Now look at a nearby houseplant, or out the window at a tree. Those green organisms contain carbons and oxygens that were once present in your body and the cells of other animals, fungi, or microorganisms. Aerobic respiration and photosynthesis are not simply chemical processes with reciprocal equations. Most autotrophs fix carbon dioxide into organic compounds and release oxygen. Most heterotrophs break down those compounds and in the process consume oxygen and release carbon dioxide.
Autotrophs fix the released carbon dioxide once again, release new oxygen, and the cycle continues.
These cyclic events ultimately involve all of the trillions of tons of carbon on our planet, whether it is currently tied up in wood, leaves, animal tissues, microorganisms, or rotting soil particles, and whether it is floating in the atmosphere as carbon dioxide gas, dissolved in seawater, or tied up as carbonates in rocks. If all autotrophs suddenly stopped functioning today, biologists estimate that all organic compounds would be metabolized to carbon dioxide within 300 years, and no free oxygen would remain in the oceans and atmosphere after just 2000 years. This is an astonishingly short time, considering that it took many millions of years for oxygen to accumulate to its current levels in air. In the greenhouse effect, gases trap heat from sunlight beneath the atmospheric layers, just the way the glass windows of a greenhouse trap heat inside the greenhouse. This trapped heat may in turn be leading to increased global temperatures, changing climate patterns, additional photosynthesis by autotrophs, and a slight melting of the polar ice caps.