http://www.newton.dep.anl.gov/askasci/wea00/wea00296.htm
The duration period for carbon dioxide molecules in the
atmosphere is somewhere between 100 and 500
years. Obviously, not all carbon dioxide molecules
will stay in the atmosphere that long, but on average
the duration may be around 200-300 years. Some scientists
believe that it could be longer than that, others
believe that the duration is shorter. Presently,
there is some uncertainty in those figures.
The most important thing concerning CO2 duration is that
its large concentration plus its long duration in the
atmosphere make it the most important greenhouse gas
after water vapor.
Some other greenhouse gases also have similarly long
durations in the atmosphere, but their concentrations
are much smaller than CO2 and thus they are less
important (but not unimportant) contributors to
warming.
Although water vapor is the most effective greenhouse
gas, it has a duration in the atmosphere of only 3-7 days
and its concentration will likely only increase if atmospheric
temperature increases. This is a double whammy that
most climate scientists are concerned about. If increasing
concentrations of CO2 result in warmer atmospheric
temperatures, that will likely result in higher
water vapor concentrations in the atmosphere and thus
further enhance atmospheric warming, assuming that the
increased water vapor concentration does not lead to
increased cloudiness (which may reduce warming in
some regions of the world, but increase warming in
others).
David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory