I was reading this article today, and I saw this chart of atmospheric carbon dioxide levels for the past year:
The chart shows a decline in atmospheric carbon dioxide levels from roughly June-October, a cycle that appears to be consistent across many years:
Why does this cycle occur? It looks like the period of decline coincides with the season of greatest plant growth in the northern hemisphere. Or is it that energy demand rises so much during the northern-hemisphere winter that the CO2 levels for the entire atmosphere are quickly changed?