In my post of a week ago, I raised the issue of climate change, and argued that because there is a net power input to the surface now, due to reduced cooling caused by the blanket effect of the so-called greenhouse gases, even if we stopped producing such gases right now, we would still have serious problems because the current rate of net ice melting would continue. Now, it is all very well to moan about it, but the question is, what should our response be? This is too complicated for one post, so this will start a small sequence, although not all will be consecutive.
The easiest response is to do nothing and keep going as we are. Eventually, the sea will rise by about 60 meters. That would drown London, Beijing, and a number of other cities, in fact almost every port city, and it would remove a huge amount of prime agricultural land. Suppose we do not wish that, what can we do? In logic, there are four main options: lower the heat input; raise the heat output; store input energy as chemical energy; increase snow precipitation on polar regions so that it makes up for the increased melting. The last option means we accept everything else, such as increased temperatures and worse storms, but we protect our land. Obviously, we should also reduce our output of so-called greenhouse gases, because while even stopping this output does not solve the problem, at least it stops making the problem increasingly more difficult.
You may argue that such options suffer from failure to be practical. Possibly, but unless we investigate, how do you know? Another argument sometimes put forward is we should not do anything because there could be unintended consequences. That too is true, but is drowning London and starving a great fraction of the population a desired consequence, because that is what happens if we do nothing?
Lowering the heat input is most easily achieved by reflecting more radiation to space i.e. increase the albedo of the planet or place reflectors in space. Increasing the albedo is probably most easily done by increasing cloud cover. One proposal I have seen to do that is to spray seawater into the air. The biggest single problem with this proposal is that there appear to be no readily available analysis of the costs and benefits. How would we power the sprays? If that were done through solar, or wind energy, that would be more helpful than doing it by burning diesel. How long would such salt-laden clouds last? We simply don’t know. Some might argue that clouds contain water, which is itself a powerful blanket material. That is true, and it is why cloudy nights are warmer than cloudless ones, nevertheless there should still be a significant net benefit, because the reflection to space is of visible and even ultraviolet light, whereas the blanket effect merely affects infrared light, of a moderate frequency range, although it does it 24 hrs/day.
How about space reflectors? The cost would be enormous, although there is one possibility. Suppose one could develop solar-powered lasers that were sufficiently powerful to ablate space junk. You do not need a major mirror, but merely a large surface area. If you could boil away the metal and condense it as dust, that would still qualify as area. As an aside, it does not need to be that bright, although it should be. If sunlight is absorbed in space, that is almost as effective because the dust then re-radiates the energy as heat, and most will be directed to space.
It is also possible that there could be other minor ways of contributing. Thus the concept of everyone painting their roof white, as suggested by physics Nobel laureate Steven Chu, or even using aluminium for roofs is often rejected as making contributions that are too small, nevertheless, every ordinary householder still has to paint their roof or replace it at some time, and does it hurt to be helpful?
Another possibility might be to inject something into the exhaust of jet engines at high altitude. The point here is the jets are flying anyway, and you would end with micron-sized white dust in the contrails. Materials have to be chosen so they do not form slags in the engines, hence the choice depends on technical details of which I am unaware. Materials, in order of higher melting point dust, might range from a mercaptan or dialkyl sulphide (no solid, but would produce sulphuric acid on oxidation, which would condense water vapour and make clouds), diethyl zinc (which would produce white zinc oxide, melting point 1975 oC, and hence would remain as a dust in any working engine) or alkyl silanes (which would produce silicon dioxide, similar to volcanic ash, with a melting point above 1600 oC. The actual melting point depends on the form of the solid).
Finally, there is also the possibility of growing certain crops that give off gases that may increase cloud cover. Thus certain marine algae are reported to give off mercaptans, which would be photooxidised to sulphuric acid and thus form clouds, and also each molecule would remove some number of photons from the solar input. Removing ultraviolet also removes the corresponding heat input.
An important point to consider is that all light that is not reflected to space is either converted to heat eventually, or is locked away as chemical energy. The Earth continually presents to the sun a cross-sectional area of about 40.5 x 10^12 square meters. You can work out for yourself the area required to reduce the solar input by whatever per centage you wish, after correcting for whatever efficiency you choose, but as you can see, it is a very large area, no matter what.
It may strike you that trying to solve this problem this way is simply too difficult and expensive. Possibly, but my argument is we are wrong to rely on one king hit. For me, this is the problem to be solved by a thousand cuts, so to speak. In later posts I shall add thoughts on the other alternatives. However, the above thoughts seem to me to form the start of a concept. There are some things we might try that either might have other benefits or are reasonably cheap to put into practice, and these should take some form of precedence. But the overall conclusion is clear: there is simply insufficient data available to reach any reasonable conclusion.