and rainy conditions in late December and the first few days of January. The Alps experienced record high temperatures over Christmas and New Year, reaching 20.9C (70F) in northwest Switzerland. "It was exceptionally warm over Christmas and New Year," says Stephanie Dijkman, director of Anzère tourism. "Nearly all the snow in the village was gone. I was quite worried." People were unable to ski down to the village, she adds. Fortunately, it started snowing again in early January, just in time for the arrival of the "real skiing fans" who book their trips outside of the holiday season and are hoping for "very good conditions", she says. But the recent warm weather and winter washout throughout the Alps is an omen of what awaits the ski industry. Many resorts are aware that they only have two options: close or adapt their business model to cope with mounting climate threats. From the Swiss Alps to the Rocky Mountains, resorts are grappling with the impacts of climate change by trying to provide sufficient snow cover and rapidly adapt to the changing conditions, while envisaging what their future looks like in a warming world. Besides measures to reduce its own emissions, such as by investing in renewable energy and building Europe's largest wood-fired heating plant, Anzère is planning ahead for a time where skiing may no longer be viable. "We're aware that we need to adapt the resort for the years to come," says Dijkman. "We're investing more in bike trails, in climbing [routes] and walking paths and highlighting all that we have to offer, whether it's in the summer or winter." Climate change is turning skiing into an endangered pastime. Ski seasons are becoming shorter and slopes are turning green as temperatures rise. Heavy rainfall in Alpine resorts contributed to snow melting and slushy conditions. When temperatures rise, the atmosphere ends up holding more water vapour which leads to more rainfall, says Marie Cavitte, a glaciologist and climate researcher at the Catholic University of Louvain in Belgium. "[When] temperatures increase, [water vapour] falls as rain instead of falling as snow," she adds. "That is what is happening at low altitude ski stations, which are below 1,600m (5,250ft). There we are seeing a lot more rain on snow events which increase the melting of existing snow." At low altitude resorts in Europe, snow depth is shrinking by 3-4cm (1.2-1.6in) every 10 years, says Cavitte. Ski seasons are becoming shorter and slopes are turning green as temperatures rise (Credit: Dietmar Stiplovsek / Getty Images) Ski seasons are becoming shorter and slopes are turning green as temperatures rise (Credit: Dietmar Stiplovsek / Getty Images) Another major concern is the rapid disappearance of glaciers. Alpine resorts such as Tignes in France rely on glaciers for snow cover and water supply. In the European Alps, glaciers are expected to vanish almost completely by 2100, with the UN Intergovernmental Panel on Climate Change warning that they could lose 94% of their 2017 volume by the end of the century. As temperatures rise, glacial ice melts faster and there is less fresh snow. Glaciers melting could lead to severe flooding and erosion in the valley below and increase the risk of avalanches, destroying critical infrastructure as well as significantly reducing water supplies for local mountain communities. According to a 2017 study by the Institute for Snow and Avalanche Research, the Alps could lose up to 70% of their snow cover by 2100, if global temperatures rise in line with business-as-usual. The world is currently on track for an average of 2.7C (4.9F) of warming by 2100. If global temperature rise is kept below 2C (3.6F), though, the reduction in snow cover will be limited to 30%, according to the study. Rising temperatures and decreased snowfall are shortening the winter ski season, a concerning trend for the mountain communities whose livelihood depends on the winter sports industry. In a swathe of US ski resorts covering a combined area the size of Virginia, the ski season shrank by an average of 34 days between 1982 and 2016, according to a 2018 study. Anzère's winter season has shrunk by just a few days over the past 30 years, says Dijkman. "But in some European resorts, [the season] has shortened by weeks and that is obviously going to have quite a big impact." A lack of snow isn't the only challenge that ski resorts are contending with. "In the United States, wildfires are almost a greater threat," says Auden Schendler, senior vice president of sustainability at Aspen Skiing Company, which operates the Aspen Snowmass resort in Colorado. Over the past five years, wildfires have come dangerously close to towns and shut down major highways in the ski region. "We're already seeing fires at ski resorts. They're as big a threat as loss of snow," says Schendler, adding that in recent years the resort has also experienced mudslides caused by extreme rainfall. Carbon Count The emissions from travel it took to report this story were 0kg CO2. The