One of the challenges about geoengineering--this is ideas like injecting stratospheric aerosols to cool the planet in the event of a climate emergency--is that we don’t really understand exactly what the climate response to geoengineering would be. And obviously, the first step is to do a lot of computer modeling. But eventually if we ever were serious about thinking that implementing some form of geoengineering might be better than not implementing it, we’d like to know now whether it’s actually going to be possible to test it. So we looked at this question of whether you can test geoengineering at all.
The first observation is, it’s a lot easier to distinguish the signal from your geoengineering test against the background natural climate variability if you use some modulated forcing. And the one issue with that that we need to think about is that the climate’s response to some sort of a short period forcing, so some four-year period, isn’t going to be the same as the climate’s response to sustained forcing. So you still need to extrapolate to what happens at longer time scales.
The second question is, how long does it take to do the experiment and learn what you’d like to learn? The short answer there is, if you do a test with, for example, one watt per square meter of radiated forcing, which is a fair amount, and you ran that test for about 20 years, then you would learn the global climate sensitivity to about 25% accuracy and much, much worse accuracy for trying to see what the response is over places like India.