SANTAREM, Brazil As the light plane banked left, the smell of smoke reached the cockpit. The landscape below was an ashen green, the sun above an orange glow behind sooty billows of gray.
The Amazon forest was burning, and it was more than a sign of human encroachment. It was also the sight and scent of a dangerous chemistry, of tons of carbon dioxide — transformed from wood and leaf — rising into an atmosphere already loaded with it.
In cooler confines some days later, at an international climate conference in Argentina, British scientists told of a different, slow-motion kind of chemistry in the tropical forest, one foreseen by supercomputers running intricately programmed models of global warming.
“In the Amazon, the vegetation dies back because there won’t be enough rain,” explained climatologist Vicky Pope, detailing one of the most sophisticated studies yet — by Britain’s Hadley Centre — of what a warmer world would mean.
For South America’s rain forests, such a “dieback” would mean steady decomposition of dead vegetation and the release into the atmosphere of massive amounts of carbon dioxide, the “greenhouse gas” that itself is blamed for much of climate change.
Whether it unfolds quickly by fire or slowly through global warming, the future of this forested river basin is a key to the future of Earth’s climate. Hundreds of scientists are working overtime to understand that critical relationship — between the atmosphere and the region known as Amazonia, more than 11 times the size of Texas and home to one-third of the world’s species.
“Scientifically, we’re not 100 percent confident we know all the processes,” said Brazilian physicist Paulo Artaxo, who has studied the Amazon for 20 years. “There are thousands of different critical effects. There’s much more scientific work that needs to be done.”
Artaxo is a lead scientist among 1,700 researchers, from 200 universities and other institutions, who have joined during the past six years in an unprecedented undertaking called “LBA” — for Large-Scale Biosphere-Atmosphere Experiment in Amazonia — a Brazilian-U.S.-European scientific assault on the unknowns of what some call the planet’s “lung,” the Amazon’s breathing green vastness.
The respiratory process is well known: Trees absorb carbon dioxide through their leaves, use it to build themselves, and emit oxygen into the air.
That keeps an atmospheric balance, but industrialized man has thrown the balance off by burning fossil fuels and producing excess carbon dioxide, which traps heat that otherwise would escape into space.
Scientists believe the 1-degree-Fahrenheit rise in global temperatures the past century was largely a result of those emissions, and they project higher temperatures and climate disruptions to come. The international pact called the Kyoto Protocol, a limited first step in trying to control emissions, takes effect Thursday, though without participation of the United States, the biggest emitter, which maintains the emissions cutbacks would set back its economy too much.
The forest’s basic science may be well known. But the details are immensely complex — of soils and nutrients, evaporation and precipitation, rivers and lakes, gas flows and root systems, and of the hard numbers of deforestation, which pours carbon dioxide into the atmosphere at the same time that it kills off carbon-absorbing vegetation.
But whatever the unknowns, “we already know enough to make policy decisions. The important thing is to stop deforestation,” American ecologist Philip M. Fearnside told a reporter, as a tropical downpour beat on the roof of his institute in Manaus, the Amazon’s major riverside city.
Satellite reconnaissance showed that 600 fires were started in the region each day on average last year, the Brazilian government reports. The rate of destruction has almost doubled in the past decade, to 9,000 square miles over 12 months of 2003-2004 — an area about the size of New Hampshire.
Forest is being destroyed by cattle ranchers, by landless peasants slashing and burning to create cropland, by illegal lumbering, and increasingly by large agribusinesses planting lucrative soybean. The fires seen everywhere from the air outside Santarem, a rough-edged town 500 miles up the Amazon from the Atlantic, were mostly set to create giant fields for soy.
The government’s own plans to pave 2,100 miles of additional road through the wilderness could lead to clearing of up to 70,000 square miles of forest over 30 years, it was estimated by Fearnside’s Amazon Environmental Research Institute.
Judging from experience, “paving increases the deforestation rate in a strip along the highway, to a depth of 50 kilometers” — 30 miles — “on each side,” said Fearnside, who has pioneered Amazon research for three decades.
What might this mean in a time of climate change?
Worldwide deforestation is now believed to contribute under 20 percent of manmade emissions of carbon dioxide, said Artaxo, of Brazil’s University of Sao Paulo. And the Amazon forest is believed to remain a “sink” — still absorbing slightly more carbon than it emits.
But scientists say the feedback loops of a warming world might change that picture in mere decades.
For one thing, computer modeling foresees a warmer Pacific Ocean stirring more frequent and intense El Ninos, the climate phenomenon that tends to dry the eastern Amazon. Rising temperatures themselves would also help dry vegetation. In addition, deforested terrain sends less moisture — via plants’ “evapotranspiration” — into the air to fall as rain. Dead trees then add more carbon dioxide to the atmosphere, further heightening warming in a destructive cycle.
The Intergovernmental Panel on Climate Change, a U.N.-organized science network, concluded in its latest assessment that the remaining Amazon “is threatened by the combination of human disturbance, increases in fire frequency and scale, and decreased precipitation from evapotranspiration loss, global warming, and El Nino.”
Much more remains to be learned, even including how much “biomass” an average acre contains in this highly diverse forest — a number crucial to knowing how much carbon a dying forest would release.
The NASA orbiter ICESat has been working on that. Designed to monitor melting polar icecaps, the satellite’s sensitive laser eye was turned to measuring tree heights in the Amazon to help scientists judge how much wood is down there.
Closer to home, the LBA scientists have deployed research boats, balloons and aircraft, are monitoring 200 forest plots for growth and other factors, and have raised 16 instrument-packed towers across the Amazon, up to 210 feet high, to better measure and understand the interplay between forest and atmosphere.
In one of their most elaborate experiments, they have produced a vision of a bleak, dry future 30 miles south of here, on a patch of the Tapajos National Forest, a shadowy green world of barking monkeys, iridescent butterflies and whistling Captain-of-the-Woods birds, and of ropey vines hanging from hundreds of tree species.
At this Seca Floresta site — “Dry Forest” — scientists for four years have deprived 2 1/2 acres of Amazonia of half its normal rain by stretching a plastic roof of 5,600 polyethylene panels over the soil.
Towers, catwalks and instrumentation allowed researchers to closely monitor tree metabolism and respiration, groundwater and gas flows and other processes as this piece of forest succumbed to what the computer models foretell for much of the Amazon.
“We got some amazing results,” research manager Paulo Brando told a visiting journalist. “We expected the smaller trees to die more. But it was the larger trees. They’re dying at a rate of 9 percent a year” — several times the natural rate.
At treetop level, from a 90-foot tower, leafless, desiccated spots could be seen in the canopy. Carpets of leaves and fallen limbs littered the forest floor.
“If you have large trees dying, you have a lot of carbon being released,” Brando said.
If they’re dying across great stretches of Amazon, in the “dieback” of the supercomputers, these scientists say, the cost to the world could be incalculable.