digital emissions from this story are an estimated 1.2g to 3.6g CO2 per page view. Find out more about how we calculated this figure here. Ski resorts are scrambling to ensure that they can stay open for their entire season and provide tourists with the snow-covered, picturesque settings they expect. To do this, they are harnessing huge snowmaking machines which cover the pistes in fresh white powder. A study by the University of Basel in Switzerland warns that resorts situated below 1,800-2,000m (5,900-6,600ft), will increasingly have to rely on artificial snow to keep just their higher ski slopes open for up to 100 consecutive days, while their lower slopes may not be possible to save. This would raise their water consumption by 79% by 2100. During an average winter towards the end of the century, a resort would consume about 540 million litres (119 million gallons) of water, compared to today's 300 million litres (66 million gallons). In the French Alps, water consumption could increase ninefold by 2100, according to the study. This could lead to conflict between the skiing industry and local communities who may rely on the water for hydropower, the researchers warned. (Read about how small hydropower plants have long sustained remote communities in the Alps). The study also predicts a huge rise in operational costs for ski resorts if they are forced to use artificial snowmaking to maintain their slopes. This is because the cannons that distribute the snow and the ploughs that groom the pistes are incredibly energy-intensive and run on polluting fossil fuels. There are currently no snow-making machines that run on renewables and plans to reduce overall emissions are focused around improving efficiency. Solutions to adapt to the changing climate in the mountains therefore "ironically add more CO2 to the atmosphere and are making the climate problem worse", says Cavitte. Even today, snow ploughs on average account for roughly 60% of a resort's carbon emissions, while snow cannons contribute about 25%, says Cavitte. Ski lifts aren't as polluting, she says, since many resorts are trying to use renewable electricity. Besides the climate impact of snow machines, there is also an operational challenge as artificial snow can only be produced when temperatures stay below 1C (34F). The air has to be cold enough that the water droplets which the machines expel freeze and turn into snow particles. Over Christmas and New Year, many Alpine resorts were forced to close their slopes as the temperatures were too high to generate artificial snow. Ski resorts are investing in mountain bike trails, walking paths and climbing routes to encourage tourists to visit in the summer (Credit: Alamy) Ski resorts are investing in mountain bike trails, walking paths and climbing routes to encourage tourists to visit in the summer (Credit: Alamy) Meanwhile, some have resorted to covering glaciers in protective blankets to stop them from melting in the summer. Made out of white UV-resistant synthetic material, the blankets shield the thick winter snow from the Sun's rays during the warmer summer months. According to a 2021 study, the technique can reduce the melting of snow and ice by 50-70%, compared to unprotected surfaces. But it is a costly process, the study's authors warn. Covering all of Switzerland's 1,000 largest glaciers would cost about 1.4bn Swiss Francs ($1.5bn; £1.2bn) annually, they estimate. There are also negative environmental consequences associated with this adaptation measure, warns Cavitte. Polluting machines are used to put the blankets down and remove them. "And when they take them off, there's always pieces of plastic that are left behind which contaminate the glacier and surrounding land," she says. There are also concerns about how this measure will impact local biodiversity and wildlife, she adds. These short-term measures will not shield the industry from the looming climate threat. "The ski industry is not going to be able to save itself," acknowledges Schendler. Despite this bleak outlook, many resorts have adopted ambitious, long-term sustainability goals in a bid to reduce their emissions and protect their natural resources. Big Sky resort, which sits at an elevation of 2,200m (7,200ft) in the Rocky Mountains in southern Montana, is aiming for net zero emissions by 2030, under its Forever Project, which was launched in 2021. The resort has introduced a wide range of sustainability measures, including a newly installed 32 kilowatt (kW) solar array, improving the efficiency of buildings, reducing its water usage and protecting its forests, says Amy Fonte, sustainability specialist and head of the Forever Project. The resort also purchases renewable energy credits for the remaining electricity it uses, including for its 38 chairlifts and its housing, says Fonte. Aspen Skiing Company, meanwhile, is aiming to source 100% renewable electricity to power all its operations by 2030. "Ski resorts use fossil fuels and a lot of energy. It would be completely