This process occurring around the very middle of the Earth is the driving force in the world's climate. It is directly responsible for much of the rainfall in South America and even, scientists have discovered, in faraway places such as the corn belt in the US mid-west. If there is poor rainfall in the Amazon's rainy season (November to February), then the US is likely to have a drought four months later during its all-important growing season.
What happens to the Amazon affects the rest of the planet, and scientists have long been aware that if too much forest is felled by loggers, cattle rearers and soya farmers, the convection process will be disrupted, with disastrous global consequences.
"If the Amazon loses more than 40 percent of its forest cover, we will reach a turning point where the world's largest forest will begin an irreversible process of savannization," says Carlos Nobre, a senior scientist at INPE, the Brazilian institute of space research, as well as a leading climatologist.
The consensus is that a smaller forest would be unable to sustain the convection process and will start drying out. A vicious spiral will begin in which the drier forest becomes more vulnerable to forest fires and the fires, by destroying vegetation, will make the Amazon more vulnerable to drought.
Until last week it was widely assumed that only 17 percent of the forest -- an area larger than the size of France -- had been felled.
This is well short of the critical 40 percent postulated by Nobre, but an article printed in Science journal last month shows that the satellite images on which this figure was based were not telling the full story.
The images, the scientists said, were detecting only clear-cut swaths of land, where all the trees had been removed. They were failing to detect so-called "selective logging," where timber companies go into a forest under the canopy and take out valuable hard timber. The scientists reckoned that this kind of activity was destroying on average an additional 15,500km2 of forest each year. This means that the destruction, previously put at about 20,000km2 a year, is almost twice as serious as had been thought.
Dorothy Stang, an American nun who was killed in February by gunmen sent in by local landowners, was a passionate environmentalist. Living in the Amazon basin for over 20 years, she complained in one of her last filmed interviews of the devastation caused by loggers and cattle farmers, which, she believed, was changing the climate.
"It used to rain steadily for nine months," she said. "Now it rains for six or seven months at best. The destruction is killing the forest. We've got to stop it."
Sue Branford was the BBC's Brazil correspondent.