Why do atmospheric carbon dioxide concentrations fluctuate seasonally?

Atmospheric carbon dioxide concentrations fluctuate seasonally due to the natural processes of photosynthesis and respiration in plants. During the growing season, plants take in carbon dioxide for photosynthesis, which reduces the amount of CO2 in the atmosphere. In the winter, when plants are dormant, there is less photosynthesis and more respiration, leading to an increase in atmospheric CO2 levels.