hypocritical if those resorts were not working to fix the system," says Schendler. Meanwhile, a wood pellet plant heats 600 apartments, two hotels and a public swimming pool in Anzère, saving the village 1.5 million litres (330,000 gallons) of oil a year. Most of the resort already runs on hydroelectricity, which is generated at the nearby Tseuzier dam. Anzère is also car-free and offers free public transport to all visitors, in a bid to encourage people to travel more sustainably. Tourists also have a role to play in helping reduce emissions and preserving the mountains, says Dijkman. "It's a mindset that needs to not only be present among local businesses but also among the people that come here on holiday." This starts with choosing how they travel to the ski resort. According to a recent survey by the Ski Club of Great Britain, just 2% of British travellers take the train to go skiing, compared to 72% who fly. (Read more about the climate impact of flying). For people in Europe looking to travel in a more sustainable way to the Alps, there are plenty of options ranging from the Alpen Express, an overnight train travelling from the Netherlands via Cologne to Austrian resorts, or the TravelSki Express which runs between London and the French Alps, via a Eurostar to Paris. Even if they are unable to change their own fate, resorts can still play an important role in the fight against climate change, says Schendler. "The role of the industry is to help the public understand what they stand to lose from climate change and to advocate for solutions." Ski resorts are the "perfect messenger" for highlighting the reality of the climate crisis, says Schendler. "People love what skiing offers them today. The threat of that disappearing is the kind of visceral hit that people need to catapult themselves into action." The ski industry is not going to be able to save itself – Auden Schendler Fonte agrees that the ski industry "can make significant behaviour changes…We can really help mitigate the impacts of climate change," she says. "I'd like to think that we're going to live in a world where skiing will still be around." But this looks like an unlikely reality for many resorts, especially lower-lying ones in Europe. Many are seeking to pivot their business model and ensure that they have a future if there is no more snow. Some ski resorts are covering glaciers in protective blankets to stop them from melting in the summer (Credit: Miguel Medina / Getty Images) Some ski resorts are covering glaciers in protective blankets to stop them from melting in the summer (Credit: Miguel Medina / Getty Images) "We're now investing more in summer [activities] than we're investing in winter [ones]," says Dijkman. The plan is to highlight "all the activities that the mountains have to offer", without turning the resort "into a Disney [theme] park", she says. Schendler says a future without skiing will be very difficult for resorts. Even with increased investment, they will struggle to generate a similar stream of revenue during the summer, he argues. "Skiing is a huge business," he says. "The [economic] throughput in the summer is going to be much lower. You can't really build a business around hiking." But according to Rachael Carver, who surveyed ski tourists at the Austrian resort Stubai Glacier for her dissertation at the University of Staffordshire in the UK, people see the value of visiting the mountains even if there is no snow. Some 70% of those surveyed said they would return to the resort if the glaciers vanished, citing the beautiful mountain scenery and hiking opportunities as the main reasons. Carver also observed an increase in "last-chance tourism" at Stubai Glacier, with people visiting the resort before the glacier vanishes completely. "Many people are aware that glaciers are melting and that ski resorts are disappearing. They want to see these sites before they [vanish] completely," she says. This increased interest in visiting glaciers provides resorts with an opportunity to educate people about climate change, she notes. But this surge in "last-chance tourism" is also unsustainable and placing even more pressure on an already fragile ecosystem, says Carver. Adapting the business model and pivoting to new mountain activities is the only choice, says Dijkman. "We have to become greener if we want to keep living here," she says. She is hopeful that tourists will continue visiting the mountains even if there is no snow. "Our mountains are naturally stunning and we already have absolutely everything to make holidays perfect, whether it's in the winter or summer."

