Water Vapor Hasn't increased in Arid Regions
Those in the know here (just a few?) understand that CO2 alone cannot explain the global temperature increase. The radiant exchange effect (i.e. greenhouse gas) is well-known and undisputed, but explains less than half the measured global air temperature rise. Best to use change since 1958 since when accepted CO2 global measurements began at the Mauna Loa Station. Earlier measurements around the globe by chemical methods are widely disputed. Regardless, CO2 alone can't explain the temperature rise for any period.
Climate models include "additional effects", triggered by an initial rise from a CO2 increase. The biggest effect is an increase in water vapor, since the dominant greenhouse gas in the atmosphere. This goes back to the 1800's, where the Clausius-Clapeyron relation was used to calculate an increase in water vapor due to an increased air temperature. That is for steady-state conditions, with unlimited liquid water available to evaporate. That is true over the oceans, yet air coming off large areas of ocean averages only 70% relative humidity (at all air temperatures), so isn't saturated (100% r.h.), which the C-C relation requires. I haven't researched the reason, but expect it has to do with hot humid air constantly rising to colder elevations where it condenses and rains out.
Regardless of theories, has water vapor actually increased?
A Jan 2024 paper found that water vapor hasn't changed in arid regions. A view of the globe shows most of the land is arid (Western North and South America, most of Africa and Mediterranean, Central Asia, Australia). Summary for the public:
[https://news.ucar.edu/132936/climate-change-isnt-producing-expected-increase-atmospheric-moisture-over-dry-regions#:\~:text=A%20basic%20rule%20of%20climate,as%20the%20planet%20becomes%20warmer](https://news.ucar.edu/132936/climate-change-isnt-producing-expected-increase-atmospheric-moisture-over-dry-regions#:~:text=A%20basic%20rule%20of%20climate,as%20the%20planet%20becomes%20warmer)
The academic paper:
[https://www.pnas.org/doi/10.1073/pnas.2302480120#:\~:text=In%20observations%2C%20this%20increase%20in,arid%2Fsemi%2Darid%20regions](https://www.pnas.org/doi/10.1073/pnas.2302480120#:~:text=In%20observations%2C%20this%20increase%20in,arid%2Fsemi%2Darid%20regions)
The actual data is much different than climate model predictions, which likely means they over-estimate future global warming. One possible reason is that there isn't sufficient liquid water to evaporate and keep the r.h. constant at the higher air temperature. If true, that would increase the specific humidity, which is what matters in radiant exchange (total mass of water vapor).
I also found these interesting earlier articles and papers:
2022 General description for the public (and any reader not already up to speed on the narrative):
[https://science.nasa.gov/earth/climate-change/steamy-relationships-how-atmospheric-water-vapor-amplifies-earths-greenhouse-effect/](https://science.nasa.gov/earth/climate-change/steamy-relationships-how-atmospheric-water-vapor-amplifies-earths-greenhouse-effect/)
A June 2022 paper found water vapor has increased \~1%/decade:
[https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022JD036728](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022JD036728)
They make this strange comment about the data sources which don't fit the narrative:
"... it seems possible that decreases in humidity in some data sets may not be real and we are confident that water vapor is increasing with warming of climate."
So the data doesn't verify it? Need to rely on "confident"?
A July 2023 paper which found water vapor increased in the troposphere from 1980-2020:
[https://spj.science.org/doi/10.34133/olar.0015](https://spj.science.org/doi/10.34133/olar.0015)
An article ca 2009 speculating that a decrease in water vapor explains a decreased temperature rise rate back then, though not based on any actual measurements of water vapor:
[https://www.aaas.org/news/science-slowing-rise-global-temperature-linked-declining-stratospheric-water-vapor](https://www.aaas.org/news/science-slowing-rise-global-temperature-linked-declining-stratospheric-water-vapor)