Why are global carbon dioxide levels highest in May and lowest in October each year?

Answer 1

The Northern Hemisphere has much more land for vegetation to grow on compared to the Southern Hemisphere. Carbon dioxide (CO2) builds up slowly during the northern winter, when trees and plants are dormant. Then in May everything begins to grow, and CO2 is taken out of the atmosphere, so the levels start coming down. In October and November vegetation stops growing, and in its dormant state it stops absorbing CO2, so the carbon in the atmosphere increases. These levels go up and down like this every year.

The readings for 1958, 1959 and 1960 show this:

1960: May: 320.5 ppm: October: 314.5 ppm

1959: May: 320.0 ppm: October: 313.5 ppm

1958: May: 318.0 ppm: October: 313.0 ppm

Ppm means parts per million, so 320 ppm is the same as 0.032 percent (per hundred).

The recent readings for May are:

2011: 394.35 ppm

2010: 393.22 ppm

2009: 390.18 ppm

See the graph at the link below.

Answer 2

Global carbon dioxide levels are highest in May and lowest in October due to the natural cycle of photosynthesis in the Northern Hemisphere. During the spring and summer months, plants actively photosynthesize and absorb carbon dioxide, reducing its concentration in the atmosphere. In the fall and winter, when plants shed leaves and go dormant, carbon dioxide levels rise as less is being absorbed.