Rainforests provide important ecological services. Once thought to be a major source of oxygen and known as the lungs of the world, it is now clear that rainforests are at best a marginal net provider of oxygen. They do hold an important stock of carbon (were the Amazon to be deforested it would be equivalent to 15 years of current annual increase in greenhouse gases) and may actual remove a small amount of CO2 from the atmosphere every year.
The Amazon forest has the unusual characteristic of making about half its own rainfall. Moisture is brought in on westward winds from the Atlantic and then recycles as it moves toward the Andes. Rain that falls on the forest evaporates off the forestís complex surfaces and is transpired by the leaves, so that it returns to the air mass and can form the basis for rain farther to the west. Until this was determined the concept was that vegetation depends on climate and that it has no effect on climate. We now know that is a false assumption for the Amazon and probably at least for other rainforests.
Recently it became clear that when the moisture recycled across the Amazon basin reaches the high wall of the Andes it is deflected. A significant portion goes south and provides important rainfall for south central Brazil and northern Argentina. So the newly recognized Amazon rain machine is making a vital contribution to the Brazilian economy through its benefits to agro-industry and some hydro-electric facilities.
Further the Amazon rain machine is vulnerable to continued deforestation. That can also be reinforced by the effects of El Nino now known to reach across South America to create drought in Brazil. In addition there is a separate drought inducing phenomenon that occurred in 2005: the same change in the Atlantic circulation that spawned Katrina also produced massive drought deep into the heart of the Amazon. This latter event could be a preview of what one of the major climate models predicts in terms of Amazon dieback if energy use and climate change continues along their current paths.