Here's a report on a brand-spanking-new model intercomparison project looking at the impacts of stratospheric aerosol release on the global climate. It's using a completely new data set, too. Here are some thoughts.
They ran the simulation from 2020 to 2099, with a short and steep ramp-up and ramp-down of aerosol release near the beginning and end of the range (respectively) to simulate a relatively rapid start and stop (I guess they're optimistically supposing that we'll have this all wrapped up by 2100). They assumed a maximal 4x CO2 concentration (!!!) over pre-industrial levels.
The interesting novel consequences, as I see them:
* This tactic reduces the "extreme temperature and precipitation changes" compared to control model runs with the same CO2 increase and no SRM. That seems important, since the rate at which some of this stuff changes is at least as important as the absolute change in some cases, at least with regard to adaptation and other social planning. Rapid changes are hard to deal with, and a really quick change might be more damaging overall than a slower change with a higher absolute magnitude.
* There's a significantly larger change in overall radiative response (and an associated slowdown in the hydrological cycle) compared to other kinds of SRM. I guess they mean something like the "space mirror" approach here. We already knew that reducing incoming solar radiation wholesale would yield different results than reducing surface radiation via aerosols (because of uneven atmospheric heating and evaporation), but this looks like the most specific and significant quantification of that prediction that I've found.
The efficiency of the cooling effect resulting from sulfuric aerosols drops rather alarmingly as the amount of those aerosols in the atmosphere increases. The more of these compounds there are up there, apparently, the larger the average particle size becomes. This suggests a natural limit for the effectiveness of this technique, which is really important--we can't rely on this forever without the side-effects ramping up to unacceptable levels. Even if everything works out ideally, this is not a permanent fix: the more GHG we put out, the less effective this will be.
* Even this simulation didn't include a lot of small-scale stuff, like inter-layer transport of aerosols within the atmosphere. Since those are important for things like cloud formation, it seems plausible that the estimates of precipitation impact here aren't entirely correct.
This all seems relevant for the project that I'm pursuing, especially considering the fact that there's no mention of the ways in which this might constrain other viable policy options. Given the explicitly short/medium-term effectiveness of this strategy, we'd need to have something else cooking while we're implementing it--preferably a strong mitigation strategy. However, implementing this plan will engender all kinds of complications for popular mitigation strategies, it seems to me. More grist for the mill. #geoengineering #climateengineering #climatechange #climatescience