Brazil — A few months ago, a team of scientists walked into a stretch ofAmazon forest and purposely burned it. The scientists want to understand howburning forests contribute to climate change and they want to know how toslow or stop those fires.
But fire is everywhere in the Amazon. It’s the way people clear land foragriculture and livestock. It also burns down a lot of forest that isn’t meantto be cleared.
A patch of isolated forest in the middle of a giant soybean farm on thesouthern edge of the Amazon basin was set aside just for this experiment.Ecologist Dan Nepstad of the Woods Hole Research Center in Massachusetts is theproject’s mastermind. He has a theory.
“Fire begets fire,” he says. “Once a fire goes through aforest, trees die. It becomes more susceptible to further burning.”
And further burning eventually destroys the forest.
But there’s much more at stake than just these forests. What happens to themaffects both the local and the Earth’s climate. Nepstad says the more theforests burn, the more carbon goes up into the atmosphere. That contributes toglobal warming. At the same time, global warming seems to be making the Amazondrier and more prone to burning. And Nepstad says the world can’t do withoutthese forests.
“The Amazon is like a giant air conditioner,” he says as his crewstarts lighting the leaf litter along the edges of the fire lines. It’s like theway people sweat when they’re hot. Moisture is drawn up through the trees andleaves and then into the air, which cools the atmosphere. But the forest can’tdo that very well during droughts. And global warming could make droughts worse.
That’s why 30 researchers are now darting though this smoking forest. They’vecovered their faces with bandanas and carry notebooks. For three days they’llset fires, map how the fires spread, count the trees that die and those thatlive. Some crew members are local farmhands, others come from Americanuniversities.
Jennifer Balch from Yale University’s School of Forestry is in charge of theburn this year. She reassures the crew that she’s well prepared lots ofpencils and snake bite kits.
Crewmembers hoist fuel canisters onto their backs. The fuel is forced into aflaming wand. They drag the wand through the leaf litter and the fire catches,crawling across the forest floor. Balch and her team race ahead of the fire.They collect dead leaves, measure their moisture content and calculate windspeed and relative humidity.
They also want to know how much carbon is in the leaf litter and deadbranches the “biomass” that a fire consumes. Half the biomass iscarbon. They calculate how much biomass is in the plot before the burn, thenagain afterwards.
“The difference,” says Balch, “gives us the amount of carbonthat goes from these types of under-story fires immediately to the atmospherethrough combustion.”
All told, it’s a lot of carbon. A big tree here stores about 1.3 tons ofcarbon. Worldwide, the cutting and burning of forests contributes aboutone-fifth of the carbon going into the atmosphere.
The fire burns overnight a surprise to the scientists. Usually thehumidity rises at night and puts the fires out. In other places the flames dostop, though. Sometimes it’s as simple as an ant trail a natural fire-breakcreated by leaf-cutter ants that sweep the ground as clean as new asphalt.
After three days, it’s clear the fires are burning hotter and faster thanever before, especially in a plot that was burned once three years ago and thenleft alone. Nepstad is pleased with the results as much as he can be as hewatches a forest burn.
Fire begets more fire, just as he suspected. And one reason is the grass.
Where grass has invaded the forest, the fire explodes. Fleeing insects smackyou in the face. Hawks gather along the edge of these hot fires, waiting topounce on escaping animals.
Nepstad says grass follows people, when they log the forest or burn it tomake room for crops.
“All these things converge into what we call a brush-land scenario,”Nepstad says. “We believe by the year 2030, half the Amazon will be clearedor severely damaged by either drought, fire or logging.”
And if the brush-land scenario comes true, he adds, that means more fireswill affect the Earth’s climate. “If we have 250 billion tons of carbon inthese ecosystems stored,” he says, “and they’re currently a fifth ofthe annual flow of carbon to the atmosphere, that’s going to get worse. It couldbe big enough to undo the gain we get from lowering emissions from industry andcars elsewhere in the world.”
Yet the scientists here know they can’t eliminate fire in the Amazon. It’s atool that people have always used to sculpt their environment, Balch says.
“It can be argued that human evolution was facilitated by capturing fireas a tool,” she says.
Brazilian law requires landowners in the Amazon to keep up to 80 percent oftheir land in forest. But out-of-control fires eat up a lot of that. The task inthe Amazon is Promethean: figure out how people can harness fire to clear theland they need, but keep fire out of protected forests. Otherwise, fire willconsume the Amazon and change the climate.