Over just a few decades in the mid-20th century, this small country chopped down a majority of its ancient forests. But after a huge conservation push and a wave of forest regrowth, trees now blanket more than half of Costa Rica.

Far to the south, the Amazon forest was once being quickly cleared to make way for farming, but Brazil has slowed the loss so much that it has done more than any other country to limit the emissions leading to global warming.

And on the other side of the world, in Indonesia, bold new promises have been made in the past few months to halt the rampant cutting of that country’s forests, backed by business interests with the clout to make it happen.

In the battle to limit the risks of climate change, it has been clear for decades that focusing on the world’s immense tropical forests — saving the ones that are left, and perhaps letting new ones grow — is the single most promising near-term strategy.

That is because of the large role that forests play in what is called the carbon cycle of the planet. Trees pull the main greenhouse gas, carbon dioxide, out of the air and lock the carbon away in their wood and in the soil beneath them. Destroying them, typically by burning, pumps much of the carbon back into the air, contributing to climate change.

Over time, humans have cut down or damaged at least three-quarters of the world’s forests, and that destruction has accounted for much of the excess carbon that is warming the planet.

But now, driven by a growing environmental movement in countries that are home to tropical forests, and by mounting pressure from Western consumers who care about sustainable practices, corporate and government leaders are making a fresh push to slow the cutting — and eventually to halt it. In addition, plans are being made by some of those same leaders to encourage forest regrowth on such a giant scale that it might actually pull a sizable fraction of human-released carbon dioxide out of the air and lock it into long-term storage.

With the recent signs of progress, long-wary environmental groups are permitting themselves a burst of optimism about the world’s forests.

“The public should take heart,” said Rolf Skar, who helps lead forest conservation work for the environmental group Greenpeace. “We are at a potentially historic moment where the world is starting to wake up to this issue, and to apply real solutions.”

Still, Greenpeace and other groups expect years of hard work as they try to hold business leaders and politicians accountable for the torrent of promises they have made lately. The momentum to slow or halt deforestation is fragile, for many reasons. And even though rich Western governments have hinted for years that they might be willing to spend tens of billions of dollars to help poor countries save their forests, they have allocated only a few billion dollars.

Around the world, trees are often cut down to make room for farming, and so the single biggest threat to forests remains the need to feed growing populations, particularly an expanding global middle class with the means to eat better. Saving forests, if it can be done, will require producing food much more intensively, on less land.

“For thousands of years, the march of civilization has been associated with converting natural ecosystems to crops that serve only man,” said Glenn Hurowitz, a managing director at Climate Advisers, a consultancy in Washington.

“What’s happening now is that we are trying to break that paradigm. If that succeeds, it’s going to be a major development in human history.”

A Remarkable Comeback

Deep inside a Costa Rican rain forest, white-faced capuchin monkeys leapt through the tree tops. Nunbirds and toucans flew overhead, and a huge butterfly, flashing wings of an iridescent blue, fluttered through the air.

Ignoring the profusion of life around him, Bernal Paniagua Guerrero focused his gaze on a single 20-foot tree, placing a tape measure around the spindly trunk and calling a number out to his sister, Jeanette Paniagua Guerrero, who recorded it on a clipboard.

With that, the tree, a black manú just over two inches in diameter, entered the database of the world’s scientific knowledge. Its growth will be tracked year by year until it dies a natural death — or somebody decides to chop it down for the valuable, rot-resistant wood.

The Paniaguas and their co-worker, Enrique Salicette Nelson, work for an American scientist, Robin Chazdon, helping her chronicle a remarkable comeback.

Cuatro Rios, the forest they were standing in one recent day, looked, to a casual eye, as if it must have been there forever. Trees stretched as high as 100 feet, and a closed canopy of leaves cast the understory into deep shade — one hallmark of a healthy tropical rain forest.

In fact, the land was a cattle pasture only 45 years ago. When the market for beef fell, the owners let the forest reclaim it. Now the Cuatro Rios forest, near the tiny village of La Virgen, is a study plot for Dr. Chazdon, an ecologist from the University of Connecticut, who has become a leading voice in arguing that large-scale forest regrowth can help to solve some of the world’s problems.

