Final answer:
Carbon is returned to the atmosphere primarily through combustion, which is part of the biogeochemical carbon cycle and includes the burning of organic matter like fossil fuels, leading to the release of carbon dioxide.
Step-by-step explanation:
Carbon is returned to the atmosphere primarily through the process of combustion. Combustion occurs when organic matter, such as fossil fuels or biomass, is burned, releasing carbon dioxide gas back into the atmosphere. This process is a key part of the biogeochemical carbon cycle, which also includes photosynthesis, respiration, and geological activities like volcanic eruptions. In the carbon cycle, plants absorb carbon dioxide through photosynthesis, converting it to organic carbon, which is used to build plant tissues. When animals consume plants, they use the carbon for their own needs. Carbon is then released back into the atmosphere as carbon dioxide through the process of respiration by both plants and animals. Additional sources that return carbon to the atmosphere include the decomposition of dead organisms and the extraction and burning of fossil fuels.