Denier misdirection and garbage modded insightful.
Water vapor is short lived and exists in dynamic equilibrium, regularly falling out of the air as precipitation in a very short cycle. It may be the largest holder of heat in the atmosphere (indeed the planet if you count the oceans) but it is not the largest driver. Left to its own devices it would not cause significant warming. The effect of water vapor is a reaction to other forces (such as CO2), not a first cause in itself. That is how a minor gas, CO2, can be the main driver of warming even while being a rather small portion of the atmosphere.
When skeptics use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as powerful as water vapor, which there's already a lot of, adding a little more CO2 couldn't be that bad, right? What this argument misses is the fact that water vapor creates what scientists call a 'positive feedback loop' in the atmosphere — making any temperature changes larger than they would be otherwise.
How does this work? The amount of water vapor in the atmosphere exists in direct relation to the temperature. If you increase the temperature, more water evaporates and becomes vapor, and vice versa. So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even further—a positive feedback.
How much does water vapor amplify CO2 warming? Studies show that water vapor feedback roughly doubles the amount of warming caused by CO2. So if there is a 1C change caused by CO2, the water vapor will cause the temperature to go up another 1C. When other feedback loops are included, the total warming from a potential 1C change caused by CO2 is, in reality, as much as 3C.
The other factor to consider is that water is evaporated from the land and sea and falls as rain or snow all the time. Thus the amount held in the atmosphere as water vapor varies greatly in just hours and days as result of the prevailing weather in any location. So even though water vapor is the greatest greenhouse gas, it is relatively short-lived. On the other hand, CO2 is removed from the air by natural geological-scale processes and these take a long time to work. Consequently CO2 stays in our atmosphere for years and even centuries. A small additional amount has a much more long-term effect.
So skeptics are right in saying that water vapor is the dominant greenhouse gas. What they don't mention is that the water vapor feedback loop actually makes temperature changes caused by CO2 even bigger.
Firstly, the Earth has far more CO2 than Mars, both in raw mass, and per unit area. But while you're wrong there, its also not that relevant: Mar's climate, such as it is, is not driven by the sun or its atmospheric content of gases (CO2 or other), but by dust and albedo. , not the content of its atmosphere.
It's also much further from the sun, receiving far far less energy from it. If the energy on a unit area of Earth is taken as a unitary 1, then the relative energy intensity on the same unit are of Mars is 0.44, or 44%. Less than half as much energy per unit area. Additionally Mars has only 1/4th (roughly) the surface area of the Earth, which combined with that 44% of the energy per unit area, means the planet Mars as a whole is only receiving 11% as much energy as the Earth does. .
To expect the same driving patterns and effect is silly.
BTW, your arguments about water vapor and mars CO2 content actually rather undercut each other!!
The myth that Mars has more CO2 than Earth
One would hope that a patently false claim would just die rather than being continuously resurrected. Such is the case with the "Mars has more CO2 than Earth and is colder than Earth so CO2 can't be causing global warming" claim. This one is usually based on the fact that Mars has an atmosphere that is 95.32% CO2 by volume whereas Earth's atmosphere is only 0.04% CO2. Unfortunately, all that claim reveals is that whoever is making it didn't get very far in a science class.
If Mars and Earth had the same size atmosphere, then a direct comparison between percentages would be valid. They don't. Measurements show that the Martian atmosphere has a mass of 2.5 x 1016 kg whereas Earth's atmosphere has a mass of 5.1 x 1018 kg. That's right—Earth's atmosphere is 204x more massive than the Martian atmosphere. So we must account for that difference in size before comparing the amount of gas in the atmosphere. The way to do this is simple. Convert percentage volume to percentage mass using moles, then multiply the percentage mass by the total mass of the atmosphere. For those who have forgotten what a mole is, you can partially resurrect those long buried nightmares of general chemistry here.
Given the following
1 mole CO2 = 44.0095 g/mole
Total atmosphere mass: 2.5 x 1016 kg
Mean molecular mass of atmosphere: 43.34 g/mole
% volume CO2: 95.32%
The total mass of CO2 in the Martian atmosphere is
95.32% volume x (44.0095/43.34) = 96.79% by mass CO2
96.79% mass x 2.5 x 1016 kg = 2.383 x 1016 kg CO2
The equivalent calculation for Earth is
Total atmosphere mass: 5.1 x 1018 kg
Mean molecular mass of atmosphere: 28.97 g/mole
% volume CO2: 0.04%
0.04% volume x (44.0095/28.97) = 0.0608% mass CO2
0.0608% mass x 5.1 x 1018 kg = 3.101 x 1017 kg CO2
Last time I checked, 3.101 x 1017 kg is larger than 2.383 x 1016 kg by over 13x.
Now I can hear someone saying "Wait a second. Earth is larger than Mars." Ok. We can normalize the amount by dividing it by the surface area of the respective planet. Given
Martian equatorial radius = 3396.2 km
Earth equatorial radius = 6378.1 km
Surface area of a sphere = 4r2,
the surface area of Mars is 144,942,710.74 km2 and the surface area of Earth is 511,201,962.3 km2. Dividing the amount of CO2 by the surface area gives us
Mars: 164,409,785.62 kg CO2 per km2
Earth: 606,609,565.04 kg CO2 per km2
Which means Earth has 3.69x more CO2 than Mars, even after accounting for the different sizes of the planets. No matter how you want to look at it, the claim that Mars has more CO2 than Earth should be just plain dead.
So if Earth has 3.69x more CO2 per unit area than Mars, why is CO2 such a large part of the Martian atmosphere and such a small portion of Earth's atmosphere? The answer is nitrogen, oxygen, and argon. Those gases comprise >99.5% of Earth's atmosphere, virtually none of Mars'. Strip away the nitrogen, oxygen, and argon from Earth's atmosphere and of the gases that would be left, CO2 would make up 99% of the atmosphere by volume, an even higher percentage than on Mars.