Complex sentence: Atmospheric concentrations of carbon dioxide fluctuate slightly with the seasons, falling during the Northern Hemisphere spring and summer as plants consume the gas and rising during northern autumn and winter as plants go dormant or die and decay.
Split into simpler sentences: Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons. Concentrations fall during the Northern Hemisphere spring and summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant or die and decay.