The earth's atmosphere is about 10,000 ppm water vapor, while only (currently) 400 ppm CO2. That is a 25/1 ratio. Furthermore, about twice the greenhouse effect comes from water vapor than from CO2. This suggests the warming effect of CO2 is about 12.5/1 stronger than water vapor.
Many people confuse the overall greenhouse effect with our recent amplification of it (which is still quite small), but they are two very different things, and it is important to understand why.
The strength of a greenhouse gas was given by Arrhenius in the late 1800s by the equation:
Radiative Forcing = a * ln(X/X0), where a is a constant which depends on the particular gas, and ln(X/X0) is the natural logarithm of the current gas level divided by the original level. In the case of CO2, a ~ 6, and if X0 = ~300 ppm) (about the beginning of the 20th century) and X, the current level is ~ 400 ppm). In this case the calculation yields about 1.7 watts/meter squared, which matches measurements pretty well.
In the case of water vapor, a = 6/12.5 ~ 0.3. Now, using a rule of thumb from chemistry, a 10 C difference in air temperature results in a doubling of the amount of water vapor it can hold. For a 1 C rise in temperature, the difference between 1900 and 2000 thus calculates to a 7% increase in atmospheric water vapor.
Let's use Arrhenius for water vapor. Radiative Forcing = 0.3 * ln(10,007/10,000) = 0.02 watts/square meter, or about 1% of the warming produced by CO2.
If my reasoning and calculations are correct, then water vapor is not a significant contributor to AGW, even though it is the major greenhouse absorber in our atmosphere.
