What is the justification for assuming CO2 is the primary driver of the GH effect when H20 is far more abundant and a much stronger GHG?

First, the largest contribution to the atmospheric greenhouse effect is water--both as vapor and clouds combined. The two together account for about 81% of the present day greenhouse effect. Add in CO2 and you've accounted for about 95% of the total effect.

Now I'm not sure what you mean by "primary driver", but the reason that CO2 is crucial is because it has a relatively long residence time in the atmosphere compared to H2O. Also, the amount of H2O in the atmosphere is strongly correlated with temperature. Frankly, almost everyone knows that. During the summer we notice that the atmosphere gets "muggy" as water vapor increases; during the winter our lips get chapped and our skin dries out because there is a lot less water vapor in the atmosphere. If you look at the amount of water vapor above any location (called the "precipitable water") the seasonal cycle is obvious. So we don't see water vapor as a "driver" of climate change because unless you fundamentally change the boundary conditions (distribution of land masses, temperature, wind speed) you're not going to change the amount of water vapor in the atmosphere substantially.

However, if we increase the Carbon dioxide the atmosphere, some of it DOES stay there--all you need to do is look at the Keeling Curve to see that. So as you increase the CO2, you will increase the Earth's greenhouse effect and raise the temperature. If you raise the temperature, you WILL increase the amount of water vapor in the atmosphere. Quantitatively we expect the global mean concentration of water vapor to go up about 7% for each kelvin of temperature increase. That will further increase the greenhouse effect and temperature. We don't expect that the temperature will rise indefinitely, though, because there are other feedback that will limit the temperature rise. Instead you'll have a new equilibrium temperature established.

You also say that H2O is "far more abundant". To be precise, it's about 10 times as abundant. By volume, CO2 is about 0.04% of the atmosphere while H2O is 0.4%, certainly that's a big difference, but not big enough to allow us to neglect changes to the amount of CO2.

EDIT: "Anonymous" gives an amusing answer blaming climate change on the Atlantic Multidecadal Oscillation (AMO), and showing an amazing correlation between the global mean temperature with the AMO index. What a shocker: global temperature is highly correlated with temperature over a large fraction of the globe! What's next, we he show us a correlation between mean temperature and daytime temperature for some place?

H20 probably hasn't changed the climate significantly in the last 100 years IMO. It tends to precipitate excess in a matter of a few days. CO2 may or may not have a short residence time but it is much longer than water's. I think that climates varied considerably in the past based on a bunch of things. I am skeptical that water vapor was a significant driver but I can't say for sure it isn't. I think there isn't much evidence that CO2 was a driver of climate change in the past. In the past it has always been a follower. Alarmists want us to believe that this time, CO2 jumped out in front and is now driving the GW convoy. I think climate probably is about the same as what it would have been without our CO2 emissions but bumped up slightly in temperature.

H20 probably hasn't changed the climate significantly in the last 100 years IMO. It tends to precipitate excess in a matter of a few days. CO2 may or may not have a short residence time but it is much longer than water's. I think that climates varied considerably in the past based on a bunch of things. I am skeptical that water vapor was a significant driver but I can't say for sure it isn't. I think there isn't much evidence that CO2 was a driver of climate change in the past. In the past it has always been a follower.

It's not just CO2, it's all of the various greenhouse gases that are accumulating in our paper thin atmosphere. Also, it takes heat to evaporate water, but when the water vapor condenses it released that heat back into the atmosphere...it's call the latent head of condensation. The ONLY way that solar heat leaves the atmosphere for good is by radiating it back into space. The problem here is that an ever heavier load of greenhouse gases prevent enough heat to radiate back into space in any given 24 hour period. The retained heat seeks a cooler venue..that being seawater and ice. The seawater warms and the ice melts. The data confirms that....thus 'global warming' and climate change.

That is certainly a possibility. I tend to think, however, that the extreme views of CO2 is the driver or H2O is the driver, are simply not correct. There is an interaction occurring and this whole idea of one driving the other, has never really made any sense to me.

This is why I tend to hold the opinion that AGW could be a problem and we certainly have reason to reduce our emissions, but it certainly is NOT some impending catastrophe.

Edit:

Also, what ray said is complete crap.

Edit:

This whole driver idea is crap. I think saying "CO2 is the driver? We need to let Jesus take the wheel." makes about as much sense as calling CO2 a "driver" in the first place.

Because it's the existing driver that we know is rising rapidly and we have some control over it. Once there's an increase in atmospheric heat, water vapor in the atmosphere has to increase.

without co2, water vapor would not exist.................................................................................................................. the earth would be frozen...............................................................................................................

without co2, water vapor would not exist.................................................................................................................. the earth would be frozen...............................................................................................................

without co2, water vapor would not exist.................................................................................................................. the earth would be frozen...............................................................................................................

without co2, water vapor would not exist.................................................................................................................. the earth would be frozen...............................................................................................................