In the Al Hajar Mountains of northern Oman, at the eastern edge of the Arabian Desert, high above the white terraces and minarets of Muscat, rain comes rarely and then in floods. Hajar means “rock” in Arabic, and the mountains are made of little else—a fractal landscape of umber and dusty limestone, thrust from the sea more than sixty-five million years ago and still shaped more by salt water than by sweet. When the clouds burst, as they do a few times a year, the rain skitters from the slopes like oil from a griddle, gathers into rivulets and swiftly moving sheets, and tumbles into the wadies that wind between peaks. The ancient Omanis built networks of aqueducts and underground falajes to funnel the water to their crops. Oases of mango, date palm, sweet lemon, and lime still survive on this system, their fruit knuckled in on itself against the heat, smaller and more pungent than their Indian ancestors. But on most slopes the only traces of green are a few umbrella-thorn trees, Acacia tortilis, anchored to the bare rock. Their roots can descend more than a hundred feet in search of groundwater.

“It used to be much wetter here when I was a boy,” Hamad Reesi said, as our S.U.V. lurched up a gravel switchback in the foothills. “You never had to buy fodder for your goats.” Ali al-Abdullatif nodded, then yanked the steering wheel to one side to avoid a dropoff. Next to him, Pieter Hoff dozed in the passenger seat. Abdullatif is the chairman of the Horticultural Association of Oman, a slender, cultivated man more comfortable potting plants than going on desert excursions. Hoff is a Dutch inventor and former tulip and lily grower who had come to Oman to test an experimental tree-planting device. We’d spent the past few hours bumping over back roads, stopping every few minutes to look at trees that might be good for Hoff’s project: hardy natives like Ziziphus spina-christi, said to have provided the thorns for Jesus’ crown, and Salvadora persica, the toothbrush tree. Its fibrous twigs were laced with fluoride and antiseptics. Word had it that this area was home to one of the last baobabs in northern Oman, but we’d got lost trying to find it and had picked up Reesi, a local farmer, as a guide. The great tree was deep in the mountains, he said. We would never reach it on our own.

Abdullatif and Reesi wore the traditional white robes and embroidered prayer caps of Omani Muslims. They were born and raised here—although Abdullatif had done his horticultural training in England, at Canterbury College, in Kent—and had seen the country transformed, in forty years, from a near-medieval land of warring tribes to a unified and oil-rich sultanate. When Abdullatif was a boy, firewood was still gathered by Bedouin nomads and brought in by camel; water arrived by donkey in goatskin bags and barrels once filled with ghee. Now the coast was dotted with desalinization plants and it was sometimes hard to tell that Oman was a desert nation. Along the boulevards and highways of Muscat, the medians were as lush as croquet lawns. Weeping casuarina trees lined the shoulders between beds of petunia, bougainvillea, and topiary trimmed like battlements. The sultan was said to be an environmentalist—he’d recently decreed the construction of the country’s first botanical garden—and he wanted his capital green.

The illusion didn’t last beyond the city limits. Most of Oman averages less than six inches of rain a year—barely enough to sustain native plants, much less thirsty exotics like the petunias. On the northern coast, long known as the country’s fertile crescent, so much groundwater has been tapped for farms, orchards, and date-palm plantations that salt water has seeped into the aquifers. “Look at this,” Abdullatif told us at one point, gesturing at a line of dead palms along the road, their fronds decaying to dust on the ground. “Complete destruction.”

When we reached the top of the first pass, Abdullatif pulled onto an overlook and killed the engine. The sun was setting, the road getting harder to follow, and he seemed ready to turn back. “I don’t like the drops on the sides here,” he said, as he got out of the car. “These sheer drops. They do not make me feel very secure.” After a while, Hoff shook himself awake and joined Abdullatif outside. Tall and pale, with a bladelike nose and a thinning crown of blond hair, he was built for cooler climes. Perched beside the dusky, heavy-lidded Abdullatif, he looked like an egret about to snack on a lizard.

“What if the car stops?” he said. “Is there a hotel here?”

“Yes, a very big, open hotel. We have one banana left. We share it.”

Hoff laughed. To the west, the high peaks of the Al Hajar rose rank upon rank into the coppery sky, the empty plains half in shadow below them. “We have a saying in Holland,” he said. “ ‘If you call out in the desert, no one will hear you.’ ” But I knew what he was thinking: not so long ago, these mountains were covered with desert junipers and groves of bitter olive. What would it take to bring them back?

