As you may have noticed when not concerned about a certain virus, global warming has not gone away. The virus did some good. I live on a hill and can look down on some roads, and during our lock-down the roads were strangely empty. Some people seemed to think we had found the answer to global warming, as much less petrol was bing burnt, but the fact is, even if nobody drove we were still producing net amounts of CO2 and other greenhouse gases, and even if we were not doing that, the amounts currently in the air are still out of equilibrium and would continue to melt ice and lead to high temperatures. In the northern hemisphere now you have a summer so maybe you notice.
So, what can we do? One proposal is to shade the Earth’s surface. The idea is that if you can reflect more incoming solar radiation back to space there is less energy on the surface and . . . Yes, it is the ‘and’ wherein lies the difficulties. We get less radiation striking the surface, so we cool the surface, but then what? According to one paper recently published in Geophysical Research Letters (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020GL087348 ) the answer is not good news. They have produced simulations of models, and focus on what are called storm tracks, which are relatively narrow zones in oceans where storms such as tropical cyclones and mid-latitude cyclones travel through prevailing winds. Such geoengineering, according to the models, would weaken these storms. Exactly why this is bad eludes me. I would have thought lower energy storms would be good; why do we want hundreds of thousands of citizens have their properties leveled by hurricanes, typhoons, or simply tropical cyclones as they are known in the Southern Hemisphere? This weakening happens through a smaller pole to equator temperature difference because most of the light reflected is over the tropics. Storms are heat engines at work, and the greater the temperature difference, the more force can be generated. The second law of thermodynamics at work. Fine. We are cooling the surface, and while it may seem that we are ignoring the ice melting of the polar regions, we are not because most of the heat comes from ocean currents, and they are heated by the tropics.
More examples: we would reduce wind extremes in midlatitudes, possibly lead to less efficient ventilation of air pollution, may possibly decrease low cloud cover the storm‐track regions and weaken poleward energy transport. In short, a reasonable amount of that is what we want to do anyway. It is also claimed we would get increased heat waves. I find that suspicious, given that less heat is available. It is claimed that such activities would alter the climate. Yes, but that is what we would be trying to do, namely alter it from what it might have been. It is also claimed that the models show there could possibly be regional reductions in rainfall. Perhaps, but that sort of thing is happening anyway. Australia had dreadful bushfires this year. I gather forest fires were going well in North America also.
One aspect of this type of study that bothers me is it is all based on models. The words like ‘may’, ‘could’ and ‘possibly’ turn up frequently. That, to me, indicates the modelers don’t actually have a lot of confidence in their models. The second thing that bothers me is they have not looked at nature. Consider the data from Travis et al.(2002) Nature 418, 601. For the three days 11-14 Sept. 2001 the average diurnal temperature ranges averaged from 4000 weather stations across the US increased on average 1.1 degrees C above the average from 1971 – 2000, with the highest temperatures on the 14th. They were on average 1.8 degrees C greater than the average for the two adjacent three-day periods. The three days with the increase were, of course, the days when all US aircraft were grounded and there were no jet contrails. Notice that this is the difference between day and night; at night the contrails retain heat better, while in daytime they reflect sunlight. Unfortunately, what was not stated in the paper was what the temperatures were. One argument is that models show while the contrails reflect more light during the day, they keep in more heat during the night. Instead of calculations, why not show the actual data?
The second piece of information is that the eruption of Mount Pinatubo sent aerosols into the atmosphere and for about a year the average global temperature dropped 1 degree C. Most of that ash was at low latitudes in the northern hemisphere. There are weather reports from this period so that should give clues as to what would happen if we tried this geoengineering. This overall cooling was real and the world economies did not come to an end. The data from that could contribute to addressing the unkn owns.
So, what is the answer? In my opinion, the only real answer is to try it out for a short period and see what happens. Once the outcomes are evaluated we can then decide what to do. The advantage of sending dust into the stratosphere is it does not stay there. If it does not turn out well, it will not be worse than what volcanoes do anyway. The disadvantage is to be effective we have to keep doing it. Maybe from various points of view it is a bad idea, but let us make up our minds from evaluating proper information and not rely on models that are no better than the assumptions used. Which choice we make should be based on data, not on emotion.