What explains the annual fluctuation in carbon dioxide levels?

The annual fluctuation in carbon dioxide levels is primarily influenced by the seasonal photosynthetic activity of plants. During the growing season, typically in the spring and summer, plants absorb significant amounts of carbon dioxide (CO2) from the atmosphere as they photosynthesize. This process leads to a notable decrease in atmospheric CO2 levels. Conversely, in the fall and winter months, many plants enter dormancy, and photosynthetic activity decreases substantially, resulting in a rise in CO2 levels as the plants are not actively removing it from the atmosphere.

This cyclical nature of plant growth and dormancy creates a pattern of CO2 concentration that peaks in late winter and early spring when plant activity is at its lowest and declines through the summer and early fall when plant activity is at its highest. Thus, the photosynthetic behavior of terrestrial vegetation is a significant driver behind the observed annual variations in atmospheric carbon dioxide.