[counters]

Well, the problem is that what we would need to geoengineer the climate would be equivalent to a continuous, yearly sequence of large volcanic eruptions. So the analogy starts to breakdown, because the handful of examples we have of these sorts of periods with high volcanic activity were actually pretty bad for civilization at the time:

1. 530's-540's Cluster - contemporaneous historical notes over both the far East and Western civilizations clearly illustrate widespread famine due to crop failures, most likely due to the cooling that this period sustained (sometimes called the "Little Antique Ice Age"). The famous Plague of Justinian also occurred in this period, and was likely exacerbated by famine. There's also the Norse "Fimbulwinter" mythos - a period preceding Gotterdamurang - likely inspired by this period.

2. 1250's-1280's Cluster - Suspected to have triggered the "Little Ice Age", and triggered contemporaneous crop failures in both South America and Europe. 1258 is known as one of the "Years Without A Summer."

3. 1808-1815 Tambora Cluster - Culprit behind the even more well-known "Year Without a Summer" in 1816, which produced one the more recent great famines in Western Europe in Switzerland. Agriculture-induced famines led to a wave of civil unrest across Europe.

So yeah - we obviously survived these periods. But I wouldn't exactly cite them as endorsements for any sort of geoengineering activity analogous to vulcanism.

[red75prime]

At least those show that a stratospheric injection doesn't persist for too long. 200 years of heightened volcanic activity was certainly a problem that eventually resolved itself.

[counters]

The important, missing detail that breaks down this analogy is that we don't have a reference for a long period of vulcanism while anthropogenic emissions of greenhouse gases continue.

This is where the "termination shock" issue comes in. Given current CO2 emission rates, a 50 year geoengineering strategy would mask an additional 100-125 ppm of CO2 added to the atmosphere. If the geoengineering scheme was suddenly stopped, it's not entirely obvious what the response trajectory would be of the climate system.

[red75prime]

Yeah, stratospheric seeding is mostly a stopgap measure that needs to be used in conjunction with other geoengineering and political projects that strive to reduce/reverse CO2 emissions.

[counters]

And therein arises the "moral hazard" issue. There is a legitimate concern that geoengineering could abate some of the concern over climate change and lead to further delays to reduce GHG emissions. And this is a serious problem because while we might mask global temperature change with these Approaches, they don't help resolve issues like ocean acidification.