Final answer:
Carbon enters the atmosphere primarily as carbon dioxide through the burning of fossil fuels and cellular respiration, with additional contributions from decomposition, geological activity, and deforestation.
Step-by-step explanation:
Carbon enters the atmosphere through several key processes. Carbon accumulates as carbon dioxide (CO₂), particularly when fossil fuels such as coal, oil, and natural gas are burned. This process releases a significant amount of carbon dioxide, with coal burning releasing the most CO₂. Another major way carbon is released is through cellular respiration, where living organisms, including humans, exhale carbon dioxide as a waste product. Additionally, the natural decomposition of organic matter and geological activities like volcanic eruptions contribute to the atmospheric carbon dioxide levels. Deforestation also plays a role by reducing the number of trees that can absorb CO₂, thus increasing its concentration in the atmosphere.
Other processes include the release of methane from livestock and landfills, combustion from wildfires, geological processes such as sedimentation, and the release of dissolved CO₂ from ocean waters. All of these sources and sinks are integral to the biogeochemical carbon cycle, where carbon is exchanged among the atmosphere, land, and water reservoirs on Earth.