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A deadly and hard-to-detect disease has been ravaging the treasured olive trees of southern Italy for 10 years. A highly trained squad of super-sniffer dogs could save them. O On a sunny winter morning, the dog trainer Mario Fortebraccio slowly bends toward a line of potted olive trees and indicates it with his hand. Waiting for that signal, Paco, a three-year-old white Labrador, rushes through the row of plants with his head tilted, sniffing each pot at the root, the rhythm of his inhaling echoing through the greenhouse. The dog is carefully scouting for something humans can't sense. "They don't do anything if there is no reward," Fortebraccio tells me with a smile. After a few seconds, having completed his task, Paco returned to the trainer, lifted his leg to urinate on a nearby plant, wagged his tail, and claimed a little crunchy treat. At Vivai Giuranna, an extensive commercial greenhouse with over one million plants in Parabita, in the southern Italian region of Puglia, Paco is searching for Xylella fastidiosa, a type of bacterium that has been ravaging southern Italy's olive fields for the past decade. Paco and a few other four-legged colleagues make up the highly trained Xylella Detection Dogs team. "These dogs have got something unique," says Angelo Delle Donne, the head plant health inspector for the government of the province of Lecce, who has been battling Xylella since it was discovered in Puglia in 2013. Paco, a very good boy, gets a pat on the head amidst his efforts to detect Xylella (Credit: Agostino Petroni) Paco, a very good boy, gets a pat on the head amidst his efforts to detect Xylella (Credit: Agostino Petroni) Xylella fastiodiosa is a bacterium that clogs the xylem (the vessels that carry water from the roots to the leaves) of trees and other woody plants and slowly chokes them to death. Spittlebugs, a common insect, spread the disease: when they bite an infected leaf, the bacteria move into their saliva, and the bugs transmit the disease when they feed on their next healthy plant. There are no known cures for this disease, and once infected, the plant slowly dries up (though some infected plants manage to survive without showing symptoms). There are several strains of Xylella, and together they affect 595 plant species worldwide at the last count. Over the past century, Xylella has decimated orange fields in Brazil, vineyards in southern California, and pear trees in Taiwan. Then, 10 years ago, Xylella reached Puglia's olive trees. You might also like: Italy's plan to save Venice from sinking The lost generation of ancient trees The vegan leather made from flowers With its 60 million olive trees, Puglia used to produce up to 50% of Italy's olive oil, but in just a few years, Xylella infected and killed 21 million trees, many of which were several centuries old. Today, an endless sea of dead, grey tree trunks covers the lower part of the region, dotted with what's left of thousands of small-scale farms, olive mills, and greenhouses. Mauro Giuranna, the owner of Vivai Giuranna, has personal experience of a Xylella attack. When plant inspectors found infected plants in his greenhouse, he had to dispose of about €1m ($1m/£900,000) worth of plants. "We were too superficial [in countering Xylella] in the first years," Giuranna says. "There are no more monumental olive trees left." He wishes controls had been tighter and faster. However, the regional governor President Michele Emiliano was initially sceptical about a link between Xylella and the rapid desiccation of olive trees. The scientists working on trying to stop the bacteria were put on trial, accused of spreading the bacteria themselves (eventually, all charges were dropped). Italy was investigated by the European Commission for an inadequate response. A spokesperson for Emiliano told Future Planet that he had never endorsed anti-scientific or conspiracy theories. "The President has launched an important action of listening to everyone, organising public assemblies, personally participating in all the events to which he was invited, to bring an issue as complex, dramatic and divisive as Xylella into the context of a civil dialogue," the spokesperson said. Italy's response has now improved since the early days after the infection, taking measures such as large-scale disease monitoring. However, Xylella keeps spreading through the region's olive fields. "We are always chasing the disease," says Delle Donne. The Labrador is not the only breed to make an excellent super-sniffer, as Ellis the seven-year-old springer spaniel can attest (Credit: Agostino Petroni) The Labrador is not the only breed to make an excellent super-sniffer, as Ellis the seven-year-old springer spaniel can attest (Credit: Agostino Petroni) As the bacteria keeps spreading northward at a rate of about 20km (12 miles) per year, popping up in other regions of Italy and Europe too, governments are concerned, while scientists and plant inspectors are racing to contain the disease and prevent it from spreading further. Once an infected tree is identified close to healthy trees, it has to be uprooted and any other trees in a 50m (160ft) vicinity are carefully inspected. Nicola di Noia, an agronomist from Taranto and the general director of Unaprol, Italy's largest olive oil producers' consortium, understands the danger of Xylella well and loudly advocates