TCS Daily

A Case of the Vapors

By Vincent Gray - April 5, 2004 12:00 AM

Water vapor is the most important greenhouse gas, a fact which is agreed by all climate scientists, but is concealed or covered up by politicians, environmentalists, journalists and scientists alike. It is difficult to give a figure on how important water vapor is, since the infra red absorption spectrum of water vapor overlaps that of carbon dioxide. Published figures range from estimates of between 60% and 90% of the total greenhouse effect coming from water vapor.

Apart from its role as vapor, water has a large influence on the climate from its condensed forms (clouds and precipitation such as rain and snow); on latent heat change when it condenses or evaporates; and on change of albedo on a surface as it freezes or thaws.

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 vapor concentration and its variability with height, region and other circumstances before the era of weather balloons and satellites, so we are unable to judge its past influence.

Since radiosondes became available in the late 1920s and particularly with the use of satellites since the 1970s we now have a very large number of measurements of water vapor concentration in different parts of the atmosphere. The results are very complex, and vary with time, location and level. It is almost impossible to provide such a thing as a global average or a meaningful trend over time.

Climate change alarmists 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 and ozone, because we are able to measure them. and work out a global average and a trend from them, which, on its own, is supposed to dominate the climate.

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 vapor influence which they cannot measure properly. They have decided to treat water vapor and clouds as "feedbacks" which can be assumed to bear fixed mathematical relationships to the minor greenhouse gases. Since these relationships cannot be tested against actual climate measurements they get away with the assumption that there is a large "positive feedback" from water vapor which can provide frightening predictions of future global warming. Because the effects of water vapor and clouds are only "feedbacks" they can leave them out altogether when they present graphs showing the various kinds of "radiative forcing," the extra radiant effects from supposedly human greenhouse gas emissions.

A recent paper on the subject -- Minschwaner, K., and A.E. Dessler, "Water vapor feedback in the tropical upper troposphere: Model results and observations," Journal of Climate March 15, 2004 -- takes the satellite measurements of water vapor concentration in the tropical upper troposphere made by two instruments, the Microwave Limb Sounder (MLS) and HALogen Occultation Experiment (HALOE) which are located on the NASA URAS satellite, launched in late 1991. In their paper they compare changes in these measurements with changes in tropical Sea Surface Temperatures since that date (see Figure 1).

Fig 1. Graph of deviations in water vapor measurement in the Tropical Upper Troposphere (Dq in parts per million by volume) plotted against changes in Sea Surface Temperature (DSST) in degrees Kelvin. (modified from Minschwaner and Dessler)

The actual increases in humidity are well below those assumed by the "constant humidity" assumption of all the computer models (dashed red line). In order to try and make their results more general, the authors have provided an alternative model of their own for the atmosphere above the tropics. This model, as shown by the blue line, is not all that marvellous either, but if it were accepted it would cause a large drop in the future temperature predictions of the climate activists.

These measurements deal with only one small part of the atmosphere. There are some authorities, such as Professor Richard Lindzen, of the Massachusetts Institute of Technology, and recent papers by Karner and Douglass who believe that the overall "feedback" for water vapor should be negative, thus reducing the supposed heating effect of carbon dioxide even more.

Then there are the effects of clouds, which are also a poorly known "feedback" with measurements which cannot easily be averaged. Their cooling effects are supposed to increase over industrial regions. The trouble with this theory is that global temperature increases since 1979, as measured by NASA satellites, have taken place exclusively over land, but not over the sea, the opposite to what the theory predicts.

Dr Vincent Gray is a research scientist with a wide experience in five countries (UK, France, Canada, New Zealand and China). He is an expert reviewer for the Intergovernmental Panel on Climate Change. He is the author of "The Greenhouse Delusion" published by


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