Andy Marshall, a biologist, yanks on the steering wheel of a battered Nissan station wagon and swings it off a track in the Kilombero Valley of southern Tanzania. Rain from the night before has left hubcap-deep puddles across the road. Mr. Marshall downshifts, swerves onto a recently harvested field of sugar cane, and parks on the furrows. The Nissan shudders for an instant before going quiet.

The biologist – a researcher on the staffs of the University of the Sunshine Coast in Queensland, Australia and the University of York in England – and three Tanzanian villagers slog a short distance through dirt clods and stubble toward a tall leafy wall of deep green: the Magombera Forest. Cradled at the base of the Udzungwa Mountains, the Magombera is one of the most biologically diverse habitats in Africa. Many large mammals, birds, and reptiles inhabit the emerald woods, including elephants, waterbucks, buffaloes, bush pigs, wart hogs, aardvarks, porcupines, and three monkey species. Marshall himself has discovered a new species of chameleon here: the Kinyongia magomberae. An unusual mixture of East African trees normally not found together shade the forest floor. The canopy towers 100 feet above the ground.

Until recently, the Magombera carpeted about six square miles of mostly flat land in the valley. But in the past three decades, half of the forest plain has been cleared, primarily for farming. The jungle that remains has been seriously degraded – selectively logged for construction timber – leaving gaping holes in the high, green canopy.

Carl de Souza/AFP/Getty Images Parts of the Western Amazon rainforest in Brazil have suffered heavy deforestation in the past year as an economic crisis has contributed to illegal logging.

Marshall wants to patch Magombera’s wounds. The unnatural holes in the forest’s fabric lessen the trees’ capacity to soak up and store carbon dioxide, the gas that’s warming the planet and turning the weather chaotic. Forest gaps also reduce the jungle’s suitability for some of its rare wildlife. If only he can cure this small woodland’s ills, Marshall says, his method might then revive millions more acres of unhealthy forest around the world – and perhaps make a significant contribution to slowing global warming. There’s “huge potential,” he says. He needs only one simple tool: a sharp machete.

Marshall’s method of stemming greenhouse gases – by pruning excessive undergrowth that prevents forests from flourishing – is one of a slew of quixotic ideas being worked on by scientists and researchers around the world to help solve what could be the dominant issue of the next 100 years.

While most of the attention in curbing global warming focuses on lowering emissions, many people are trying to solve the problem from the other side – by preserving the “lungs of the Earth” that absorb and sequester harmful gases. Though some of the initiatives may be more notional than forest-ready, experts believe it will ultimately take a host of different approaches to avert worsening superstorms and to keep rising seas from coursing through coastal cities from Miami to Ho Chi Minh City, Vietnam.

Every year humans disgorge 36 billion metric tons of carbon dioxide – almost enough to fill up all of the Great Lakes – out of tailpipes and smokestacks. Fortunately, only about half of this planet-insulating gas stays in the atmosphere. Otherwise, Earth would be warming at an even faster rate. Ocean water and vegetation on land absorb the other half, in equal parts. Forests alone soak up one-quarter of the torrent of CO 2 that people pump into the air. “We are talking about a free 25 percent emissions reduction,” says Scott Denning, a climate scientist at Colorado State University in Fort Collins. “It’s awesome.”

Preserving the health of forests is one of the best ways to slow global warming, says Professor Denning, especially in the band of productive tropical jungle that encircles the globe from the Amazon to Central Africa, through Southeast Asia and Indonesia. But humans are doing the opposite. They’re clearing these forests at a furious clip. In the thousands of years since humans discovered fire and invented agriculture and axes, people have chopped down, burned off, and cleared away a third of the woods that once carpeted the earth. The world has lost a forested area twice the size of the United States. After accelerating for centuries, the rate of forest loss has slowed slightly in recent decades. Still, every year loggers and farmers cut down a West Virginia-size area, almost all in tropical South America, Africa, and Asia.

