A few years later Keeling observed a second cycle: a hemispherical seasonal cycle of CO2, low in summer and peaking in the winter for the same reason CO2 peaks at night: no greens at work to eat it.

• 几年后基林观察到了二氧化碳的第二个循环特征：二氧化碳的半球状季节性循环，，夏低冬高，且基于同样原理，在夜晚达到峰值：因为没有绿色植物捕食它。