The desert is a good place for visionaries. It can flower in the mind even as it withers at your feet. About a third of all land on the planet has been claimed by it—almost twenty million square miles—and the percentage increases every year. Where rain is scarce and the ground is stripped of trees, where soil is eroded by the steady beat of sun, hooves, and seasonal farming, a landscape can turn to dust in a generation. “These are real deserts that are being born today, under our eyes,” the French botanist André Aubréville warned in 1949, when he popularized the term “desertification.” “The desert always menaces.” In the past century, over most of the globe, the amount of dust in the air has doubled.

It’s an old story in some ways. Deserts have been advancing and retreating for much of the earth’s history, driven by tectonic shifts and planetary wobbles beyond our control. The Sahara and the Arabian Peninsula haven’t been green for thousands of years. What has changed is the fact that global warming is making climates more extreme. Regional rainfall is hard to predict in the long term, but most models agree on the over-all pattern. “The wet will get wetter and the dry will get dryer,” Isaac Held, a research scientist with the National Oceanic and Atmospheric Administration, told me. By the end of the century, according to the Intergovernmental Panel on Climate Change, rainfall could decrease by fifteen to twenty per cent in the Middle East and by twenty-five per cent in North Africa. “That’s a lot,” Held said. The recent drought and famine in Somalia, which has killed tens of thousands of people and driven many more into Kenya and Ethiopia, is a preview of things to come.

To Hoff, the solution seems straightforward. If we can replant the forests lost to desertification, he says, we can provide food, fuel, shade, and shelter on an enormous scale. We can conserve water, fertilize the soil, protect wildlife, and cool the atmosphere. Every year, human industry sends about nine billion tons of carbon into the air. An acre of trees, planted in a desert, could pull two to three tons of that carbon back down. “Multiplied by five billion, we have solved the problem,” Hoff says.

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The math is a little fuzzy, admittedly. Five billion acres is an area twice the size of Europe. Even if all of it could be reforested, the trees would gradually stop sequestering carbon as they matured. Still, the benefits would be dramatic and the idea isn’t as far-fetched as it seems. Since the mid-nineteen-sixties, Israel has forested tens of thousands of acres of the Negev Desert, using simple irrigation systems to collect and distribute the rainfall. In Kenya, the Green Belt Movement founded by the late Wangari Maathai, who won the Nobel Peace Prize in 2004, has planted more than forty-five million trees. And the Chinese have outdone everyone. Since 1982, they have planted more than forty billion trees, many of them in a nearly three-thousand-mile strip along the southern edge of the Gobi Desert. Forests that were clear-cut for agriculture during the Great Leap Forward, fed into furnaces for the ironworks of the Cultural Revolution, or sawed up for housing and other needs (chopsticks alone consume nearly an acre of trees a day) have been replanted on an equally epic scale. By 2020, the Chinese plan to add another hundred million acres of trees—an area larger than Germany.

As global temperatures rise, reforestation schemes seem to grow ever more extravagant—fever dreams of the desert’s future. One project, proposed three years ago by a group of British and Norwegian designers, would consist of long chains of greenhouses and orchards, running for miles across the Sahara. The trees and crops would subsist on seawater pumped from the coast and desalinated using heat and power from huge solar arrays. Another proposal, from the Swedish architect Magnus Larsson, would make use of an organism called Bacillus pasteurii, which can turn sand into sandstone. In Larsson’s scheme, great masses of the bacteria would be injected into dunes across the breadth of the Sahara, creating a bulwark against the sand and solid footing for a shelterbelt of trees. Water would collect in the sandstone’s cool, porous substructure, sustaining the trees’ roots and any settlers who wished to move inside. In Larsson’s drawings, the underground rooms have the groovy, biomorphic look of an old Yes album cover.

George Taylor, a former agriculture and environment officer for U.S.A.I.D., remembers fielding a number of such proposals when he worked in Africa in the nineteen-eighties and nineties. “Delusional development,” he calls them. Yet a variation on Larsson’s idea has attracted substantial political and financial backing. The Great Green Wall, as it’s known, was first proposed in the mid-eighties and finally approved by the African Union in 2007. The exact shape it will take is still a matter for debate, but the idea, in its original form, is thrillingly simple. To halt the spread of the Sahara, eleven African nations have agreed to erect a wall of trees across the dusty shoulders of the continent. It will stretch from the Atlantic coast of Senegal to the east coast of Djibouti, across sand and scrub and desiccated grassland, in a column nine miles wide and almost five thousand miles long. If and when it’s completed, it will be the largest feat of horticulture in human history. “The desert is a spreading cancer,” Abdoulaye Wade, the President of Senegal, declared at a summit in Chad in 2010. “We must fight it. That is why we have decided to join in this titanic battle.”