Indeed, forests are already playing an outsize role in limiting the damage humans are doing to the planet.

For the entire geologic history of the earth, carbon in various forms has flowed between the ground, the air and the ocean. A large body of scientific evidence shows that the amount of carbon in the air at any given time, in the form of carbon dioxide, largely determines the planet’s temperature.

The burning of coal, oil and natural gas effectively moves carbon out of the ground and into the active carbon cycle operating at the earth’s surface, causing a warming of the globe that scientists believe is more rapid now than in any similar period of geologic history.

Though the higher temperatures are causing extensive problems, including heat waves and rising seas, the increasing carbon dioxide also acts as a sort of plant fertilizer. The gas is the primary source of the carbon that plants, using the energy of sunlight, turn into sugars and woody tissue.

Scientific reports suggest that 20 percent to 25 percent of the carbon dioxide that people are pumping into the air is being absorbed by trees and other plants, which keep taking up more and more even as human emissions keep rising.

But when people damage or destroy forests, that puts carbon dioxide into the air, worsening the warming problem. Historically, forests have been chopped down all over the planet. Now they are actually regrowing across large stretches of the Northern Hemisphere, and the most worrisome destruction is occurring in relatively poor countries in the tropics.

Scientists concluded decades ago that deforestation must be stopped, both to limit climate change and to conserve the world’s biological diversity. These days, they are also coming to understand the huge potential of new or recovering forests to help pull dangerous emissions out of the air.

“Every time I hear about a government program that is going to spend billions of dollars on some carbon capture and storage program, I just laugh and think, what is wrong with a tree?” said Nigel Sizer, director of forest programs at the World Resources Institute, a think tank in Washington. “All you have to do is look out the window, and the answer is there.”

Scientists are still trying to figure out how much of a difference an ambitious forest regrowth strategy could make. But a leading figure in the discussion — Richard A. Houghton, acting president of the Woods Hole Research Center in Massachusetts — has argued for turning some 1.2 billion acres of degraded or marginally productive agricultural land into forests.

That is an exceedingly ambitious figure, equal to about half the land in the United States. But researchers say it would be possible, in principle, if farming in poor countries became far more efficient. Some countries have already pledged to restore tens of millions of acres.

Dr. Houghton believes that if his target were pursued aggressively, and coupled with stronger efforts to protect existing forests, the rapid growth of carbon dioxide in the atmosphere could be slowed sharply or possibly even halted.

That, he believes, would give the world a few decades for an orderly transition away from fossil fuels. “This is not a solution, but it would help us buy some time,” Dr. Houghton said.

Finding an Effective Tactic

The Amazon, spreading across nine countries of South America, is the world’s largest tropical forest. The majority of the Amazon is in Brazil, which for decades treated it as a limitless resource.

Sometimes aided by United States government funding for development, Brazil encouraged road construction that effectively opened the forest to settlement, including illegal land grabs. Crews harvested select trees for timber and then cut or burned the rest to make room for cattle ranching and soybean farming.

Deforestation was so rampant that by the middle of the last decade, 17 percent of the Amazon had been cut, and millions more acres had been damaged. Environmental groups worldwide sounded the alarm, as did indigenous and traditional peoples whose ancestors had lived in the forest for thousands of years.

As deforestation hit a peak in 2004, the Brazilian government came under international condemnation, and it began trying to halt the destruction. In 2006, environmental groups found a way to bring marketplace pressure to bear.

Crops grown on deforested land, notably soybeans, were being used to produce meat for Western companies like McDonald’s, creating a potential liability in the eyes of their customers. Greenpeace invaded McDonald’s restaurants and plastered posters of Ronald McDonald wielding a chain saw. That company and others responded by pressuring their suppliers, who imposed a moratorium on products linked to deforestation.

