Now that we have identified the major processes in the short-term (days to decades) carbon cycle, we can examine what makes atmospheric CO2 level oscillate seasonally. Except in a narrow band around the equator, photosynthesis is seasonal, with higher rates in the summer and lower rates in the winter. Respiration, in contrast, remains more or less constant through the year.
Even cursory examination of the Earth shows that land is distributed asymmetrically, with more land—and hence more plants—in the Northern Hemisphere than in the Southern Hemisphere. For this reason, global atmospheric CO2 declines through the northern summer, when the ratio of photosynthesis to respiration is highest, and then increases through fall and winter, when the ratio is reversed. The result is the seasonal oscillation of atmospheric CO2 levels documented by Keeling.