Extreme situations are claimed to demand extreme measures, and none more so than those in the realm of geoengineering. The latest of the many highly questionable and probably dangerous strategies proposed is to pump sulphur dioxide (SO2) into the stratosphere, where it is expected to form an aerosol of sulphuric acid droplets. Tom Wigley, of the U.S. National Centre for Atmospheric Research, has used computer models to try and predict the influence of injecting "sulphate" particles at intervals of between one and four years, which he concludes would be similar to the cooling effect of the 1991 eruption of Mount Pinatubo in the Philippines. The idea of injecting sulphates into the stratosphere at around 16 km above the Earth's surface was first proposed around thirty years ago but was almost immediately rejected as a dangerous tinkering with the natural Earth systems.
This cannot be overstated. The Earth is a highly complex machine. By this I do not mean that it is merely complicated, i.e. hard to comprehend because there is a lot to it, but "complex" in the mathematical sense, where the minutiae components function together to produce complexity, akin to chaos theory, where an exact outcome cannot be computed or selected from any number of outcomes for which the nature of each can be clearly predicted. It is this that bedevils predictions about climate change, made from computer models, for example there are large variations in the extent of warming to be expected as CO2 levels rise, ranging between about one and six degrees C over the span of this century.
The cooling effect of an aerosol in the high atmosphere occurs because it reflects the sunlight and hence reduces the amount that reaches the Earth's surface. According to Wigner, the most practical way to get the sulphur dioxide into the atmosphere at the required altitude is to send up fleets of planes - indeed in greater number than the entire world's commercial airline capacity - to take it there. "There" is an interesting point, since "it" lies at around 16 km, which is about the cruising altitude of the Concorde aircraft (scuttled since 9/11), and most aircraft could not fly here, being designed for lower altitudes. We would then, require the wholesale construction of a new purpose-designed fleet of planes to meet the task. Here we go again, another "solution" that seems to demand engineering on the large scale once the idea is addressed a little more closely than is allowed by circumspect reflection. Another method proposed is to mix sulphur compounds into aviation fuel, hence emitting SO2 along with their exhaust gases: but only if they flew at high altitudes, otherwise there would be an impact on the chemistry of the troposphere, with unknown consequences, other than that the sulphuric acid would likely be rained out onto the surface: "acid rain"! Dead lakes and architectural heritage etched away. I have worked as a consultant on the properties of fuel-additives, and I am fairly certain that the presence of sulphur compounds in a fuel would impact significantly and badly on their performance in terms of miles-per-gallon. Stratospheric chemistry is also impacted upon by the presence of water vapour exhausted by planes, which is one argument for curtailing aviation use, and the proposal would require an endorsement that the benefits of injecting SO2 here outweigh those detriments from increased fuel combustion in the higher atmosphere. There is no evidence that it would.
I wonder too, what effect the sudden presence of the unnatural levels of SO2 would have on the essential chemistry of the ozone layer. It could be a molecular equivalent of releasing mink into a land where they have no predators to keep them in check, and decimating indigenous species. So, it is likely that as is the case with nitric acid hydrates in the stratosphere, the sulphuric acid based aerosol would provide ample surfaces on which to catalyse the decomposition of ozone, and so having cut-back on CFC use to repair the ozone layer, we would simply be substituting this form of pollution with another: back to square one, with the ozone-hole stretching elastically overhead, and admitting more UVB and skin-cancers.
Rather like the planes proposed to deliver the putative SO2 "seed" into such elevated climes, I doubt this idea will ever fly! We are again left with the single option of curbing fuel use, and especially aviation fuel, to cut CO2 emissions in an effort to mitigate global warming and the destructive effects of climate change. This is no back-door entrance policy for the aircraft industry.