19TH MARCH 2004
Water vapour is the most important greenhouse gas, a fact which is concealed or covered up by politicians, environmentalists and scientists alike. It is difficult to give a figure on how important since the infra red absorption spectrum of water vapour overlaps that of carbon dioxide. Figures I have been able to find range from 75 to 90% of the total greenhouse effect coming from water vapour..
Apart from its role as vapour, water has a large influence on the climate from its condensed forms, clouds and precipitation (rain and snow), and on  latent heat change, and change of albedo as it freezes..
You would think, then, that investigations on "climate change" should place an emphasis on this overwhelmingly major influence.
Unfortunately, we have next to no reliable measurements of atmospheric water vapour concentration and its variability with height, region and other circumstances. We are making a start, but past information is forever lost.
Climate change enthusiasts have therefore chosen to follow the example of the man who looked under the lamp post for his lost wallet because that was where the light was. They concentrate almost exclusively on the minor greenhouse gases, such as carbon dioxide, methane abd ozone, because we are able to measure them.
Climate models, based on the ridiculous assumption that greenhouse gases are the only influence on the climate, are faced with the problem of what to do with the major water vapour influence which they cannot measure. They have decided to treat water vapour, and clouds as "feedbacks" which can be assumed to bear a fixed mathematical relationship to the minor greenhouse gases. Since these relationships cannot be tested they get away with it easily, they can assume a large  "positive feedback" and they can leave water vapour and clouds out altogether when they present graphs showing the various kinds of "radiative forcing"
A recent paper on the subject has been accepted for the Journal "Climate Change"

Minschwaner, K., and A.E. Dessler, "Water vapor feedback in the tropical upper troposphere: Model results and observations",  Journal of Climate  March 15 2004

The preprint is available from <>

This paper takes the satellite measurements of water vapour concentration in the tropical upper  troposphere from the Microwave Limb Sounder (MLS) and HALogen Occultation Experiment (HALOE) which are instruments located on the URAS satellite which was launched in late 1991. These measurements were compared with Sea Surface Temperatures  over the same period (attachment WVapour.gif). This shows that the actual increases are well below those assumed by the "constant humidity" assumption of all the  computer models.The above authors have provided an alternative model of their own which, as shown by the dotted line (added by my correspondent, Doug Hoyt) is not all that marvellous either.

Doug Hoyt comments that these measurements deal with only  one "feedback loop", and it is still possible that the net "feedback" from water vapour could be negative, as has been argued by Professor Lindzen, the subject of my last newsletter.

Vincent Gray
75 Silverstream Road
Crofton Downs
Wellington 6004
New Zealand
Phone/Fax (064) 4 9735939
"It's not the things you don't know that fool you.
It's the things you do know that ain't so"
Josh Billings

water vapour feed back