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 = 4πr2,
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.