One important question is how the extra sulphur compounds put into the stratosphere would interact with the ozone layer. The most comprehensive study concluded that injecting enough sulphur to suppress the warming associated with a doubling of carbon dioxide would indeed deplete ozone in polar regions, delaying the recovery of the Antarctic ozone hole by 30 to 70 years.
Other studies indicate that the Indian monsoon could be seriously disrupted, affecting food supplies for up to two billion people, although the disruption may be less than in a scenario of warming without the solar filter.
Even so, our understanding of what influences the monsoon is weak, our knowledge of how global warming would change the monsoon is weaker, and trying to estimate the combined influence of warming and solar radiation management is little more than educated guesswork. Who knows what would happen to rainfall patterns, but if catastrophe ensued after sulphate spraying at least we would know whom to blame. Or would we?
Playing God – and risking the planet
Here we get to one of the strongest objections to sulphate aerosol spraying. We cannot know how it would affect the global climate system through models or even by conducting experiments. Only by full-scale implementation could we get a clear idea of its impacts.
Even then we would need at least ten years of global climate data before we had enough information to separate out the effects of sulphate aerosol spraying from natural climate variability and, indeed, from the effects of human-induced climate change. The levels of omniscience and omnipotence required to make it work really would have us playing God.
To compound the risks, if after ten years, when we accumulated enough data to decide that our intervention was not a good idea it may be impossible to terminate the solar shield. Why should this be so?
For some time ecologists have known that the rate at which the globe warms is a greater threat to ecosystems than the amount of warming because a slower rate of warming gives plant and animal communities more time to adapt.
According to one study, if sulphate aerosol spraying began in 2020 and had to be stopped after 40 years, we would see a surge in average temperature by a scorching 1.3ºC in the first decade, falling back to 0.33ºC in the following decade.
Few ecosystems could survive the first decade of rapid heating after the solar shield had been turned off. So once deployed it is likely that we would become dependent on our solar filter, the more so if we failed to take the opportunity while it was in place to cut greenhouse gas emissions sharply. This is perhaps the solar filter’s most dangerous drawback.
Bill Gates’ climate warriors: The geo-lobby
A constituency advocating investment in a major research program has now emerged, and is gaining influence. At the centre of this network is a pair of North American scientists actively engaged in geoengineering research – David Keith, a Harvard physicist, and Ken Caldeira, an atmospheric scientist based at Stanford University.
For some years they have been Bill Gates’ principal source of expert knowledge on climate change. Gates was persuaded to commit several million dollars to finance research into geoengineering. (Richard Branson is also promoting geoengineering as a response to climate change.)