Final answer:
The natural fluctuation of atmospheric CO2 concentrations occurs each year during spring and autumn months, in correspondence with the plant growth and decay cycle affecting CO2 levels.
Step-by-step explanation:
Records of CO2 concentrations in the atmosphere show a natural fluctuation each year during spring and autumn months. During these seasons, plants undergo a cycle of growth and decay, which has a significant impact on atmospheric CO2 levels.
In spring, plants begin to grow, absorbing CO2 for photosynthesis and thus reducing the amount of CO2 in the atmosphere. In autumn, as plants die and decay, they release CO2 back into the atmosphere, causing levels to rise again.
Natural fluctuations in CO2 are different from the steady increase observed over the last century due to human activities. The historical cycle of atmospheric CO2 levels has varied between 180 and 300 ppm but has increased significantly due to industrialization and the use of fossil fuels.