Therefore the average residence time for an individual CO2 molecule (be it natural or human) is 3.8 years. (joannenova.com.au)
The average residence time for water vapour in the atmosphere is about 7 days. (tamino.wordpress.com)
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. (en.wikipedia.org)