Seasonal variation in carbon uptake, because most of the worlds landmass is in the northern hemisphere. More details from https://en.wikipedia.org/wiki/Keeling_

Seasonal variation in carbon uptake, because most of the worlds landmass is in the northern hemisphere. More details from https://en.wikipedia.org/wiki/Keeling_Curve https://en.wikipedia.org/wiki/Keeling_Curve > The Keeling Curve also shows a cyclic variation of about 6 ppmv each year corresponding to the seasonal change in uptake of CO2 by the world's land vegetation. Most of this vegetation is in the Northern hemisphere where most of the land is located. From a maximum in May, the level decreases during the northern spring and summer as new plant growth takes CO2 out of the atmosphere through photosynthesis. After reaching a minimum in September, the level rises again in the northern fall and winter as plants and leaves die off and decay, releasing CO2 back into the atmosphere.