In 2014, diplomats from 36 countries, including the US, many European nations, and Japan, signed the New York Declaration on Forests, an agreement intended to halt deforestation by 2030. They pledged to restore and reforest 865 million acres – an area larger than India – as well. That is a monumental logistical challenge. “2030 is only 12 years away,” says Stephen Elliott, a biologist at Thailand’s Chiang Mai University. Mr. Elliott, director of his university’s Forest Restoration Research Unit, is among the hundreds of scientists and policymakers around the world looking for ways to renew the vitality of land degraded by wholesale and selective logging, and protect the endangered woods that remain.

Daniel Grossman The Magombera Forest, one of the most biologically diverse habitats in Africa, lies at the base of the Udzungwa Mountains, shown here in the distance.

The world hears about advances in driverless car technology every day, and Elliott says the same autonomous navigation techniques might someday help to achieve the ambitious objectives of the New York Declaration. “I don’t think we can do an area the size of India by 2030 manually,” he says.

By “manually,” Elliott means how people restore forests today. He says that in Thailand, and in most other tropical countries, forest crews work with tools and apply techniques that would be familiar to their ancestors, “using Iron Age hoes and Stone Age baskets.” Sturdy farmworkers haul heavy hampers of nursery-raised saplings into clearings. They insert root balls into shallow holes cut through unyielding soil mats.

Such backbreaking work is expensive, even where labor is cheap. It’s slow, too. Farmers and ranchers already occupy the most accessible, easily worked parcels – flat areas near roads. Politically and economically, these plots are not open for reforestation. Roads don’t go where most of the available land is. Steep slopes, untamed rivers, and other obstacles also hinder access, multiplying the difficulty and expense.

Reforestation is “the only agricultural and horticultural activity that hasn’t been automated,” Elliott says. In 2015, he set out to change that, with help from autonomous drones. He invited an interdisciplinary group of 80 scientists and engineers from around the world to meet up in northern Thailand where he studies reforestation methods.

They bantered and brainstormed for four days about how drone squadrons might reconnoiter over restoration plots, pluck seed-laden fruit from treetops, shoot those seeds into the soil, and care for the seedlings that later emerged. Freewheeling discussions on how aerial robots could cut down fruit with mini chain saws, ferry this harvest in nets, and ward off rodents with urine-soaked cat litter, lived up to a conference slogan: “The craziest ideas are best.” “It was the best fun I’ve ever had,” says Elliott.

Carl de Souza/AFP/Getty Images Felipe Spina Avino, a conservationist with the World Wildlife Fund, uses a drone to map an area of rainforest in the Western Amazon region of Brazil.

Many researchers are withholding judgment about the potential for drone restoration of forests, though. Robin Chazdon, a biologist at the University of Connecticut in Storrs, says Elliott’s idea for robotic weeding “raised my eyebrows a bit.” Professor Chazdon edited a 2016 paper in the journal Biotropica where Elliott laid out his ideas. “There are a lot of issues that remain to be worked out,” she says. Not the least of these is how to induce air-dropped seeds to germinate and how to repel seed-hungry herbivores.

Other ways to preserve the carbon sequestering ability of forests focus on preventing the trees from being cut down to begin with. This isn’t easy, either. Any attempt to silence chain saws and the thwack of axes must answer to a litany of powerful interests craving new land and fresh wood. Farmers want more acreage for crops. Ranchers want new pastures. Developers want lots to build on. And both timber companies and small-scale loggers want lumber.

Farmers in the Hoima and northern Kibaale districts in western Uganda are clearing trees – mostly for subsistence farming and to sell wood for timber and fuel – faster than almost anywhere else on Earth. In 2011, Seema Jayachandran, an economics professor at Northwestern University in Evanston, Ill., began an unusual investigation: how to entice small landowners in Uganda into protecting land, not clearing it.

With collaborators in the US, Belgium, and the Netherlands, Professor Jayachandran recruited 1,099 Ugandan families for a study of whether modest cash rewards could sway them. Her results, published last July in the journal Science, has attracted worldwide attention from forest restoration experts.

In the past two decades, farmers in many countries have been offered such payments to refrain from clearing jungle. The idea has been tried in countries as far-flung as Vietnam and Costa Rica.

