[Petrochemicals (OP)]

 How much extra water vapour is put into the atmosphere due to human activity?  It seems over the last 50 years man's activities have led to a far greater ammount of water vapour potential, irrigation in deserts, bathing water, hydrocarbon combustion itself, deforestation, industrial cooling. The human populace itself has increaced rapidly in number, so the number of the potential sources of water vapour has increaced by sheer numbers, let alone technological advancements.

[alancalverd]

Difficult to say, but the worrying question is where is it? As mean atmospheric temperature rises, so the water gets distributed at higher levels, and instead of localised, dense, shortlived cumulus clouds, it forms altostratus and eventually cirrus. These have very different influences on insolation and radiative transfer. Not well understood but their effect is clearly demonstrated in the prehistoric temperature record.

[evan_au]

On a more down-to-Earth level, increasing sea surface temperatures produces increased evaporation, which is predicted to lead to more hurricanes. The hurricane results in heavy rain over a large area, so some of this moisture will return to the sea, and some will end up on land.

From memory, hurricanes are likely to form in the Atlantic when the sea surface temperature rises above about 26C.
- The hurricane season is likely to start earlier, and run later.
- The zone of hurricane formation is likely to extend even further from the equator towards the poles 

[Bored chemist]

The principle  factor that determines the amount of water in the atmosphere is the temperature.
If that's "too low" the water falls out again.
The observant among us will be familiar with rain.

Very nearly all the water in the atmosphere is there as a consequence of solar driven evaporation.
Since we don't affect the sunshine, we won't have changed it much, directly.
We will have increased it via  anthropogenic global warming.

The worrying thing is the potential for a positive feedback system where increased water also raises the temperature...

[alancalverd]

On a more down-to-Earth level, increasing sea surface temperatures produces increased evaporation, which is predicted to lead to more hurricanes.

Oddly enough, this is at variance with the finding that the increase in  night temperatures exceeds the increase in day temperatures. Hurricanes, and indeed almost all weather events, are driven by temperature differences and although the atmosphere is getting hotter, it is also becoming less heterogeneous.

Atlantic hurricane data is unreliable "pre-satellite" as sensible aviators and seafarers avoided  the likely "hotspots", but if anything the number of high-category events making landfall in the USA (a reliably-recorded occurrence) has decreased over the last 100 years, in line with the above expectation. That said, the damage done by those that do make landfall, has increased due to the expansion of human habitation on the southeast coast and the Caribbean islands.

[Petrochemicals (OP)]

On a more down-to-Earth level, increasing sea surface temperatures produces increased evaporation, which is predicted to lead to more hurricanes. The hurricane results in heavy rain over a large area, so some of this moisture will return to the sea, and some will end up on land.

Quite true Evan, warming air temperatures will also increace evaporation. More water on land is more surface area wetted. Similar to the colorado river and many many like it all over the earth being diverted to places like the arizona desert.

[Petrochemicals (OP)]

Difficult to say, but the worrying question is where is it? As mean atmospheric temperature rises, so the water gets distributed at higher levels, and instead of localised, dense, shortlived cumulus clouds, it forms altostratus and eventually cirrus. These have very different influences on insolation and radiative transfer. Not well understood but their effect is clearly demonstrated in the prehistoric temperature record.

Good point, i  imagine the atmosphere is expanding so not only more high level water but also thicker quantities. Going by the shortest path, earth emitted energy has double trouble.

[Bored chemist]

Are the deserts shrinking as a consequence of the additional water in the sky?

i  imagine the atmosphere is expanding

It must be fascinating to have such a vivid imagination.
By how much?

[chiralSPO]

if anything the number of high-category events making landfall in the USA (a reliably-recorded occurrence) has decreased over the last 100 years,

You may wish to refresh yourself with the data. (available here: https://www.nhc.noaa.gov/climo/ )

And here is the relevant graphic:

Atlantic_Storm_Count.jpeg (761.71 kB . 1310x932 - viewed 2249 times)

[alancalverd]

Whereas the figures that I quoted were about hurricanes making landfall

https://www.statista.com/statistics/621238/number-of-hurricanes-that-made-landfall-in-the-us/