Climate Letter #1864

I have repeatedly made the claim that the greenhouse effect of the complex material we call precipitable water (PW) can be treated holistically, regardless of any differences in the proportions of the materials from which it may anywhere be constituted. This claim needs to be justified in order to verify the truth of any general rule purporting to describe how changes in PW affect surface temperatures. That is, a general rule should effectively take those proportions into account along with the assessment of effects due only to volume changes.

The general rule I have repeatedly referred to, presuming it to be true, has several parts. One is that PW’s effects are expressed by nature on a smooth logarithmic scale. For example, each double in PW concentration should therefore produce the same amount of energy effect on surface temperatures. Climate scientists regularly apply this same rule to CO2 concentrations, but are basically reticent with respect to assignment of similar logarithmic powers to any of the other greenhouse gas concentrations—for reasons I don’t understand.

The logarithmic rule, in all cases, can have upper and lower limitations with respect to the size of the scale of coverage. It cannot under any circumstances be infinitely high or low from a practical standpoint. For PW I have been using a range running from about 15 grams to 30 or 40 kilograms (regular measures of total weight per vertical square meter), making room for at most a dozen doubles inside the scale. All of these doubles and their effects can actually be observed as occurrences in modern experience. The range for usual consideration of realistic doubling numbers in the case of CO2, expressed in ppm, has fairly wide historical precedents but is much tighter than this for currently practical reasons.

Based on observations that are admittedly imperfect, I have been claiming that every double of PW concentration supplies enough new longwave energy to immediately add approximately ten degrees C to surface air temperatures, when the surface itself is mainly composed of solid materials. (Deep-water surfaces have a much greater capacity for energy absorption, thus lowering the result for energy effectiveness at the overlying air level.) I have not been able to determine whether the 10C estimate would be significantly altered by having better knowledge of differences in the proportions of materials in the PW concentrations that are reported.

PW always originates as gaseous water vapor, and always maintains a large proportion of vapor content, which by itself has known powers of greenhouse energy effect. When those molecules condense, as they all do eventually, they combine with others in forming tiny water droplets. There is no change in the weight volume of PW when this occurs but we can reasonably suspect that the net greenhouse power of the PW may have changed. These very abundant droplets, now tending to make up the visible bodies of clouds, are known to produce their own greenhouse effect, but how does it compare with that of the vapor molecules that have been replaced? This is a good question, with no definitive answer. We can go on from there and investigate further changes that may occur when the tiny droplets are eventually replaced by the larger particles that are prone to precipitate. The presumed shorter lifetime of these larger particles should reduce the significance of this change, allowing it to be set aside as we focus on the larger one.

I think my 10C per double of PW estimate, even if imperfect, will eventually be put to use by the sciences because it has so much utility in explaining the details of how and where global warming actually builds up on a daily basis, and perhaps also how the warming could accelerate through feedback effects as observed in the upper atmosphere. Any internal variations in PW greenhouse effects should not prove to be great enough to justify ignoring these benefits, but a more comprehensive investigation giving support to this conclusion would still be helpful. And is overdue.

Carl