Therefore
the average residence time for an individual CO2 molecule (be it natural or human) is 3.8 years.
The average residence time for water vapour in the atmosphere is about 7 days.
The average residence time of a water molecule in the atmosphere is only about nine days, compared to years or centuries for other greenhouse gases such as CH 4 and CO2.
The average residence time of a CO2 molecule in the Earth's atmosphere is about four to seven years.