Chad Deering trudges up a dry river channel on the north side of Rincón de la Vieja, one of Costa Rica’s active volcanoes. He wears a baseball cap emblazoned with the phrase Semper Fi, a token of his tour of duty with the Marines, and lugs a peculiar apparatus, part of a sensitive gas-testing kit, that looks more like a metal mixing bowl. The bedrock here is smooth lava, a lifeless tear in the rainforest that blankets Rincón de la Vieja’s flanks.

Along with two teams of scientists, Deering is pursuing not potential volcanic drama but something imperceptibly gradual — carbon dioxide seeping invisibly from cracks in the volcano’s foundation and exposing the surrounding environment. The question is whether that elevated exposure is a positive, a negative or neither — and what it might mean for the fate of tropical forests globally.

The stability of the world’s climate depends in part on these areas.

Every year, tropical forests soak up more than 2 billion tons of carbon dioxide, a substantial share of what’s emitted by power plants, industrial smokestacks and vehicle exhaust pipes. Yet how increasing temperatures and decreasing rainfall will affect them long term remains unclear.

While many climate scientists believe that tropical forests will begin to absorb progressively less CO2, other research suggests that higher concentrations of the gas could actually protect them, an idea dubbed carbon fertilization.

In Costa Rica’s natural laboratory, a dense, steamy tangle in the country’s northwest corner, the teams hope to get closer to the answer. The issue is "one of the biggest uncertainties in climate projections of the fate of the planet," says NASA scientist Josh Fisher, the ecologist leading the trek. He believes the study "could be a game changer."

If extra carbon dioxide revs up Rincón de La Vieja’s jungle, the teams should find bigger trees, more carbon-dense species or some combination where gas levels are particularly high. One group is working on the volcano’s wetter north side and the other on its drier south side, to better assess and then compare two different ecosystems.

But how to tease out other conditions that also can affect tree growth and the species mix, including altitude and soil moisture? U.S. Forest Service biologist Michael Keller, a tropical forests expert who, though unaffiliated with the project, is following it closely, says such confounding factors make the research a "high risk" experiment.

Still, he considers it a creative approach to the urgent problem of forecasting the tropical "carbon sink."

Research needs data. And on this day, the north-flank team has hit a snag getting it. The river bed has become a deep canyon that abruptly ends beneath sheer walls. The one woman and five men huddle over a computer tablet with a high-resolution map of their intended route.

Deering, a volcanologist from Michigan Technological University, stabs a finger swollen with bug bites at a spot tantalizingly close but inaccessible. "I want to be here," he says.

A crazy idea

Another NASA scientist had been the first one to propose studying carbon fertilization on a tropical volcano’s shoulders. Several years earlier, Florian Schwandner had helped the Philippines set up a successful network for detecting early symptoms of eruptions of 8,000-foot Mount Mayon, with sensors to track the flow of carbon dioxide from faults in its foundation. (One telltale sign of an oncoming eruption is when that flow suddenly increases.)

At the space agency’s Jet Propulsion Lab in California, the volcanologist hoped satellite-based measurements of carbon dioxide releases would provide early warnings around the world. His research group was filled with ecologists and frequent discussion of trees’ carbon sink, although nobody knew how to forecast the sink’s future.

A certain kind of experiment often came up in conversation: spraying extra carbon dioxide into a forest parcel to study how trees respond. Such carbon-enhancement trials had been run often in the United States and in other temperate regions and had shown that extra carbon dioxide sometimes increased forest growth.

The studies’ relevance for tropical forests was uncertain, but the huge logistical costs of trying to replicate them in remote equatorial areas had been prohibitive.

An alternative solution dawned on Schwandner in 2016. The constant low-level discharges of carbon dioxide from volcanoes might bathe surrounding forests in enough gas to run an enhancement experiment "for free." He emailed Fisher, proposing a "compellingly crazy carbon fertilization idea." Four years later, with funding from NASA, it was finally a go.

Schwandner, Fisher, and several other scientists and graduate students recruited for the project spent months scouring geological studies and satellite images of Costa Rica, hunting for faults and vents where the 6,286-foot-tall Rincón de la Vieja might be exhaling CO2 onto its rainforest carpet. They pinpointed 16 likely regions.

Stewing in CO2

Deep in the jungle, Deering’s team has doubled back, retracing their steps along the river bed and away from the canyon walls. They soon discover a trail near the spot he pointed out. Their local guide, a botanist, says a tapir probably made it foraging for fruit and leaves.

Deering and graduate student Jacob Bonessi are taking dozens of CO2 measurements daily. They stop not far from a pile of fresh tapir dung. Deering tightly clamps the metal chamber he carries onto a patch of damp jungle soil. An umbilical cord of hoses channels soil exhalations into the apparatus on his back. Buzzing over bird calls, a pump inside the case draws the gas into an instrument that computes the concentration of carbon dioxide wafting up from the ground.

The pair gaze for a few minutes at the forest’s emerald palms and twined strangler figs. A troop of howler monkeys can be heard in the distance.

Bonessi checks the reading, displayed on a tablet linked by Bluetooth to the electronics on Deering’s back.

"What you got?" asks Deering. "One point one four six," Bonessi answers. "Big time!"

Deering whoops his enthusiasm. The number is one of the highest they’ve seen.

All over the planet, soil exudes carbon dioxide. It’s a waste product that microbes and subterranean fauna churn out while generating energy from oxygen and nutrients. But what the scientists have detected is well above the background level seeping from the soil here. This is the type of spot, infused with extra carbon dioxide from the volcano’s fractured rock, they were looking for. The trees here are stewing in it.

The team — traveling only with small backpacks stuffed with lunch, gear for measuring trees and bug repellent — heads to another targeted destination a few minutes uphill. Fisher dons a pair of snake-resistant chaps after a close call with a rattlesnake. Fina Soper, an ecologist and professor at McGill University, wears a custom neckerchief to protect against mosquitoes and ticks. "Badass Biogeochemists," it reads.

At each stop, they record the diameter of all trees bigger than a sapling inside a plot the shape and size of a soccer pitch’s center circle. Soper struggles one afternoon with an uncooperative tape measure, a special forester’s rule purchased just for this trip. She loops the metal ribbon around a trunk as broad and true as a Greek temple’s column. But the band won’t retract.

"It figures that I’d break the most low-tech device I’ve used in my life," she mutters, tugging on loops of the snarled steel.

By gathering detailed observations from many sites — each exposed to a unique combination of influences — she and the others are trying to account or control for the factors that influence tree heft. They’ll then tease out the effect of each factor, especially the one that concerns them most: greater carbon dioxide.

Fisher hopes to vastly ramp up their observations, if this initial expedition pans out, with return trips using one of the most advanced drones flown by NASA. Meanwhile, they painstakingly probe Rincón de la Vieja’s secrets. Ten days of slashing and slogging will yield the diameters of 952 trees between the two groups of scientists. Back home, they’ll calculate the mass of carbon stored in the wood of each plot using standard formulas.

"It’s good," Fisher says halfway through the expedition. "Everyone’s healthy. Everyone’s happy. Equipment is working." But as he well knows, a technical problem could upend the good fortune at any time. And then, he adds with a laugh, "We’ll all be fighting with each other. And everything will go to hell."