for its containment. "It is an unprecedented ecological, environmental disaster," he says. "We can't just get caught up in environmentalist passion. We have to be scientific." In 2020, he thought of his past experience as a carabiniere (an officer for Italy's gendarmerie), working with molecular-detection dogs for uncovering drugs and explosives and remembered their incredible olfactory skills. What if they could detect Xylella too? "We began looking for similar works done by dogs on plants," Di Noia says. And they found that a team of Californian experts had figured out a way to use smell to detect bacteria on citrus fruits. Excited by the possibility, Di Noia spoke about this idea with the Ente Nazionale della Cinofilia Italiana (ENCI, the national organisation responsible for the recognition, standardisation, and registration of pedigree dogs in Italy) and with the head researchers on Xylella in Bari, Puglia's capital, at the National Research Council. He put together the funds and decided to call in the cavalry – the Xylella Detection Dogs. Dogs like Ellis have exceptional noses but they also have other crucial traits, such as intelligence and a keen desire to play (Credit: Agostino Petroni) Dogs like Ellis have exceptional noses but they also have other crucial traits, such as intelligence and a keen desire to play (Credit: Agostino Petroni) A dog's nose, in principle, works the same way as a human's does. As we inhale and exhale, receptors in the nose detect molecules in the air and send the information to the brain. A dog's nose has a few different features that make the animal a super-smeller. Its front part of the nose serves to humidify the incoming air which aids olfaction, and the air is then pumped to the lungs, and in part, into an olfactory chamber packed with receptors to catch odorants. And that's where dogs outclass the human nose: dogs have 20 times more olfactory receptors than humans that send signals to their brains. When the dog exhales, the air goes out through the two side slits of the nose, not through the nostrils as we do: this is why dogs can sniff in a continuous cycle, catching large volumes of air and odorants. "There are animals that see a lot more colours than we do," said Adee Schoon, an independent Dutch biologist who has been working with scent detection by animals for the past three decades. "If you take that analogy and use it with dogs, you can see that we are definitely odour-blind compared to dogs." Most dogs can navigate the scent world in ways we cannot fathom, but it takes special individuals and a lot of training to become detection dogs. Schoon used to work with detection dogs in forensics, in particular rape cases for the Dutch police, to identify rapists by their semen. She says to think of detection dogs as highly trained specialists who recognised scents in the way humans recognise people's faces. Both Paco and Ellis were able to reliably spot an infected plant and freeze beside it (Credit: Agostino Petroni) Both Paco and Ellis were able to reliably spot an infected plant and freeze beside it (Credit: Agostino Petroni) However, training a new dog is not easy and can take some time. According to Serena Donnini, a dog trainer for ENCI and the coordinator of the Xylella Detection Dogs experimental programme, there are some dog breeds, such as the springer spaniel, German shepherd, cocker spaniel, and Labrador, that, thanks to their larger nose and chest space, are more likely to become good super smellers and work longer shifts in a self-directed way, often pursuing a scent for hours. But that's not enough, Donnini says, because a dog's personality is important too: to pass all the exams to become a detection dogs, the animals must love to play and eat. "This is important in order to develop a reward system," Donnini says. The more a dog loves to catch a ball, and the more they become obsessed with it, the more likely they'll look for it. "Until we have something that the dog wants so strongly that he would almost be willing to kill to get it, we can't move forward to train him." A common object that Donnini and her colleagues often work with is a hollow rubber toy. After letting the dog initially play with it, trainers start hiding it to work on the dog's searching skills. Every time the dogs find the rubber, they receive a food reward. "It must think, 'working here is great because I found my toy'," Donnini says. To the dogs, the rubber has a very specific odour, so the more the training advances, the more the trainer breaks the rubber into smaller pieces until they become the size of a lentil. The smaller the rubber fragment, the more the dog concentrates on finding it and speeds up its sniffing frequency. Once the dog is taught how to indicate it has found the toy by freezing, barking or sitting, trainers insert the target smell. According to Donnini, there are different ways to do this, but there are two most common methods. The first is pairing – putting together target smell and toy, rewarding the