Let me start by referencing [ldischler]'s Idea, "Cooling the earth with contrails", linked below. I'm also copying one of the links from that Idea, about how after Sept 11 2001, for three days there was no significant air traffic in the U.S., making contrails, so scientists got lots of good data about
the effect of contrails. I remember reading about that at the time the results first came out, about a year later.
Just like clouds, contrails are. Same albedo (reflectance) and composition (fog) and so on. So they reflect sunlight during the day, helping to prevent excess warming, and during the night, if they haven't dissipated, they reflect infrared back toward the ground, helping to prevent excess cooling. Deserts are famous for being cold at night, because there is so little water in the air above them (even when not condensed into fog/clouds, water vapor counts as a greenhouse gas).
It seems to me that What We Need is a way of increasing the amount of water vapor in the air by day, in the form of clouds, to directly fight warming, and decreasing it at night, to let the heat out. This Idea relates only to the first thing, just like [ldischler]'s. I suppose sometime I'll have to concoct some wild notion to accomplish the second thing (I'm not sure that there ARE any such notions ever suggested along that line).
I'm writing this partly because of a flaw I see in [ldischler]'s notion of adding a chemical to make contrails last longer. If they last longer, such as overnight, then they are at that time NOT working to help cool the earth. Also, remember that as jets become more fuel-efficient, each jet causes less contrail than in earlier years. We need more jet flights to compensate, but global petroleum supplies, from which jet fuel is made, is starting to fail to keep up with demand. In the not-too-distant future, there may be no jet contrails, right when we need them most!
Okay, so you see that this Idea has a title and subtitle about really tall glass smoke stacks for steam. "Really Tall" needs to be near-stratospheric level (more than 5,000 meters or 16,400 ft). Some sort of supporting strutwork/truss made of titanium will probably be required, like a giant Eiffel Tower. And we need a bunch of these things.
At the base of each Glass Steam SuperStack is a glass-covered area, over seawater. We will have an actual ordinary Greenhouse Effect here, when the sun shines, so a lot of water will be warmed and will evaporate.
The rising vapor is funneled into our Glass Steam SuperStack. Since it is made of glass, the Greenhouse Effect continues to warm the vapor! (We may encourage this by having a movable black light-absorber inside the stack, so that sunlight that enters doesn't pass all the way through.) By the time it rises to the top of the Stack, the vapor should be moving fast enough to keep on rising well into the stratosphere. It will also expand and cool, and VERY PROBABLY will condense into a cloud form, very much like a jet contrail, as the high winds at that altitude carry it away from the Glass Steam SuperStack.
Quite a lot of cloud cover should be created continously by these things, at no cost in jet fuel or special chemicals (a major advantage over [ldischler]'s Idea). This Idea is designed to inject massive amounts of water vapor into the stratosphere in the daytime.
Now all we need is a way to remove it at night!