Climate Letter #1976

How is Earth’s greenhouse energy amplified by precipitable water (PW) contents in the upper atmosphere? This is a good question, one that, if true, deserves a better answer than any I have offered so far in these letters. I want to dig more deeply into the details. Without a good answer, parts 2 and 3 of Carl’s theory may still be of interest but they become inconsequential as valid indicators of acceleration in the speed of climate change. The easiest approach to answering would be to blame an acceleration of raw supply of PW to the upper atmosphere, with more and more water vapor entering the streams that are swiftly lofted upward from points of origin at the surface. This could be true, and could make a difference, but factual evidence is lacking, and anyway it’s not what the theory says. The theory is about how fixed concentrations of PW gain leverage that adds to their warming effect when they are allowed to increase their freedom of movement. The theory and actual practice both need to be described in terms of maximum clarity and rationality.

There is a presumption, based on theories presented in part 1, that any doubling of total PW content in the atmosphere overhead, however accomplished, will quickly add about 10C to surface temperatures via additional greenhouse energy effects. That claim can be challenged elsewhere, but for this purpose acceptance can be given. Here we are saying, based on evidence from imagery, that PW stream concentrations, upon arrival in the upper atmosphere, are of considerably high strength to begin with, having densities that may account for as much as 20 or 30kg, per square meter, of the ensuing total. The lower part of the atmosphere might very well end up containing a share that is less than that, by weight, in the same square meter. The theory proceeds from this entry point based on the idea that these high-altitude concentrations are moving rapidly, and never stop moving if no barriers are encountered. Moreover, again barring any kind of interference, the direction of movement will always be toward higher and higher latitudes. PW concentrations that exist closer to the surface have no such option. They have no particular momentum or freedom of movement to begin with, no sense of direction, and may not be moving at all. Whatever is in place is largely derived from local sources, such as they are, of a widely assorted composition. There is a pronounced tendency for these totals to continuously decline with with rising latitudes. As an ultimate consequence, total PW values, as gathered from both atmospheric levels, declines from around 40-50kg near the edge of the tropics all the way down to less than 1kg in the heart of the polar zones.

If relatively high concentrations of PW in the upper atmosphere were to stay at a constant level, and have complete freedom of movement, in a direction that carries them over progressively lower and lower PW concentrations in the lower atmosphere, and if the full value of these concentrations is always added together in the making of a determination of surface temperatures, it seems unavoidable that changes from leverage must be taken into account as a result of this movement. The low ambient PW values found in upper latitudes will be more affected by the overhead passage of a constant high-altitude PW value than will higher ambient values at a lower latitude by overhead passage of this same PW concentration as the stream which carries it moves across one and then the other in quick succession. Of course nature is more complicated than this with respect to the overhead passage but the basic principle will still be there when high-altitude concentrations do not remain constant as we know to be the usual case. (To be continued.)

Carl