The Brazilian government used satellites to step up its monitoring, cut off loans to some farmers in counties with high deforestation rates, and used aggressive police tactics against illegal logging and clearing. Brazilian state governments and large business groups, including some beef producers, joined the efforts.

The intense pressure resulted in a sharp drop in deforestation, by 83 percent, over the past decade. Moreover, the Brazilian ministry of agriculture began to focus on helping farmers raise yields without needing additional land.

Not only were millions of acres of forest saved, but the carbon dioxide kept out of the air by Brazil’s success also far exceeded anything any other country had ever done to slow global warming. Norway put up substantial funds to aid the effort, but otherwise, Brazil did it without much international help.

With so little money from abroad, the gains in Brazil are considered fragile, especially if a future government were to lose interest in forest protection. Daniel C. Nepstad, an American forest scientist who has worked in Brazil for decades and now heads a group called the Earth Innovation Institute, said, “We could still see a huge slide backward.”

The Next Big Test

With deforestation somewhat under control in Brazil, Indonesia is becoming a big test of the environmental groups’ strategy.

Deforestation is rampant there, with people chopping down even national forests with impunity. The biggest reason is to clear land for the lucrative production of vegetable oil from the fruit of a type of palm tree.

Just a handful of companies sell the oil — used in a wide array of consumer goods like soap, ice cream, confections and lipstick — into global markets, and the environmental groups have been targeting these big middlemen. Companies controlling the bulk of the global palm-oil trade have recently signed no-deforestation pledges, and Indonesia’s influential chamber of commerce recently threw its weight behind a demand for new forest legislation in the country.

But even if Indonesia takes strong action, there are fears that the gains could prove fleeting. The economic incentive to chop down forests remains powerful, and crackdowns on deforestation can just spur profiteers to go elsewhere.

“Asian companies are rushing into Africa and grabbing as much land as possible,” said Mr. Hurowitz, of Climate Advisers. “That’s kind of scary.”

Still, with hopes running high that the world may finally be rounding a corner on the deforestation problem, attention is turning to the possibility of large-scale forest regrowth.

Dr. Chazdon believes strongly in halting deforestation, but she says that many of the plots of old-growth forest that have already been saved are too small to ensure the long-term survival of the plants and animals in them. Forest expansion onto nearby land could help to conserve that biological diversity, in addition to pulling carbon dioxide out of the air.

But the strategy presents many challenges. It will require abandoning marginal agricultural land, meaning the remaining farms will have to become more efficient to keep up with demand for food, as well as a growing demand for biofuels. And some scientists have warned that if the strategy is poorly executed, agriculture could merely be pushed away from forests into grasslands or savannas, which themselves contain huge amounts of carbon that could escape into the atmosphere.

Costa Rica, a “green republic” famous worldwide for its efforts to protect forests, shows how difficult a forest restoration strategy can be in practice.

Legal protection is minimal for much of the forest that has grown there in recent decades. The workers who help Dr. Chazdon track her plots often see telltale signs of illegal hunting and logging, and they say the authorities are lax about stopping it. “So many ugly things happen that we just lose a little faith,” said Mr. Paniagua, one of the workers.

Moreover, a wave of pineapple production to supply a growing world market is sweeping the country, tempting many owners to reclear their land. Growing Chinese demand, in particular, has raised the fear that “the whole of Costa Rica will be paved in pineapples,” said Carlos de la Rosa, director of La Selva Biological Station, a famed research outpost where Dr. Chazdon does much of her work.

But for now, the second-growth forests of Costa Rica, covering roughly 14 percent of the land area of the country, at least show what may be possible if the world gets more ambitious about tackling global warming. Brazil, too, is beginning to see regrowth on a large scale in the Amazon, and is spending millions to restore forests along its Atlantic coast.

Decades of watching the Costa Rican forests recover have taught Dr. Chazdon that, at least in areas that still have healthy forests nearby to supply seeds, the main thing human beings need to do is just get out of the way. After all, forests were recovering from fires and other natural calamities long before people ever came along to chop them down.

“The forests know how to do this,” Dr. Chazdon said. “They’ve been doing it forever, growing back.”