It's a Gas!

Just for a moment let's believe that CO2 in the atmosphere does cause warming of the planet.

People have been saying for a long time that water vapour is a more significant greenhouse gas than CO2.

So what happens to the water vapour as the CO2 increases?

It looks like it actually decreases, reducing the effect of the CO2. The graph below shows the average humidity in the atmosphere at different pressures (equivalent to height above sea level).



So maybe that's part of the reason why there has been no warming since 1998?

This interaction of CO2 and H2O isn't yet established beyond reasonable doubt, but it's looking good.

More detail here