TANGURO RANCH, Brazil -- On an Amazon plantation the size of Cape Cod, a Massachusetts forest ecologist and his team are setting the rain forest on fire.
After measuring the humidity of the air and the density of the forest canopy, workers pour kerosene in neat trails and torch the underbrush. Researchers, their brows dripping with sweat, measure the height and width of the flames and later determine how far into the forest the fire traveled.
A month later, the team counts the remaining trees to determine how many died.
The big surprise in initial burns, said the lead researcher, Daniel Nepstad of the Woods Hole Research Center in Falmouth, is "that quite a lot of big trees survive. That's good news: this is a tough forest."
The bad news is that fire-promoting droughts have become increasingly common here, taking a terrible toll on the rain forest -- and eventually, Nepstad and many other scientists believe, on the climate of the rest of the world.
Whenever a tree dies and decays, its carbon is taken up by microbes and other organisms in the soil and eventually released as the greenhouse gas carbon dioxide. During last year's harshest Amazon dry season in 40 years, drought and accidental fires killed half a billion metric tons of trees in Brazil, according to conservative estimates -- trees storing the carbon equivalent to the annual emissions of California and New York state combined.
Brazil is one of the world's 10 worst carbon polluters. In the United States, China, and other countries, fossil fuels are the major source of emissions, but 70 percent of Brazil's greenhouse gases come from clearcutting and forest fires, according to the Amazon Institute for Environmental Research.
Nepstad is one of numerous researchers who say a vicious cycle of drought and the fires it triggers could speed the transformation of one-third of the Amazon into scrub vegetation in the coming decades.
"People tend to focus on glaciers and the poles as early warning signs of global warming, but tropical rain forests are also showing signs. The process is underway, and it's alarming," he said. "Tree loss in the faraway Amazon affects New Englanders because it has the potential to speed global warming worldwide."
Earlier this month at climate change negotiations in Nairobi, Kenya, Brazilian officials proposed the creation of a fund to compensate tropical rain forest nations for keeping forests intact. Under the Kyoto Protocol to reduce greenhouse gas emissions, countries can earn "carbon credits" for planting new forests, which take years to absorb significant quantities of carbon. But there is no reward for protecting old forest before it is burned or chopped down.
Nepstad, meanwhile, is trying with increasing urgency to determine whether the forest has a "tipping point" after which too much drought and fire would have irreversible consequences. Historical records suggest that big fires swept the Amazon every several centuries in the past; now they occur every couple of decades.
On a soybean and rubber plantation in Brazil's Mato Grosso state at the southeastern edge of the Amazon, Nepstad's team has spent two years torching the forest to study its ability to survive and restore itself. His experiments on a previous drought simulation project showed that the third successive year of drought triggers significant loss of large trees. One of the most worrisome trends, he said, is the invasion into burned forest of highly flammable grasses, which increase the risk of future fires.
The Amazon is a "giant air-conditioner," in Nepstad's words, that evaporates water and cools the region. But as trees die, that cooling function disappears.
Renato Ramos da Silva, a meteorologist at the Federal University of Pará, says that when forest cover is removed, cloud formations change, decreasing rainfall and causing more-severe weather patterns, with hail and lightning sparking fires. Smoke from fires, in turn, inhibits further rainfall.
Losing the Amazon rain forest entirely could reduce rainfall in regions as far away as the central US farm belt, according to climate modeling by Roni Avissar, a climatologist at Duke University. Scientists calculate that in an average year, up to one-fourth of carbon emissions that contribute to rising global temperatures come not from cars and factories but from tropical deforestation and fires.
With an estimated 430 billion metric tons of carbon stored in tropical rain forests -- the equivalent of 50 years' worth of today's carbon emissions -- protecting rain forests could go a long way toward meeting the world's carbon-reduction targets, say scientists involved in long-term studies in the Amazon.
Conversely, if current rates of Amazon deforestation continue unabated, some 33 billion metric tons of carbon will be released into the atmosphere from dead trees by 2050.
Papua New Guinea and Costa Rica are leading a coalition of rain forest nations that advocate a carbon market within the Kyoto Protocol. It would award credits to nations that could prove they were preventing deforestation. If Brazil reduced annual deforestation by just 10 percent, it could earn $2.5 billion in carbon credits in five years under such a scheme, says Paulo Moutinho, a Brazilian scientist who has advised the government.
Protecting the Amazon could be done relatively easily and cheaply without harming the interests of Brazilian cattlemen and farmers, said Woods Hole Research Center land economist Frank Merry .
The Brazilian government could offer tax breaks, subsidies, and other incentives to encourage better land management and low-impact logging. Protected parks and reserves measurably discourage clearcutting and agricultural fires. And with a quarter of the Brazilian Amazon in private hands -- an area the size of the US Eastern Seaboard -- ranch owners must be given incentives to leave their unproductive land as forest, Merry said.
Already, commodity traders and consumer groups are beginning to seek timber, soy products, and beef produced in environmentally conscious ways. After a campaign by the environmental group Greenpeace, which targeted European McDonald's outlets for using Amazon soy to fatten chickens, the large business alliance that purchases Amazon soy beans declared a moratorium this year on buying soy grown on recently cleared forest land.
Brazilian producers are also realizing they can demand higher prices for wood or commodities produced under strict environmental standards.
Renato da Rosa, 36, a cattle rancher who owns 5,400 acres in Mato Grosso state, said he installed firebreaks and has stopped setting fires to clear land for pasture.
"These environmental trends and talk of climate change is affecting the industry," he admitted. "Consumers don't want to eat Amazon beef if they think it's killing the rain forest."