Why does the amount of carbon dioxide in the atmosphere goes down during the summer months?

The amount of atmospheric carbon dioxide follows a slow upward trend, superimposed by a saw-tooth pattern with a peak during the northern hemisphere summer and a low during the northern hemisphere winter. These short term fluctuations are due to the vigorous growth of plants during the warmer months. The sourthern hemisphere does not balance this effect because many of its evergreen trees continue to grow all year around, and because of the smaller land area.