Sulphate aerosols are the most commonly proposed substance: these would essentially mimic the effect of a large volcanic eruption, which can inject such aerosols into the stratosphere. The 1991 eruption of Mount Pinatubo led to a detectable global mean surface cooling of up to 0.5 °C, which lasted over a year.10
There is evidence that SAI can offset some of the impacts of climate change, but will come with ancillary risks of its own. Crucially, it would be possible to restore the average global temperature to pre-industrial levels if sufficient quantities of aerosols were injected in a sustained fashion.11 However, modelling studies strongly suggest that it is not possible to reset temperatures in every region of the planet. For example, while the average global temperature may be restored to pre-industrial levels, there will be some terrestrial regions where average temperatures are not fully restored. The same applies to precipitation, wind patterns and other aspects of climate. So, while SAI could lead to a more favourable outcome overall, there would inevitably be some winners and some losers.12
To date, the overwhelming majority of studies of the effectiveness and consequences of SAI have been carried out through computer modelling. A small number of field tests have been proposed, but these have generally faced opposition and been called off as a result.13 Because SAI is a global intervention by design, any significant real-world test of efficacy would be indistinguishable from actual implementation --- not least because the effects would probably take years to become detectable in global temperature data due to internal variability.
Implementing SAI would entail the development of a new class of aircraft able to carry heavy loads of sulphate aerosols to altitudes of approximately 20 kilometres. It has been estimated that it would take over $2 billion to bring such craft into use.14 Overall, it has been estimated that a global programme of SAI would cost $18 billion per year for every °C of warming offset.15 This is relatively small compared to the costs of the damages from anthropogenic climate change --- well within the reach of a coalition of governments, or even billionaire enthusiasts.