Freeman Dyson on the average lifespan of a carbon dioxide molecule in Earth's atmosphere.
Roughly, the total atmospheric carbon is eight hundred gigatons and photosynthesis absorbs seventy gigatons of carbon per year, giving a lifetime of about twelve years. This is the average time that a carbon dioxide molecule spends in the atmosphere before it is absorbed by a land plant. I used this lifetime to estimate how long it would take for a major change in the land vegetation to produce a major change in the atmosphere. This calculation completely ignores the ocean. In reality the flow of carbon dioxide into the ocean is about twice as large as the flow into land vegetation. So the lifetime of a carbon dioxide molecule in the atmosphere is really only about five years.