The graph below shows how the greenhouse effect varies with humidity during the mid-latitude summer. As you can see, there is a huge increase in downwelling longwave radiation going from no water vapor to 1% water vapor (10,000 PPM.) A little bit of humidity goes a long way towards keeping the surface warm.
One of the reasons why Antarctica is so cold is that there is very little water vapor in the interior of the continent, so little heat reaches the surface from the atmosphere. Likewise, Mars is very cold despite a large amount of CO2 in its atmosphere, due to the low atmospheric pressure and lack of water vapor.
Also note that you can’t actually go above 40,000 PPM because the air becomes saturated.