A NEW KIND OF FARM In Dadaab, now Kenya’s third-largest city, Somali refugees grow vegetables in recycled grain sacks. Photographs by Antonio Bolfo

Our driver isn’t at all happy about this. We are headed to Kibera, the notorious slum in the Kenyan capital of Nairobi, and Mary Njenga, our guide for the visit, has just suggested that maybe it would be a good idea for the men to stay behind in the car. People in Kibera can be pretty desperate, and you never know when one of them might pull a knife or a gun on you. "If it’s just the women," Njenga says, "they’ll know we’ve come to see the farmers."

We pull into an open area on the outskirts of the shantytown and, while stripping ourselves of watches and cell phones, make a plan to reconvene here in a couple of hours. (Antonio, the photographer, isn’t about to hang back, but Peter, our driver, is visibly frantic about getting himself and his treasured Toyota out of here as fast as he can.) Njenga leads us down the wide dirt road that serves as the main drag of the "informal settlement," as these places are euphemistically known, and onto a narrow path that snakes among shacks fashioned out of mud, tin, and scraps of wood and cardboard. Children poke their heads out of makeshift doorways to call "How are you?" or "Mzungu!" (Swahili for white person), as we step gingerly over shallow gullies of sewage and under drying laundry and low-hanging electric wires. The place reeks of human shit.

Njenga knows this territory well. An environmental scientist and outspoken advocate for women (and with her shaved head and vow never to marry, the most outspoken Kenyan woman I’ve met), the 40-year-old has been coming here regularly for the past decade, helping the locals figure out sustainable strategies for feeding themselves and their families. Estimates vary as to how many people live in Kibera -- some say half a million; others, a fraction of that -- but either way, at just under one square mile, the slum is among the most densely populated places on earth. And the people here are hungry. In a recent study of Kibera’s residents, more than 95 percent of those surveyed reported worrying at some point in the past 12 months that they would run out of food before finding the money to buy more. (Nearly 20 percent said they’d gone a whole day and night without eating.) Unlike those who live in the country and have land for farming, city dwellers generally have to pay for their food, sometimes spending as much as 80 percent of their incomes to do so.

But as Njenga is happy to show me, they’re finding new ways to cope. We meet up with Catherine Wangui, a friendly 25-year-old sporting a newsboy cap, who tells us how, about four years ago, representatives of the French nongovernmental organization Solidarités International, which does emergency relief and reconstruction work around the world, came here and distributed old flour sacks to some of the women. They explained how to fill them with soil and rocks before poking holes in the sides and pushing in seeds. Wangui, who grew up in Kibera, stops in front of three of these "vertical gardens" -- four-foot-tall sacks plumped out with dirt and sprouting gangly tendrils of kale and spinach. Her 5-year-old daughter, Grace, who is playing nearby in a neat dress and braids, now gets fresh vegetables every day, says Wangui, who sells some of what she grows at a little wooden kiosk that she runs. Njenga also introduces us to people who, in spaces barely the size of closets, are raising chickens and profiting from them. Not that everyone is suddenly thriving; one young woman tells us how her garden sacks have enabled her to buy sugar and cooking oil, but hits me up nonetheless for some spare shillings -- to the serious chagrin of Njenga.

Three years ago, for the first time in human history, the number of people living in cities worldwide outnumbered those living in rural areas, and the United Nations projects that by 2050, up to 65 percent of the global population will be urbanized. The rate of urban migration is particularly high in sub-Saharan Africa, where 15 million people abandon the countryside every year to move to the cities. Climate change will exacerbate the trend, as extreme events -- like the drought currently devastating the Horn of Africa -- become more frequent and more intense. Climate models predict that in the years to come, sub-Saharan Africa’s arid and semiarid areas will increase by up to 350,000 square miles, an area equal to the size of the country of Nigeria. Longer, hotter dry periods and unpredictable rainfall already are making it harder for farmers to know when to sow and harvest their crops, and in this part of the world, where high-tech irrigation is all but unheard of, the challenge is especially acute. Less arable land -- and fewer farmers -- also means less food: the Intergovernmental Panel on Climate Change has estimated that yields from rain-fed agriculture here could be cut in half by 2020, and the Washington-based International Food Policy Research Institute predicts that, as a result of climate change, output of staple crops like cassava and wheat could plunge by as much as 22 percent by 2050.

Hungry people and crowded cities, of course, make a combustible mix. Think of Paris in 1789, or St. Petersburg in 1917. As recently as 2008, the skyrocketing cost of staple foods, fueled in part by speculation in agricultural commodities markets, led to riots in no fewer than 36 countries, 21 of them in Africa.

