Every year, fire envelops the jungle, throwing up inky billows of smoke that blot out the sun.

Animals flee.

Residents for miles around cry and wheeze, while the weak and unlucky develop serious respiratory problems.

When the burning season strikes, life and health in the Amazon falter, and color drains out of the riotous green landscape as great swaths of majestic trees, creeping vines, delicate bromeliads and hardy ferns are reduced to blackened stubble.

But more than just the land, these annual blazes also lay waste to a cherished notion that has roosted in the popular mind for decades: the idea of the rain forest as the ''lungs of the world.''

Ever since saving the Amazon became a fashionable cause in the 1980s, championed by Madonna, Sting and other celebrities, the jungle has consistently been likened to an enormous recycling plant that slurps up carbon dioxide and pumps out oxygen for us all to breathe, from Los Angeles to London to Lusaka.

Think again, scientists say.

Far from cleaning up the atmosphere, the Amazon is now a major source of pollution.

Rampant burning and deforestation, mostly at the hands of illegal loggers and of ranchers, release hundreds of millions of tons of carbon dioxide into the skies each year.

''It's not the lungs of the world,'' said Daniel Nepstad, an American ecologist who has studied the Amazon for 20 years.

''It's probably burning up more oxygen now than it's producing.''

Scientists such as Nepstad prefer to think of the world's largest tropical rain forest as Earth's air conditioner.

The region's humidity is vital in climate regulation and cooling patterns in South America, and perhaps as far away as Western Europe.

The Amazon's role as a source of pollution, not a remover of it, is directly linked to the galloping rate of deforestation in the region over the past quarter-century.

The dense and steamy habitat straddles eight countries and is home to up to 20 percent of the world's fresh water and 30 percent of its plant and animal species.

Brazil's portion accounts for more than half the entire ecosystem. Official figures show that, on average, 7,500 square miles of rain forest were chopped and burned down in Brazil every year between 1979 and 2004.

Over the 25 years, it's as if a forest the size of California had disappeared from the face of the Earth.

Such encroachment on virgin land is theoretically illegal or subject to tough regulation, but the government here lacks the resources - some say the will - to enforce environmental protection laws.

Loggers are typically the first to punch through, hacking crude roads and harvesting all the precious hardwoods they can find. One gang of woodcutters, in cahoots with crooked environmental-protection officials, cut down nearly $371 million worth of timber from 1990 until it was busted in the biggest sting operation of its kind in Brazil, authorities said earlier this month.

Close on the loggers' heels are big ranchers and farmers, who torch the remaining vegetation to clear the way for cattle and crops such as soy, Brazil's new star export.

Prime burning period in the Amazon runs from July to January, the dry season. In 2004, government satellite images of the forest registered 165,440 ''hot spots,'' fires whose flames can shoot as high as 100 feet and push temperatures beyond 2,500 degrees.

These tremendous blazes spew about 200 million tons of carbon emissions into the atmosphere each year, which translates into several times that amount in actual carbon dioxide.

In contrast, Brazil's consumption of fossil fuels, the chief source of greenhouse gases worldwide, creates less than half what the fires send up.

During burning season, dark palls of smoke settle over parts of the jungle for days.

Even with the burning of the rain forest, Brazil's annual output of carbon pollutants is tiny compared with that of the United States, which produces nearly 6 billion tons.

But Brazil's share still vaults it onto the Top 10 list of polluters, ahead of industrialized nations such as Canada and Italy.

But under the international environmental treaty known as the Kyoto Protocol, Brazil and other poor countries are not required to reduce their emissions of greenhouse gases.

Nor does the accord contain financial incentives to encourage nations such as Brazil and Indonesia to rein in the destruction of their tropical forests.

''This is a very sensitive issue in Brazil and among developing countries,'' said Paulo Moutinho, research coordinator for the Amazon Institute of Environmental Studies.

''If you want to include developing countries, especially countries with large areas of tropical forests, in some kind of mechanism to mitigate climate change, you need to compensate deforestation reduction.''

In 2004, Brazil lost an estimated 10,000 square miles of forest, the second-worst year on record.

Environmentalists had hoped that the 2002 election of President Luiz Inacio Lula da Silva, Brazil's first left-leaning leader, would reverse the tide, not accelerate it.

Critics say that despite repeated promises to protect the Amazon, Lula's government has favored farming interests fueling its destruction in order to keep Brazil's economy growing and to boost his chances of re-election next year.

Researchers are trying to determine what role the Amazon plays in keeping the region cool and relatively moist, which in turn has a great beneficial effect on agriculture - ironically, the same interests trying to cut down the forest.

A shift in climate here could cause a ripple effect, disrupting weather patterns in Antarctica, the Eastern United States and even Western Europe, some scholars believe. This is what worries ecologists about the continued destruction of the rain forest: not the supposed effect on the global air supply, but rather on the weather.