Tropical forests, such as the Amazon, are an essential part of the "lungs" of the world. All trees and plants absorb CO2 as part of the photosynthesis process, providing energy for the plant and storing CO2 in their bodies by combining it with water to make cellulose. Trees also release some CO2 at night, but overall, they remove more than they give off. The sheer size of tropical forests means that they are an important part of the way our planet removes excess CO2 from the atmosphere.
Logging to harvest wood and forest clearing to make more farm land has reduced the size of forests massively. Reducing forest sizes means there are less trees to absorb CO2, while the rotting dead stumps and roots release further greenhouse gases. This loss of CO2 removal is the equivalent of adding more CO2 to the air. It is estimated that tree loss globally accounts to 12% of global greenhouse gas emissions.
The ozone layer