But so much forest is already gone that it may not make much difference. Paving over every patch of green that could have absorbed run-off is one major reason. Some 70 percent of Brazil's population lives in the coastal Atlantic forest region. Their water supply, flood control, soil conservation and regional climate all ultimately depend on this forest, which is more than 90 percent gone. Cut the forest down and there are fewer plants to hold the rain and cycle it back.
More water runs off, carrying more topsoil, leaving less to make rain.
The Amazon has about a fifth of the fresh water in the world, so it is not drying up-yet. This margin of uncertainty accounts for what biologists do not know about the plants and animals in tropical forests. Diminished forests will mean diminished biotic resources. Biologists have calculated that the greatest wave of extinction since the dinosaurs disappeared 60 million years ago is happening now because of tropical forest loss. The grand master of ecological disasters is global warming.