I think that Gaianne isn't concerned about the effects of SO2 coming out (which it would do on a steady basis, regardless), but instead about the negative effects of losing its cooling effects all at once, causing abrupt warming that might well be more ecologically damaging than slow warming.

There is a valid concern here, though I think that Gaianne gives it too much weight for several reasons:

• First, treating the collapse of civilisation as certain claims more knowledge of the future than I think anyone can have at this point.

• Second, the scenario requires that a collapse be both world-wide and remarkably thorough, because putting SO2 in the stratosphere isn't very difficult. Since a generous estimate of the cost is 1/5000 of GWP, an economic collapse could be 99% complete, yet the cost would be no more than 2% of the remaining GWP. (And the job could be done with technology at the WWII level or less.)

• Third, I don't see any snap-back effects that are likely to be worse than the cumulative effect of decades of warm conditions, for example, polar melting.

(BTW, I usually avoid claiming that I know any particular person's unspoken thoughts and motives, or predicting that they will never reconsider an opinion. I'm much more comfortable making predictions that substantial groups will react in a particular way -- for example, I think that your prediction regarding Gaianne will surely prove true of many people.)

Words and ideas I offer here may be used freely and without attribution.

You make my point for me:  

In the absence of change, we enter the scenerio implied by technopolitical's point 2).  SO2 injected into the stratosphere keeps the Earth just barely cool enough.  Making no change, we keep releasing CO2 into the lower atmosphere at an increasing rate, as a by-product of continuing and expanding industrial civilization.  As a result, the amount of SO2 that needs to be injected into the stratosphere also increases, until the precipitating SO2 DOES become an environmental problem.  We have then achieved a world that is just barely cool, and multiply polluted.  

At what point do these extra problems add to the accelerating destruction of the biosphere to the extent that it ends human life support?  That time comes.  

Fundamental change is not only necessary, it is going to happen whether or not anybody likes it.  The only question is whether there is a way to choose the better, rather than the worse, changes.  

nobody will be around to give a fsck about it.

I don't favor policies of extinction.  

The Fates are kind.