Why do CO2 concentrations cycle throughout the year?

Answer 1

If you look at a globe, you will notice that the majority of the land area of the earth lies north of the equator. As a result, more vegetation exists on the earth when it is summer in the northern hemisphere than exists when it is summer in the southern hemisphere. More vegetation means that more CO2 can be sequestered. The annual CO2 cycle is the result.

If you want a good visualization of this process: watch An Inconvenient Truth - it is all explained there

It's a bit more than that, during winter plants release CO2, so during the summer more O2 is released and CO2 sequestered, and in the winter the process is reversed (O2 being used in the respiration by detrivores). Since, as has already been stated, most land mass is in the Northern Hemisphere there is not enough Southern summer growth to balance the CO2/ O2 and so the concentrations cycle.

Answer 2

CO2 concentrations cycle throughout the year due to variations in photosynthesis and respiration rates in plants. During spring and summer, plants perform photosynthesis, removing CO2 from the atmosphere and decreasing its concentration. In fall and winter, plants go through dormancy, leading to decreased photosynthesis and increased respiration, which raises CO2 levels.