How is carbon dioxide added to the air?

When you breathe in, you breathe in oxygen, but when you breathe out, you're breathing out carbon dioxide.

Animal life uses oxygen in respiration, necessary for survival. Carbon Dioxide is released as a by-product. When a tree is burnt or rots it releases its CO2. When we burn fossil fuels (coal, oil and natural gas), for energy or transport we add carbon dioxide to the air.