Scientists disagree about how well these efforts work. One problem has been that even if land clearing slows down after payments, how can researchers be sure that the reimbursements, and not other factors, caused the change? Moreover, skeptics suggest that payment programs might simply shift cutting to other locations, with no net benefit.

Jayachandran’s experiment was novel. Unlike previous reimbursement programs for forest protection that had been studied, landowners were selected at random either to receive payments or not, creating two groups for comparison. Half of the landowners received about $12 per year for each acre they left alone. Through a local conservation group, the scientists spot-checked parcels. The research team also monitored forest cover with high-resolution satellite photos.

Jayachandran started the experiment doubting that payments could substantially reduce tree cutting. So she was surprised when a research assistant emailed her a table with preliminary findings. “Wow, this program is having a big impact,” she thought. Later verification proved that the small payments had slowed cutting substantially. People who received no money cleared 9 percent of their land in two years of observation. People paid to leave their land untouched cleared only 4 percent. The program had reduced deforestation by more than 50 percent.

Andrew Marshall/WENN.com/Newscom Australian biologist Andy Marshall discovered this new species of chameleon in the Magombera Forest in Tanzania.

Moreover, the team showed that the program was an economical way to fight global warming. Trees owned by families who received the program’s cash rewards stored 250 tons per year more CO 2 than woodlands owned by neighbors who had not chosen to receive the payments, at a cost of $105. The climate benefit could be ephemeral, unless payments continue. Still, Jayachandran has calculated that paying Ugandans to protect their land in perpetuity costs much less than putting up a solar panel or a windmill in the US with an equivalent CO 2 reduction.

Alex Pfaff, an economics professor at Duke University in Durham, N.C., praises the Uganda study for proving that “there is potential” for reducing deforestation by paying landowners not to cut trees. But he doesn’t think that means the idea will succeed everywhere. He says payments work best where a lot of forest is being cut, but not at great profit (such as in hilly terrain), and where somebody carefully monitors compliance. Otherwise, he says, people who have no plans for clearing forest might get paid for doing nothing. Or, conversely, they might get paid and then chop down forest anyway.

Professor Pfaff studied one of the world’s longest-running attempts to protect tropical forests with cash rewards, Costa Rica’s Payment for Environmental Services program. Like several other experts who’ve examined Costa Rica, he found that two decades of payments – totaling tens of millions of dollars – have saved very little forest. The payments “didn’t do much.” Deforestation declined, he says, but for other reasons.

In the Tanzanian wild, Marshall wants forests to heal themselves, with only a little help from humans. He’s cutting skeins of lianas, a variety of vine with a woody stalk – the “kind that Tarzan swings on,” says Marshall.

Gordon Wiltsie/National Geographic/Getty Images/File A Yanayacu Indian swings on a vine hanging from a massive tree near Iquitos, Peru, in November 2009.

They are native to the Magombera woods and other degraded forests that Marshall hopes to help. They often proliferate after a logger makes a clearing – stymieing regrowth of some of the world’s most lush forests and preventing trees from playing their role as Earth’s burial ground for carbon.

Marshall notes that between one- and two-thirds of all tropical forest land on the planet has suffered some form of abuse. Careful tending of ailing patches could substantially boost tree productivity. He is determined to prove his case in the Magombera.

He steps from the sugar cane plantation where he parked, baking under a sulfurous sun, into the forest. In only a few paces, the air cools noticeably and the light dims. The scientist and three helpers form a single file as they tramp deeper into the woods. Marshall wears a pair of scuffed black boots. Dirt from weeks of fieldwork clings to his pants.

“Ants!” one of the villagers yells in Swahili. A column of driver ants marches across the path. The size of a rice grain, a driver ant can bite with powerful jaws. The team starts jogging, stomping the ground with each step to prevent the insects from clinging to their shoes.

Over the years, Marshall has learned to identify most of the 500 plants in Magombera by their fruit, leaves, and flowers. “Sniff this,” he says, scraping bark from a sapling. “It smells like raw carrots.”

