Photosynthesis is the process by which plants use sunlight, carbon dioxide, and water to produce glucose and oxygen. In the spring, when leaves start to grow back on trees and other plants, there is an increase in photosynthesis. This means that more carbon dioxide is being taken in by the plants, which leads to a decrease in atmospheric carbon dioxide levels. By May, when the leaves are fully developed and photosynthesis is at its peak, the carbon dioxide levels are at their lowest.
As autumn approaches and the leaves begin to fall off trees, photosynthesis decreases significantly. With fewer leaves, there are fewer plants absorbing carbon dioxide from the atmosphere. This results in higher levels of carbon dioxide, which are measured in October. Overall, the seasonal changes in photosynthesis due to leaf growth and loss directly influence the levels of carbon dioxide in the atmosphere.