Reservoir Times
The impact of greenhouse gases on the enhanced greenhouse effect is determined by their residence time in the atmosphere.
![](http://www.ei.lehigh.edu/learners/cc/carbon/trends_thumb.png)
Atmospheric residence time represents the average amount of time the molecules of a greenhouse gas exist in air before it is removed or converted to something else. When atmospheric residence time is greater, the total impact of a greenhouse gas on global warming is larger.
The average residence time of atmospheric gases are:
![](http://www.ei.lehigh.edu/learners/cc/carbon/waterVapor.jpg)
1) water vapor (H2O)
9 days
2) carbon dioxide(CO2)
tens of thousands of years
3) methane (CH4)
12 ±3 years
4) chlorofluorocarbons (CFC)
decade to century
Consequently, anthropogenic greenhouse gases will impact global warming long after emission cut-backs are achieved.