The Great Green Wall is a collective effort, which is to say, a patchwork affair. Each country along its path will reforest its segment after its own fashion, coördinated by a pan-African agency in Chad. The Global Environmental Facility has allocated a hundred and nineteen million dollars for the countries involved in the project, but that barely counts as seed money. A desert, once established, is hard to push back. African leaders have neither the means to mobilize a billion tree-planting farmers nor the money for irrigation systems like those in Israel. How can they grow a forest in the desert?

When I asked foresters and environmentalists that question, they tended to give conflicting answers: the solution lay in technology or grass-roots activism, they said, land reform or carbon-credit financing, drought-tolerant trees or water-retaining gels. And so, this past year, I went to Oman and then to sub-Saharan Africa to look at two of the most promising, albeit contradictory, approaches. Both of them, as it happened, were espoused by Dutchmen.

Hoff’s invention, which he calls the Waterboxx, was inspired by a trip to Italy in 1994. He was driving past a barren mountain range near Naples, and he began to wonder what it would take to grow trees there. The local climate was fairly dry, but the problem was less a matter of moisture than of timing. Even deserts can get as much as twenty inches of rain a year, but it all comes down at once. The plants that survive tend to rely on condensation—“They drink from the air,” as Hoff puts it. In Africa’s Namib Desert, Welwitschia plants have been known to live for more than a thousand years on the dew that they absorb through their long, porous leaves. What if a device could be built on the same principle? Hoff thought. It could collect rain and dew, then release it to a seed or sapling one drop at a time.

Hoff had never lived in a desert. He didn’t know much about tree planting or industrial design. But he did have a good business sense and a lifetime’s experience with plants. Born in 1953, he grew up in a small farming village in West Friesland, the eldest of nine children. His father was a tulip and lily grower with a modernizing bent—he was one of the first farmers in northern Holland to own a tractor—and Pieter showed an early gift for breeding new varieties. (Most of his lilies were named after Santana songs: Moonflower, Black Magic Woman, and his best-seller, a canary-and-aubergine number called Festival.) In 1976, Hoff and two of his brothers bought the farm from their father and began to expand it. By 2003, when they sold the place, it was Holland’s largest grower of lilies.

Hoff went on to devote most of his time and the greater part of his fortune—some thirteen million dollars, at last count—to developing the Waterboxx. By the time I met him, he had spent five years shuttling from desert to desert, testing prototypes with local agronomists. That month alone, he’d been in Kenya, Kuwait, and Bahrain, with Spain and India still to come. “I always search for the most extreme places, where no one expects anything to grow,” he said. All told, he had planted some sixty thousand trees in twenty countries, with a few vegetable patches and vineyards thrown in. (Robert Mondavi Winery was testing Waterboxxes in the Napa Valley.) Oman was his most challenging site yet. If he could grow trees there, Hoff figured, he could grow them anywhere.

On the morning before our trip to find the baobab, Hoff took Abdullatif and me to see his latest plantings, in the port city of Sohar. The site was a former camel racetrack, levelled to make way for an industrial park. The soil was gray and gravelly, compacted into hardpan by bulldozers and steamrollers. A viewing platform had been erected nearby—what exactly was on view wasn’t clear—surrounded by a hurricane fence topped by razor wire. Beyond it, the land lay table flat in every direction, punctuated only by some container cranes along the coast and the distant flares of an oil refinery. Hoff knelt down and poured a handful of dust into his hand. “It’s like the moon,” he said.

The Waterboxxes were arrayed in a circle around the platform. There were forty in all—a gift from Queen Beatrix of the Netherlands to Sultan Qaboos bin Sa’id, commemorating his forty years in power. Like most good tools, they weren’t much to look at: a set of simple ideas combined to surprising effect. Each box had a round, four-gallon tank molded out of polypropylene, with an open-ended shaft in the middle where the seedlings grew. The lid was modelled on a lotus leaf, with radiating folds that collected the rain and the dew and sluiced them into a pair of drains. “If you have a rain shower of only four inches, then this is full,” Hoff said. A wick at the bottom of the tank carried the water to the root at a rate of about four tablespoons a day—a single tank could sustain a seedling for about a year without a refill. The whole box functioned as a temperature regulator, Hoff said. The water absorbed heat by day and released it by night. The shaft was shaped to let in the morning and evening sun but throw shade over the seedling at midday.

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Hoff reached down and pried the lid off one of the boxes. “Mother Nature plants trees differently than people do,” he said. “We buy a very big plant and then we dig a hole for it. Mother Nature starts with a seed.” Saplings from nurseries have well-developed secondary roots, which spread laterally through the ground. Short and densely woven, they draw maximum sustenance from the soil, but they need water right away and by the bucketful. A seed can afford to wait. Encased in dung from a passing bird or other animal, it can survive for months without rain. If the soil is dry, it can put all its energy into sending a single taproot in search of groundwater.