Briefly explain why there can be seasonal variation in carbon dioxide concentrations in the ...

Plants and algae absorb carbon dioxide in spring and summer and release carbon dioxide in fall and winter. Since the northern hemisphere has more land, and the land has higher primary productivity than the ocean, this means that the absorption of carbon dioxide during northern summer is larger than the release of carbon dioxide by the southern hemisphere at the same time, so there is net drawdown of atmospheric carbon dioxide during (northern hemisphere) summer and increase in winter. This was demonstrated in the Mauna Loa long-term studies.