Before moving on, I want to correct a dumb statement I made in yesterday’s letter, which said, “Some of the gain in solar energy would be captured by ocean surfaces, where up to half of the energy input would end up being stored in deeper waters….” If that were true the oceans would soon be boiling over. In fact only a very small fraction is stored at depth each day. That fraction is nevertheless up to fifty times greater than quantities stored below land surfaces from equal amounts of incoming energy. At one time I bore the thought that differences in subsurface storage offered the best way to explain the prominent temperature divergence we see on this chart after 1975, where the land anomaly is rising twice as fast as oceans:
Well, the continuing divergence in surface temperatures still needs a good explanation, so here goes: it all comes down to the cooling effect of evaporation. Evaporation probably begins to occur with little or no delay following an increase in the temperature of any surface and whatever water may exist on the surface. We all know about the cooling effect this produces, no matter the circumstances. Oceans have a whole lot more water on the surface than land does, so why should they not experience a whole lot more cooling? The divergence should then continue for as long as there is a continuous trend of increasing energy input from above, whether it be coming from solar or greenhouse sources—just as we see on this chart for almost fifty years now, and running. The energy that is removed by evaporation will not just disappear. The vapor will carry it away, not as sensible heat but in a “latent” form, which natural processes will restore as real heat, but only in a new location, or wherever there is vapor condensation taking place.
When ocean surfaces are cooled in this way, and affecting practically every bit of the surface as on oceanic ones, those surfaces will be able to re-emit less energy back to space than land surfaces that had the same amount of energy input. Evaporation will have already done a big part of the removal in advance. That means the air above a land surface will have a larger flux of outgoing photons to contend with, ready for capture by whatever airborne molecules are in place and have the ability to do so. The amount of outgoing flux will largely determine the temperature of the air above if everything else is the same.
There is another thing that makes this picture interesting. While water vapor is in the air, carrying latent heat energy from one location to another, it behaves much like all the ordinary greenhouse gases. It captures outgoing photons of certain wavelengths and re-emits photons of its own making. The latent heat quality does not seem to be a factor. The ability of water vapor molecules to move rapidly, travel long distances and in some cases extend their movement into high parts of the atmosphere, sometimes in the form of highly concentrated streams, does make a difference in comparison with other gases, in terms of how its greenhouse effect is variably distributed over Earth’s surface. That part of the water vapor story, familiar to regular readers of these letters, is only a sidelight today.
For today’s purposes, there is still one more thing to say about water vapor that I rally want to emphasize, so I can be fully prepared to tell the biggest story of all, which will now have to be delayed until tomorrow. Let’s go back to the scene we started with today, at the initial point of surface evaporation. An argument is made that whenever surface heat increases so does the rate of evaporation of any water that exists on that surface, probably with little or no delay. That means the surrounding air will be holding more vapor—probably moving upward because it is lighter in weight than most air molecules, and moving off in some direction too, if the wind is blowing—fully prepared to start exercising its greenhouse powers. Ocean surfaces will produce much more volume than land, but we don’t see any difference in the underlying process between these two. Now for a question of fundamental importance, under certain unusual circumstances that are known to exist. What difference could it possibly make for the evaporation process if the surface heating that made it happen were in fact caused by one special thing, as opposed to any other thing or combination of things? Tomorrow we will see why this question is so important.
Carl