By which process is carbon returned to the atmosphere?

The process of combustion is responsible for returning carbon to the atmosphere. When carbon-based materials such as fossil fuels, wood, or other organic matter are burned, they undergo a chemical reaction that releases carbon dioxide (CO2) and other byproducts into the air. This process significantly contributes to the carbon cycle, as it takes carbon stored in these materials and reintroduces it into the environment, where it can affect greenhouse gas levels and climate.

Other processes, such as transpiration, primarily involve the movement of water rather than the direct release of carbon into the atmosphere. Groundwater relates to the water cycle and does not play a direct role in the atmospheric carbon content. Photosynthesis, conversely, captures carbon dioxide from the atmosphere to produce glucose and oxygen, effectively reducing the amount of carbon in the air rather than returning it. Thus, combustion is the correct process linked to the release of carbon back into the atmosphere.