Actually _you're_ wrong Aidan. CO2 makes up about .04% of the atmosphere while water vapour is between 0.2% and 4%. So water is between 500% and 1000% more prevalent. https://sciencing.com/percentage-water-vapor-atmosphere-19385.html On top of that water absorbs more infrared energy than CO2 https://www.friendsofscience.org/assets/documents/FOS%20Essay/CO2_Versus_Water.html. Combination of these makes water vapour much more significant than CO2 in terms of warming the earth.

On top of that the ability of both to absorb IR decreases per additional unit of either molecule in the atmosphere, which means the effect is logarithmic. We could double the amount of CO2 in the atmosphere for a trivial increase in temperature of around 1.5 degrees. And then double it again for the same effect

GrahamY,
Your claim that I'm wrong is due to a lack of comprehension on your part, as I acknowledged the role of water vapour. However I concede that was rather unclear due to a typo and mouso on my part. What I meant to say was:
"Apart from water vapour (which works as a feedback mechanism because increasing temperatures increase how much of it the atmosphere can hold) CO2 is dominant because there's so much more of it."

I want to check you understand how it's a feedback mechanism: Water vapour doesn't accumulate in the atmosphere like other greenhouse gases, but instead condenses out. The warmer the atmosphere is, the more water vapour it can hold. So the warming effect of other greenhouse gases increases the amount of water vapour in the atmosphere. Indeed that's compounded by the effect of water vapour itself. Do you still follow this?

Your claim that "We could double the amount of CO2 in the atmosphere for a trivial increase in temperature of around 1.5 degrees. And then double it again for the same effect" is extremely dubious for four reasons:

1. The effect is not entirely logarithmic. Factors including doppler shifting of the frequencies absorbed in the upper atmosphere make it more complicated than that.

2. .The figure of 1.5 degrees for a doubling ignores the enormous buffering effect of the ocean. Without the ocean's huge thermal mass, the atmosphere would have warmed far more. And because of the ocean's huge thermal mass, the atmosphere will continue to warm even after we cease to increase atmospheric CO2 levels.

3. A 1.5 degree increase is far from trivial, as that's just the average, and hot conditions are disproportionately affected. Some heatwaves will exceed 50°C.

4. There are other feedback mechanisms, such as melting tundra releasing methane. AIUI the most devastating feedback mechanism of all (which is fortunately quite a long way off) is the disappearance of marine stratus, which threatens to increase temperatures by 8°C.

Sorry, forgot to include the link for that last claim:
http://www.sciencedaily.com/releases/2019/02/190225123036.htm

You keep making the same mistake. There is much more water than there is CO2 in the atmosphere. CO2 only adds a little warming, H2O the most. However, H2O also cools the planet by reflecting radiation back into space as well as condensation transferring heat from the surface up into the troposphere where the heat is released and rain comes down.

It is not at all clear whether water vapour warms or cools the planet overall. You have to pit cooling from condensation and cloud formation against warming from humidity and cloud formation.

The fact that there has been no runaway global warming effect when the earth has been a lot warmer suggests that water vapour actually provides a negative feedback. So, on an historical basis while water may be a stronger greenhouse gas when it comes to reradiation of IR, it may be the handbrake which caps the heat increase from other mechanisms like CO2 emissions, increased solar activity and increased solar insolation due to orbit.

BTW, the thermal mass of the ocean is a bit of a furphy. It is the temperature of the oceans that controls the temperature of the earth, and if the heat was trapped in the ocean then it wouldn't be escaping into the atmosphere and there would be no global warming. You can't have it both ways.

When I go scuba diving on the GBR I find the water a lot colder now than say 30 years ago. The coral aren't as colourful anymore either ! The fish have all but disappeared. The jetsam on the beaches of Cape York has turned the Peninsula coast into the last great wildermess !

There were a couple of Japanese papers recently showing that increased CO2 tended to replace water vapor thus reducing the level of water vapor in the air.

I don't see how this would happen, but the observations appeared to support the theory. The math was very heavy, & not so convincing, if I understood it fully, however if it is true, increased CO2 would be a cooling factor.

This winter in Europe, & Texas, plus our summer do agree.