RENEE MONTAGNE, host:

A few months ago, a team of scientists walked into a stretch of Amazon forest and set it on fire. The scientists wanted to understand how burning forests contribute to climate change and they want to know how to slow or stop those fires.

Every year, huge tracks of the Amazon go up in smoke. And as part of our NPR and National Geographic series Climate Connections, NPR's Christopher Joyce joined the scientists for an up-close look at how the Amazon burns.

CHRISTOPHER JOYCE: The patch of forest sits isolated in the middle of a giant soybean farm on the southern edge of the Amazon basin. It's on fire, and I'm in the middle it.

That is leaf litter burning fast and hot about three yards from me. And as the wind picks up a little bit, it's going to explode up into the canopy. And I'm getting a little toasty here, even with my bandana over my face, snake boots on my legs 'cause of the snakes come roaring out of this stuff. But as you can hear, it's crackling pretty hot.

I'm standing on a fire line, a cleared path through the forest that the fire hopefully won't cross. Rich McCorney(ph) from the Woods Hole Marine Biological Lab in Massachusetts suggests I keep moving.

Mr. RICH McCORNEY (Researcher, Woods Hole Marine Biological Laboratory, Massachusetts): Don't go back up that line. That line's dangerous now. There's a lot of fuel that accumulates along the edges of the lines, and that downed fuel can really ignite rapidly.

JOYCE: Fire is everywhere here. It's the way people clear land for agriculture and livestock, and it also burns down a lot of forests that isn't meant to be cleared. Ecologist Dan Nepstad at the Woods Hole Research Center in Massachusetts is the mastermind of this experiment. He's been burning parts of this forest for years. He has a theory.

Mr. DAN NEPSTAD (Senior Scientist, Woods Hole Research Center): Fire begets fire. Once a fire goes through a forest, trees die. It becomes more susceptible to further burning.

JOYCE: And further burning eventually destroys the forest completely. But there's much more at stake here than just these forests. What happens to them affects both the local and the world's climate. Nepstad says the more the forests burn, the more carbon goes up into the atmosphere. That contributes to global warming. And at the same time, that warming seems to be making the Amazon drier and more prone to burning. And then Nepstad says the world can't do without these forests.

Mr. NEPSTAD: The Amazon is like a giant air conditioner. Really, you know, when we're - our skin gets wet when we're hot, it evaporates, it cools us off. And that's just what the Amazon forest does, especially in the eastern zone where there's this severe seasonal droughts that could get worse with global warming.

JOYCE: That's why 30-some researchers are darting through this smoking forest. They covered their faces with bandanas and carry notebooks. For three days, they set fires, map how the fire spread, count the trees that die and those that live. Some crew members are local farmhands; other come from American universities.

Ms. JENNIFER BALCH (Researcher, Yale School of Forestry and Environmental Studies): Hi, my name is Jennifer Balch, and I'm a doctoral student at the Yale School of Forestry. This is the last burn before I finish up.

JOYCE: Balch is in charge of the burn this year. She reassures the crew that she is well prepared, lots of pencils and snake bite kits.

Ms. BALCH: They really only work in the first five minutes of a bite. After that, we get in a car and go to have the anti-venom.

(Soundbite of footsteps)

JOYCE: Balch and her team race ahead of the fire. They collect dead leaves and measure their moisture content. They calculate wind speed and relative humidity. They also want to know how much carbon is in the leaf litter and in dead branches, the biomass that the fire consumes.

Ms. GINA CARDENOU(ph) (Scientist): I'm Gina Cardenou. I'm a scientist. Before the fire, we met with the biomass, and the biomass can give us how much carbon is in these trees and woods in the ground.

JOYCE: Then after the fire has burned through, Balch takes stock of what survives the burn.

Ms. BALCH: And the difference between those measurements gives us the amount of carbon that goes from these — types of unnecessary fires, immediately to that atmosphere through combustion.

JOYCE: All told, it's a lot of carbon. A big tree here stores about 1.3 tons of carbon. Worldwide, the cutting and burning of forests contributed about one-fifth of the carbon going into the atmosphere. In some places, the fire burns through the night, a surprise to the scientists. Usually the humidity rises at night and slows down fires. In other places, the fires die out. Sometimes, it's as simple as an ant trail. Natural fire breaks created by leafcutter ants that sweep the ground as clean as new asphalt.

So, Gina Cardenou and her fire-starters come back to restart some fires. The workers straps on a heavy canister of fuel.

Ms. CARDENOU: We light the gas and walk around the line and put the fire on the ground.

JOYCE: After three days, it's clear the fires are burning hotter and faster than ever before, especially in a plot that was burned once three years ago, and then left alone. Nepstad is pleased with the results as much as he can be as he watches a forest burn. Fire begets more fire, just as he suspected.

(Soundbite of forest fire)

JOYCE: One reason is the grass. Where grass has invaded the forest, the fire explodes, fleeing insects smack you in the face. Hawks patrol the edges, awaiting to pounce on escaping animals. Then Nepstad says grass follows people when they log the forest or burn it to make room for crops.

Mr. NEPSTAD: All of these things are converging in what we call a brushland scenario. We believe that by the year 2030 half of the Amazon will be either cleared or severely damaged by either drought, fire or logging.

JOYCE: And if the brushland scenario comes true, the Earth's climate will notice.

Mr. NEPSTAD: So if we've got 250 or so billion tons of carbon stored in these ecosystems - let's say maybe a fifth of the annual flow of carbon to the atmosphere that's going to get worse. It could be big enough, I think, to undo the gains we make in lowering emissions from industry, from transportation elsewhere in the world.

JOYCE: Yet the scientists here know they can't eliminate fire in the Amazon. As Jennifer Balch observes it's a tool that people have always used to sculpt their environment.

Ms. BALCH: It can be argues that part of human evolution was facilitated in part by our being able to capture fire as a tool. And the fuel that we put into our cars is just another extension of our being with a capture of this phenomenon of combustion to make it for our own uses.

JOYCE: Brazilian law requires landowners in the Amazon to keep up to 80 percent of their land in forest. But out-of-control fires eat up a lot of that. The task in the Amazon is promethean; figure out how people can harness fire to clear the land they need, but keep fire out of protected forests. Otherwise, fire will consume the Amazon and change the climate.

Christopher Joyce, NPR News.

(Soundbite of music)

MONTAGNE: In the Amazon, fire is also used as a weapon to force ranchers off their land. Hear that story at npr.org/climateconnections. You can also get the latest climate change coverage from National Geographic magazine.

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