A lot of the world oxygen is produced in those Rain Forest. Without them, more CO2, so a bad air quality. More pollution. And eventually, if the world doesn't find a solution, no more oxygen would be naturally produced.
We will have less oxygen if the rain forests are destroyed. They account for forty percent of our oxygen. Also, deforestation is one of the factors that is contributing to global warming. It also adds more carbon dioxide in the air.
