Which statement best describes how atmospheric CO2 changes are shaped in the global carbon cycle?

Which statement best describes how atmospheric CO2 changes are shaped in the global carbon cycle?

• A. Respiration releases CO2; photosynthesis releases CO2; net trend is constant.
• B. Decomposition and photosynthesis both release CO2.
• C. Photosynthesis removes CO2; respiration and decomposition release CO2; balance shapes atmospheric CO2; net trend varies with biosphere.
• D. Atmospheric CO2 is determined only by human activities.

Answer: (C)

The main idea is that atmospheric CO2 levels are set by the balance between processes that remove CO2 from the air and processes that add CO2 to it. Photosynthesis removes CO2 as plants and other photosynthesizers convert carbon dioxide into organic matter. In contrast, respiration by organisms and microbes, along with decomposition of dead material, release CO2 back into the atmosphere. The overall atmospheric concentration depends on which side dominates at any given time, so the net trend can vary with biosphere activity, seasons, disturbances, and climate. Humans also add CO2 through burning fossil fuels, but the natural exchanges between photosynthesis and respiration/decomposition still shape the atmospheric CO2, rather than it being determined only by human activity.

The main idea is that atmospheric CO2 levels are set by the balance between processes that remove CO2 from the air and processes that add CO2 to it. Photosynthesis removes CO2 as plants and other photosynthesizers convert carbon dioxide into organic matter. In contrast, respiration by organisms and microbes, along with decomposition of dead material, release CO2 back into the atmosphere. The overall atmospheric concentration depends on which side dominates at any given time, so the net trend can vary with biosphere activity, seasons, disturbances, and climate. Humans also add CO2 through burning fossil fuels, but the natural exchanges between photosynthesis and respiration/decomposition still shape the atmospheric CO2, rather than it being determined only by human activity.