A. Producers, consumers and decomposers all respire to produce carbon dioxide. A is wrong; B. Plants absorb carbon dioxide in the atmosphere through photosynthesis and produce oxygen.

Therefore, the carbon dioxide in the atmosphere can be reduced through the process of ②. B is wrong; C. The intensification of the greenhouse effect is mainly caused by the modern carbon dioxide-based industrial society burning too much coal, oil and natural gas. After these fuels are burned, a large amount of carbon dioxide gas is released into the atmosphere.

; C is correct; D. Green plants produce organic matter and oxygen through photosynthesis, absorb carbon dioxide and water, and carbon dioxide enters the organic (biological) environment; animals and plants decompose organic matter through respiration and release carbon dioxide; bacteria and fungi decompose the remains of animals and plants into

Simple inorganic substances (carbon dioxide, water and inorganic salts) are returned to the inorganic environment, promoting the circulation of materials.

D is wrong.

So choose: C