First, the base numbers in final sentence appear to be made up but have taken root among the popular press for those opposed to the idea of man made global warming. The correct number is not 3.4% but between 9% and 30%.
Also, the underlying assumption that all greenhouse gasses are the same is patently false. Not mentioned by you, but the claim that generally goes along with the 3.4% number is that water vapor comprises 95% of greenhouse gasses and since 99.9999% of that is natural no other cause can be seen as significant.
However, adding 1 million tons of water vapor to the air and 1 million tons of carbon do not have the same effect. Because water vapor does not
cause global warming, it maintains it. Carbon dioxide, however, causes it. The amount of water vapor in the air is primarily a result of average global temperature and if too much gets in for a given temperature it will precipitate out and if too little more will evaporate and by this mechanism helps maintain the status quo.
But when something else forces an increase in warming the water vapor will adjust and help maintain that new equilibrium.
So, put a million tons more water in the air and you get a short term rise, some extra rain and then things settle back down. Put a million more tons of carbon (obviously these numbers are just made up for demonstration) and you get an increase in temperature, more evaporation, and a new equilibrium temperature.
I won't assume that you took your numbers directly from the
Fox News Junk Science column that sprouted this easily debunked idea across the internets but rather hopefully from a more reputable source that simply repeated it.
This is kind of like a claim that turning down the gas flame under your water heater won't make your bath cooler because 99% of the heat in the system at any given time is contained within tank water.
Also implicit in your post is that a large gas tax would only fight carbon emissions. It would do that directly by presumably reducing demand for gasoline (though I bet it wouldn't really) but ignores the fact that the government then has billions of dollars it can direct to fighting other pressures producing global climate change.
I'm skeptical of many of the grander claims made in the global warming argument, but both sides need to base claims on something demonstrably legitimate and I think the moderate voices of doom do a much better job of that on this particular issue.