Leaked [IPCC] climate report: Ten nuggets worth noting
The recent rate of increase in carbon dioxide, methane and nitrous oxide in the atmosphere is unprecedented in the last 22,000 years.Skeptical Science: CO2 has a short residence time
It is very likely that more than 20 percent of carbon dioxide released into the atmosphere will remain there over 20,000 years after emissions stop.
A little quick counting shows that about 200 Gt C leaves and enters the atmosphere each year. As a first approximation then, given the reservoir size of 750 Gt, we can work out that the residence time of a given molecule of CO2 is 750 Gt C / 200 Gt C y-1 = about 3-4 years.