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I would say these. The real question is how much we are accelerating the natural cycle.
Comparatively speaking, human emissions are extremely small compared to earth's ocean - land carbon cycle. It does however add up because the land and ocean cannot absorb all of the extra CO2. About 40% of this additional CO2 is absorbed. The rest remains in the atmosphere, and as a consequence, atmospheric CO2 is at its highest level in 15 to 20 million years
http://science.sciencemag.org/content/326/5958/1394
A natural change of 100ppm normally takes 5,000 to 20,000 years. The recent increase of 100ppm has taken just 120 years.