Total water vapor
Observations show that water vapor in LOWER troposphere is indeed increasing. Surface temperature increase: 0.6¡C since 1970 over global OCEANS and 4% more water vapor.
Water holding capacity of atmosphere increases about 7% per oC (4% per ¡F) increase in temperature.
From Trenberth/IPCC
Since late 1970Õs, ocean surface has been warming at ~0.14¡C/decade => ~1% WV increase/decade.  Observed WV trend since 1988 is ~1.2% per decade
Water vapor increase at higher temperatures