The four arrive at a red plastic triangle standing atop a stake in the soil. It marks a corner of one of Marshall’s 40 research plots. In a tennis-court-size clearing, a twisted sapling attracts his attention. It had grown straight like a tree then doubled over vinelike back toward the ground. What is it?

Sometimes lianas look like trees. He snaps a pencil-thick branch in two. It’s a tree, Xylopia holtzii, he announces. Liana branches are stringy and don’t break cleanly.

The distinction between liana and tree is central to Marshall’s research on how to revive degraded jungle areas such as this opening in the woods. Locals probably cleared the trees decades ago. A thick mass of leaves growing on coiled liana stalks carpets the glade now. The green, living lid shades everything below, Marshall says, stalling forest regrowth. “You can ima­gine what a tree has to go through to break through that.”

Research elsewhere grounds Marshall’s project. Scientists have long known that vines slow forest growth. More recently, biologists have put numbers on how much more carbon vine-free forests contain.

Stefan Schnitzer, a biology professor at Marquette University in Milwaukee, says that lianas ascend into forest heights, freeloading on the scaffolding of carbon-storing tree trunks. The size of their crowns often far exceeds that of the trees themselves. A group of gluttonous guests who refuse to leave once they’ve arrived, lianas suffocate the trees that host them. But liana trunks, far less hefty than those of trees, store pitifully little CO 2 , making forests webbed with the vines less efficient in hoarding the planet-warming gas.

Several years ago, Professor Schnitzer and five workers severed the trunks of every liana in 12 acres of a forest in Panama. Freed of shading and strangling vines, the trees bulked up. “It was stunning,” says Schnitzer. Three years into the experiment, the liana-free jungle was accumulating carbon in its trunks and leaves nearly twice as fast as nearby unpruned woods.

Marshall proposes putting such findings to practical – and, eventually, extensive – use. He recently published the results of his own pilot study in the Magombera Forest. He’d clipped lianas in an area of degraded jungle the size of a suburban front yard. The trees burgeoned. After five years the same land had stored eight times as much carbon as nearby control areas. Marshall says his initial results suggest that cutting lianas in a degraded forest is as effective for sequestering carbon as reforestation. And killing lianas costs 1/50th as much, he says.

Widespread liana removal awaits lar­ger-scale and longer-term trials. Schnitzer, whose research inspired Marshall, agrees that liana removal could help degraded forests store more carbon. But he fears unintended side effects of industrial-scale liana trimming. “You’re killing part of the community and you don’t know what the ramifications are,” he says.

On a walk through his research plots in Panama earlier this year, Schnitzer described an example. His colleague Steve Yanoviak, a biology professor at the University of Louisville in Kentucky, noticed that the population of one ant species increased after crews had cleared lianas.

Professor Yanoviak speculates that anteaters, predators of this arboreal ant, can’t climb into canopies to raid the insects’ nests without lianas. Even bigger impacts on denizens of the woods might remain to be discovered. This is to say nothing of the logistics of dispatching machete-swinging forest workers into every square inch of the world’s jungles. As a result, Schnitzer argues that efforts at slowing deforestation make more sense than trimming woodlands like an arboreal hedge.

Relaxing after a tiring day crawling through thickets, Marshall talks expansively about his hopes for someday thinning liana tangles far beyond the Magombera Forest. He is still smarting from an ant bite and a nasty thorn snag. Researchers often dodge degraded forests, he says, because they’re hotter and choked with undergrowth. “It’s an inhospitable place for us big humans.”

Marshall, like Schnitzer, is uneasy about the idea of widespread liana cutting. Yet he notes that no solution will be simple or without trade-offs. Replanting costs a lot. Entrenched interests resist controls on deforestation.

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Last June Marshall received a $900,000 grant from the Australian government to fund his large-scale trials. He’s now poised to turn his largest patch yet of viney jungle into a healthy forest again. “The potential carbon gain,” he says, “is colossal.”

Reporting for this story was supported by the Frank B. Mazer Foundation and the Pulitzer Center on Crisis Reporting.