The half life of a water molecule in the atmosphere is about a week, a molecule of CO2 about a century. Although molecule for molecule H2O is a more powerful greenhouse gas, it doesn't hang around nearly as long. When was the last time it hailed dry ice?
You know, that statistic about the half life of a given molecule may or may not be correct. But why is it relevant? And do you have a citation for that? (I’m curious)
Also, last time I checked dry ice hail was not a part of the carbon cycle but rain/hail/snow/sleet clearly is a part of the H2O cycle.
I know it’s early, but what’s your point?
As an aside — IF we were to largely switch from carbon based fuels to hydrogen, we’d be pumping huge volumes of water vapor into the atmosphere and probably creating microclimates around major thoroughfares.
BTW, I am all in favor of efficient catalysis to produce hydrogen.
Use of the term "half-life" to describe ingredients in a system that is largely steady-state is very simplistic. Water simply changes state regularly, and there is no chemical alteration of it. CO2 is chemically altered when it is generated by burning fuel and is therefore not in the same class as water for your analogy.