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I'm having a slightly difficult time understanding the Keeling curve, pictured here:

Atmospheric carbon dioxide

I understand that carbon dioxide (ppm) is increasing with respect to time, but why is there an oscillation between the summer months and winter months? I have two guesses but I'm not sure if they are correct:

  1. Changes in density due to temperature?
  2. More plants in the summer, resulting in less carbon.

Are these guesses correct?

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Photosynthesis increase in the northern hemisphere spring, especially in the Siberian Taiga, and carbon dioxide decrease in atmosphere. During autumn and winter, decomposition of plant material and increased combustion, bring more carbon dioxide to the atmosphere. Measurements from e.g. the South Pole shows the same pattern, but much less and in opposite phase. As the temperate and boreal landmass are larger on the northern hemisphere, the oscillation is also larger.

It's well explained here. Also check ESRL and CDIAC for more stations and downloadable data sets.

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