dog when they find them, and slowly removing the rubber of the toy. The other is contrasting – here, no toy is hidden, but as the dog urgently looks for it, when it passes by the target odour, it receives a reward. The dog soon learns to signal when it recognises the new smell, receiving a reward. Therefore, Donnini says it's crucial to train the dogs with the right scents, and this is where the scientists of the National Research Council play a fundamental role. To save trees from Xylella, that scent is the odour of an infected plant. Like humans, when plants get infected, their metabolism and scent change. Donato Boscia, a plant virologist and the head of the Bari unit of the CNR Institute for Sustainable Plant Protection in charge of studying Xylella, has been providing Donnini with certified infected olive plants. Boscia's team is currently trying to figure out the specific molecules released by the infected plants that the dogs perceive. "We don't really know, but we have to give the dog all the puzzle pieces so that he can create a precise smell image," Donnini adds. To show the dogs' precision, Donnini lays out five small potted olive trees in a row in the yard of Forestaforte, an olive mill, and the research outpost of CNR in Salento. One of the plants carries a tag with a red dot, certified as infected with Xylella by Boscia's team. Dog trainer Fortebraccio signals to Ellis, his seven-year-old dappled white and brown springer spaniel, who sprints through the plants, freezes in front of the infected plant, and wagging her tail, returns to the trainer to claim her food. No matter where the plant was positioned in the line, both Ellis and Paco could clearly spot and freeze in front of it. If Paco and his colleagues can reliably detect Xylella-infected trees on olive farms, they could stem the spread of infection and save many other trees (Credit: Agostino Petroni) If Paco and his colleagues can reliably detect Xylella-infected trees on olive farms, they could stem the spread of infection and save many other trees (Credit: Agostino Petroni) Not all plants infected by Xylella show symptoms perceptible to humans, which is one reason it has proved so challenging to contain the disease. The dogs could help halt the spread at critical strategic locations such as greenhouses and ports. It's thought that it was through an imported coffee plant from Latin America that Xylella arrived in Puglia in the first place, according to one study led by Annalisa Giampetruzzi of the University of Bari Aldo Moro. "It is precisely these [places] that have been the Trojan horse in which Xylella was introduced into a new area," Boscia says. They imagine trained dogs deployed in each entrance port that receives imported plants and others that would periodically scout the region's greenhouses. "We need to train dogs to identify Xylella-infected plants regardless of the plant species where it is found," Boscia said. The researchers still do not know if the compounds that the dogs smell come from the roots or from the branches of the tree. They are still uncertain if, with the same training, they'll be able to simultaneously uncover the bacteria in a rosemary or an oleander plant. According to Cristina Davis, a professor of mechanical and aerospace engineering at the University of California, who has been working for years to build instruments that can detect volatile organic compounds, that is a possibility. She explains that if, for example, there are 50 volatile compounds emitted by each different Xylella-infected plant species, it could be that a portion of those compounds are shared between all. "I think that there's a really reasonable expectation that you could train an animal like a dog or a sensor to be able to detect that over time," Davis says. Carbon count The emissions from travel it took to report this story were 118kg CO2, travelling by train and car. The digital emissions from this story are an estimated 1.2g to 3.6g CO2 per page view. Find out more about how we calculated this figure here. In fact, in 2014, Davis and her team managed to use an advanced instrument, a differential mobility spectrometer, to spot citrus plants infected by the bacteria Candidatus Liberibacter. As Italian scientists keep studying the infected plants, dogs could work as proof of concept to create an instrument that could help their Xylella search too. There is still a lot of work to be done, but the researchers hope the Xylella Detection Dogs will be another instrument to contrast the deadly bacteria. Besides the dire impact of Xylella on the southern Italian territory, Di Noia worries about the quantities of carbon dioxide released into the atmosphere by the endless forests of dead olive trees as they decompose. "It's a cost that we all have to pay, nationally and internationally, to contain an environmental disaster," Di Noia says. -- Join one million Future fans by liking us on Facebook, or follow us on Twitter or Instagram. 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