Sure, but it makes a difference for average global temperature.
I doubt it, 4/100ths of 1% of anything isn't enough to make a layer, or a difference (unless it's Plutonium) Given its weight, any layer of CO2 would be close to the ground. Additionally, the CO2 levels have been a lot higher when the Earth was a lot colder.
The 2% water vapor matters a lot more however.
(that's more than 100%)
Not to mention the 99% O2 and Nitrogen., without it we'd freeze instantly.
I don't deny "global warming"....it happens all the time. The CO2 is dissolved in seawater, as the oceans warm up, CO2 is released.
(that's more than 100%)
Wikipedia, FWIW, says water vapor is 0.4% on average. It actually does vary from close to zero to a maximum of a little over 4% when the dew point is maximized around 90 degrees. Even dew points less than that amount create convection and that air is replaced with less moist air, so 90 or maybe low 90's is the max. So with the water vapor varying from a little over zero to a little over 4% I said 2%. But it could be less.
The CO2 is dissolved in seawater, as the oceans warm up, CO2 is released.
Correct. But oceans are geting more acidic which means they are absorbing more CO2 than they are releasing. Also the amount of CO2 released from ocean warming is about 5-10ppm per 1C of warming. That means from the depths of the ice age at about 150 ppm the planet warmed 10C and CO2 rose to about 250 PPM. There's about 500-1000 years of lag from warming to CO2 release because it takes centuries for the warmth to percolate down to the bottom of the ocean.
If the current rise from 280 to 410 was created by ocean warming, then the ocean would have had to warm at least 13C in the past 1000 years or so. Furthermore the CO2 rise is 2-3 ppm per year. That would require past warming of 0.2 to 0.3C per year. That didn't happen. It is essentially impossible for more than a fraction of the rise of CO2 to be due to ocean warming. Maybe 5-10 ppm out of the 130 ppm was from past ocean warming.
You could argue for volcanoes but I would show evidence that volcanoes have not suddenly picked up in frequency since 1850. Your best argument would be biosphere changes particularly ocean algae which is not well measured. A large decrease in plant life will cause an increase in CO2.
The best explanation is fossil. The numbers in the rise match up quite well to the amounts determined from $$ worth of fossil burned, minus the amount of extra CO2 above equilibrium that the ocean absorbs (lowering the average pH by about 0.02 units per decade).