Trees absorb the leftover CO2 from the atmosphere therefore cutting down trees, for timber, or agriculture in large numbers (deforestation) means that the CO2 isn’t being absorbed. It floats up into the upper atmosphere and adds to the enhanced greenhouse effect.
Some 46-58 thousand square miles of forest are lost each year—equivalent to 36 football fields every minute”. Another fact is that forest are called the lungs of the world so cutting down the lungs of the world would lead to know clean air in our atmosphere.