CO2 in the atmosphere decreases during the growing season and increases during the rest of the year, which leads to maximum buildup in April and May before photosynthesis begins to take over again.
There's more carbon dioxide in the winter and a bit less in the summer. That's the collective breathing of all the plants in the Northern Hemisphere. "Plants are accumulating carbon in the spring and summer when they're active, and they're releasing carbon back to the air in the fall and winter," Graven explains.
