Trends In Atmospheric Water Vapour
The basis of IPCC predictions is that any moderate warming caused by increased CO2 levels is enhanced by more evaporation from the oceans. Water vapour is itself a strong greenhouse gas and this increase results in a large “positive feedback” boosting climate sensitivity to a doubling of CO2 as high as 6C.
This is all just theory however, so it is important to observe whether water vapour in the atmosphere has actually increased or not in response to increasing CO2. The data shown below are from the NASA NVAP-M  project based on radiosonde, TIROS, TOVS & SSM/I satellite based data. This data was kindly brought to my attention by Ken Gregory .
Fig 1: total Precipitative water vapour in 3 levels in the atmosphere im mm. The 3 curves are Northern Hemisphere, Southern Hemisphere and the “Global average” – see 2) below.
The evidence above from NVAP-M is that water vapour, if anything, has actually decreased from 1988 until 2001 at all levels in the atmosphere! If we now integrate all layers to get the total water vapour column we then get figure 2:
Figure 2 total water vapour content global averged for all levels. In red global average in black (NH+SH)/2
There is no evidence whatsoever in NVAP-M of any increase in water vapour in the atmosphere during the period 1988 – 2001 during which time CO2 levels increased by ~30 ppm (10%) and temperature anomalies by ~0.3C. The data support about a 10% reduction in water vapour including in the radiative crucial upper layers of the atmosphere. Based on this data water vapour feedback looks to be small or negative.
We can also do a simple calculation to compare the overall changes in the measurements of atmospheric CO2 and H2O.
1988-2001 increase in CO2 ~ 30 ppm = +0.5 kg/m2 (partial pressure)
1998-2001 apparent decrease in H2o ~ 4mm/m2 = -4 kg/m2
This result is evidence against significant positive feedback from water vapour to CO2 radiative forcing. Small changes in water vapour can completely offset (or enhance) any change in CO2 radiation flux to space. The evidence does not support any trend increase in water vapour with either surface temperature or CO2.