The good news is that urban gardens like Wangui’s are making a difference. And, as I realized when I rounded a corner and crashed into 34 of the things, scrunched in tight between a concrete wall and a row of connected shanties, this isn’t just some boutique trend. In Kibera -- which the Kenyan government designated a "temporary residence" for Nubian (Sudanese) soldiers after World War I and which since has drawn hundreds of thousands of squatters from other ethnic groups -- some 5,000 households currently are growing vertical gardens. (The average farming household maintains five or six of the sacks.)

And in cities across the developing world similar efforts are under way, with the poor making use of everything from used grain sacks to old tires for planting and cultivating micro-farms. The United Nations Development Program recently reported that an astonishing 800 million people worldwide are now engaged in urban agriculture, producing from 15 percent to 20 percent of the world’s food. (Many of those people are in Asia, which has a long tradition of urban farming.) Under power lines, alongside highways, down the banks of rivers -- wherever there’s unclaimed dirt to be found -- landless city dwellers are grabbing shovels and digging in. In sub-Saharan Africa alone, participation in urban farming has increased from 20 percent of the population two decades ago to nearly 70 percent today. By the year 2020, some 40 million Africans will be depending exclusively on food grown in cities.

Africa’s cities haven’t always welcomed farmers. A few days before meeting up with Njenga, Antonio and I spent an afternoon with a 56-year-old named Francis Wachira, who told us that a decade ago, when he said he wanted to grow food in the city, people looked at him as if he were crazy. Having moved to Nairobi to find work while in his twenties, Wachira spent 20 miserable years picking up the odd construction job and reselling fruit that he would buy from the central market. Finally, in 2002, though he owned no land of his own, he found an empty patch of dirt and started to plant. "Why are you farming in Nairobi?" the neighbors mocked. "Go back to the rural area."

There was prejudice at work here -- people who take up farming in the city must be poor and uneducated, the thinking went -- but there was also a perception that food produced in the polluted environment of a city was inherently unhealthy. (Given the water used for most urban crops, that perception wasn’t entirely unfounded.) And because people like Wachira were farming on public land, without any permitting involved, it galled the authorities to no end.

Wachira ignored the mockery, and today the lanky father of three actually giggled as he led us through the neat rows of kale, eggplant, spinach, and other vegetables bursting from the 6,000-square-foot plot of land in the scruffy Makadara district, across town from Kibera. "I used to grow maize," he said, "but the city council said it was a security concern." The corn grows so high, apparently, that it makes an irresistible hiding spot for the city’s legions of thieves. "You have everything here," Wachira continued as we surveyed the land adjacent to his two-room home. "You don’t have to go to a kiosk." He stooped to pick some napier grass and lettuce. "For my goat," he smiled.

Motioning to a handful of young men washing a car several yards away, Wachira, who has the professional athlete’s tic of referring to himself in the third person, led us to another patch of green. A few months earlier, he said, as the youths approached and greeted us politely one by one, he’d given them some manure and seeds and spent several afternoons teaching them to plant. Now they were out here every day, bent over their kale and sweet potatoes and snaking along a hose hooked up to the nearby public tap. "Before they started this," he said, "whenever you passed by, they were just asking for coins." Oscar Njoroge, the 32-year-old secretary of the group they’ve dubbed the All For One Youth Organization, didn’t deny that the young men had been at loose ends. In the past few years, he said, 18 youths from the immediate area had died from drugs, AIDS, or tuberculosis. "Farming has really changed our lives," agreed Erastus Maina, a 23-year-old All-For-Oner in a yellow baseball cap. "Trust has come between us. And we are being respected."

Over in Kibera, I hear similar sentiments -- about how the gardens have engendered a sense of trust among the women growing them, how they now carry soil and water for one another, and pool their money for things like pesticides. There’s also talk about how people are healthier, in part because they eat more vegetables, but also because they’ve begun to grow a wider variety of them, including traditional ones like amaranth, spider plant, and African nightshade. Over the past several decades, such plants had fallen out of favor, especially in urban areas, replaced by the easier-to-grow and cheaper kale and cabbage. (The indigenous vegetables also were associated with poor, rural people and so were looked down upon by urban consumers.) Now, thanks to campaigns sponsored by Kenya’s Ministry of Agriculture touting the nutritional benefits and drought resistance of these old vegetables, they are enjoying a renaissance throughout the country. Njenga has been working with women in Kibera to produce and sell seeds for the traditional greens, which more and more Kenyans are adopting in an effort to shield themselves from the effects of changing weather patterns.