4b. Global Carbon Cycle

Carbon dioxide is part of a natural cycle on earth. Animals (including people!) inhale atmospheric air, which contains about 21% oxygen. Once inside cells, the oxygen is used in cellular respiration, which produces carbon dioxide as a byproduct. This carbon dioxide is exhaled and released back into the air. Other natural processes, such as decomposition, also release carbon dioxide into the atmosphere.

The carbon dioxide in the air is absorbed by green plants and used inside cells for photosynthesis. A byproduct of photosynthesis is oxygen, which plants release back into the air. Carbon dioxide is also absorbed from the air by some types of marine life and the ocean. 

For most of earth’s history, the carbon dioxide cycle was balanced, keeping about 860 billion tons of CO₂ in the atmosphere. In recent decades, human activities have increased the amount of CO₂ in the air, disrupting this natural cycle. In the next sections, we’ll look at the effect of this excess CO₂.