It has seemed to me that this is the best winning argument: the science itself.
These sites are summaries:
“GREENHOUSE EFFECT IN SEMI-TRANSPARENT PLANETARY ATMOSPHERES”
Dr. Ferenc M. Miskolczi
Quarterly Journal of the Hungarian Meteorological Society, 2007
(Any imbalance our CO2 emissions caused [is] effectively countered by about 1 per cent decrease in the water vapor amount, and the system still fluctuates around its theoretical equilibrium value. His calculations on the NASA / NCAR atmospheric database proved that the Earth’s greenhouse effect does not show any steady increase, regardless of our CO2 emissions.)
Summaries of the paper:
http://www.examiner.com/examiner/x-7715-Portland-Civil-Rights-Examiner~y2009m5d31-Einstein-like-breakthrough-in-Climate-Science
This is what I keep saying! Normal fluctuations in water vapor easily counteract CO2 increases.