I frequently read CO2 has a spatially constant concentration sadly rising and at 415ppm at present. That concentration do not vary a lot spatially I think. You will find ~415ppm at Argentina or at Taiwan.
Your reading sources are overly simplistic. Carbon dioxide concentrations are not spatially constant, and the increase is not monotonic. They instead vary by several percent over the course of a year and over the globe. This is particularly the case in the Northern Hemisphere, where CO2 concentrations increase during the winter when land plants are dormant but decrease during the summer when land plants draw down CO2 levels.
The next two images portray these seasonal effects. The first image portrays CO2 concentrations over the world on 22 March 2105 while the second portrays concentrations five months later. Note that CO2 concentrations in the Northern Hemisphere dropped during this five month interval.
The net change over the course of decades is, sadly, upward, as portrayed in the graph that follows. This graph portrays CO2 concentrations as directly measured at the observatory at the top of Mauna Loa. These measurements represent the longest continuous record of direct atmospheric CO2 concentration readings. The red curve shows the monthly averages. This curve exhibits a sinusoid-like curve atop a ramp. The black curve has the seasonal variations removed, making it much closer to a monotonic ramp.
Can you explain to someone not trained in atmospheric sciences what makes H2O special?
Water is the only common substance on Earth that exists as a solid, a liquid, and as a gas in the Earth's atmosphere. While some alcohols, some oils, and elemental bromine have somewhat similar melting points and boiling points, none of these is as stable chemically as is water, and none is anywhere as close to common as is water.
Carbon dioxide remains in its gaseous state throughout all temperature and pressure ranges experienced in the Earth's atmosphere. (Aside: if CO2 levels were 2000 times what they are now, CO2 might well precipitate as CO2 snow at the Vostok Station. But that ridiculously high concentration level is not the case; CO2 will remain in its gaseous state throughout all temperature and pressure ranges experienced in the Earth's atmosphere.)
Assuming that CO2 levels remain less than 2000 times higher than present means that CO2 in the atmosphere will always be in the gaseous state, which in turn means the CO2-bearing capacity of the atmosphere is effectively infinite. On the other hand, the water-bearing capacity of the atmosphere depends very strongly on temperature. Air at 35°C at sea level can hold about 37 grams of water per kilogram of dry air before the water vapor starts to precipitate as rain. That's over 60 times the current CO2 molar concentration level. Air at -35°C can hold less than 0.2 grams of water per kilogram of dry air before the water vapor starts to precipitate as snow, which is less than the current CO2 molar concentration level.
The following graphic portrays the huge variations ("huge" compared to the several percent variations in CO2 concentration levels) in water concentration levels. The graph in question shows the monthly average total precipitable water vapor levels for May 2009.