On reflection it seems to me that there are two "stealth hinges" to the argument for the Sunscreen Defence Shield.

One is that the CO2 problem is insoluble by reduction -- i.e., that even if we stop releasing CO2 tomorrow, the global warming curve remains steep enough and continues long enough to produce disasters (pick your favourites), and feedback loops are uninterrupted.  It's too late for reduction, in other words.  Given that this argument is coming most loudly from the same folks who for the last 20 years have been denying that global warming is happening at all, then denying that it is anthropogenic, etc, I think I may be permitted a little skepticism.  They have gone from the rhetoric of ridicule to the rhetoric of denial to the rhetoric of despair, but the underlying message is simply Business As Usual, and I do mean business as usual -- fossil-fuel industrialism is the heart of capitalism, and global capitalism is what keeps today's aristocrats secure on their thrones, so they would rather die (or more like, don't care how many millions of others will die) than change the way things work.  Any argument that keeps the SUVs rolling off the dealer lots and the cheap air tickets selling like hotcakes is a fine argument by them.

So this talking point, "It's too late to do anything about CO2 reduction so never mind, party on," strikes me as one that needs closer examination.

Then there is Part 2, Assuming there are only two options (do nothing or put up an SO2 sunscreen)  which imho is a false dichotomy.  It assumes that carbon reduction is impossible, let alone ineffective, in other words that "The American Way of Life is Not Negotiable."  This sentiment is echoed elsewhere upthread in disparaging comments about "returning to the 19th century" (actually I would say the carbon economy is the one that is forever stuck in the 19th century, and some of us are urgently trying to move beyond it).  In other words, despite the legendary adaptability, creativity, flexibility and hardihood of human beings, we are supposed to believe that energy efficiency, carbon reduction and lifestyle adaptations are simply Impossible, end of story.  All of a sudden human flexibility and adaptability have ended:  it's the End of History and evolution has stopped, we can never do or live otherwise than as we do today.

If people can learn to take their shoes off on command before getting on airplanes and to meekly hand over their bottled water and shampoo based on ludicrous comic-book "terrorism" scenarios -- without rioting or rebelling -- after nothing more earth shaking than a media blitz and one (one!) successful hit on a major landmark, are we to believe they cannot learn to turn unused lights off, drive smaller cars or use public transit, eat a lower-carbon diet, adapt to lower energy use?  Americans have complacently tossed their Constitution in the rubbish bin, resigned themselves to police-state levels of surveillance, and footed the bill for the most expensive Cabinet war in history, all based on an alleged "state of emergency" far less grounded in reality than climate destabilisation and other carbon-economy side effects.  Why are we to assume that they, and others in the Consuming Nations, are an immovable rock, an intransigent, resolute and adamantine roadblock that prohibits carbon reduction?  Why is it always Politically Impossible to (say) ban SUVs, but Politically Easy to make it everyday and normal to spy on library records and rig elections with proprietary voting machines?  How is it that the most radical of authoritarian programs and rights reductions is justified (and accepted) because "everything changed after 9/11" and yet in the next breath we are told that carbon reduction is impossible because nothing can ever change?

What is insane is the AWOL energy consumption pattern, not the attempt to reduce it to a manageable footprint.  And the energy consumption pattern that drives the CO2 emissions and the climate change is only one symptom of the general consumer-industrial-capitalism problem:  the myth of infinite growth, the core/periphery dynamic with colliding peripheries of competing cores, the "necessity" of population growth to "expand markets" to absorb overproduction and ensure unrealistic (transbiotic) rates of return, the privatisation of all resources (Enclosure), etc.

Suppose the Sunscreen Defence Shield works, temporarily (it has to be temporary, we run out of sulfur to inject and fossil fuel to haul it up there after some number of centuries).  All that does is permit the continuance of the 'cheap fossil fuel' high-burn-rate economy, removing one of the primary motivations for relocalisation, bioregionalism, and other tactics needed to remedy the other pathologies of hypertrophied industrialism.  Which means that we'll hit some other wall -- the killing-the-soil wall, the depleting-the-aquifers wall, the killing-the-oceans wall, the monocropping wall, the complexity wall (cf Tainter), the population wall, etc.  Memo from the past:  "You can never do just one thing."

I have a friend who's dying of cancer.  She has cancer in her spine, in her liver, and in her lungs.  Chemotherapy has succeeded in reducing the liver and lung cancer, extending her life -- which she wanted, and which has been in some ways a blessing. However the extension of life has allowed the spinal cancer (which is less responsive) to proceed further, and now she is at risk of dying far more painfully and slowly of spinal cancer (lung or liver cancer is relatively merciful by comparison).  I see the current state of liquidationist capitalism in a similar light, since we are pursuing medical metaphors:  there is a systemic problem causing multiple symptoms, all of them bad and dangerous, all possibly lethal, some offering a more slow/painful/lingering decline, some offering a faster crash/burn.  We can mask or temporarily remedy one symptom, but the others are still present.  [There's a fundamental disconnect between capitalism and thermodynamics, and between capitalism and robust biotic systems.  Industrial capitalism as we know it is not reality-based, and therefore one of its many conflicts with reality will force a paradigm change.  Or so I suspect -- barring a cinematic miracle (friendly aliens arrive and Save Us, or the Rapture really does happen).]

Actually we are worse off than my friend the cancer patient in the sense that her tumours are growing more or less independently, each on its own clock;  but our fossil fuel binge is one of the primary drivers for our other cancers (salination, deforestation, desertification, toxification of rivers, overpaving, destruction of coastal buffers, extinctions, immiseration of peasants, concentration of capital and erosion of democracy, yada yada) so that "solving" the primary brake on the fossil fuel binge actually exacerbates the other symptoms... which in their way are as potentially lethal as climate change.

A bandaid only "buys time" if you believe that a genuine remission or cure is possible in the time that is bought.  Bought time that merely allows the patient to get sicker or suffer greater pain, is not even a real bandaid.

As was pointed out upthread, 1 bio people rely on the ocean food chain for their sustenance.  We are perilously close to destroying that food chain worldwide.  Continuing to pump CO2 into the atmosphere looks like the handiest murder weapon w/which to finish the job, and that is what the "Sunscreen" project (as sold by the Cornucopian faction) will most likely enable -- another decade or more of inaction (and profiteering), any public sense of urgency or purpose squandered.  How do we face those 1 bio people and say, "Oh goody, we've found a way to go on pumping CO2 into the oceans while protecting rich people's coastal real estate investments from destruction by extreme storms, isn't that great?"

We might also want to consider whether the cancer metaphor is really a good fit.  In using it I have myself played into Frame One above -- tsk tsk -- the notion that our situation is terminal anyway so we should buy time by any stopgap measure available to us, take Laetrile and consult faith healers, and hope for a miracle.  OTOH what if our problem is actually diet and lifestyle related, not terminal cancer but a very severe food allergy or some side effect of a sedentary lifestyle or chronic drug abuse?  In that case, we might feel very much better if we stopped eating wheat, got more exercise, or kicked our habit.  Our problem would be autopathic and it would be fairly obvious that stopgap measures are far less effective than tackling the real problem.  Advil might keep the allergenic headache at bay, but only removing wheat from the diet will really solve the problem -- and chronic Advil use has side effects :-)

Our problem is autopathic, no doubt about that.  The real question is whether it is "terminal anyway."

One of the divisions among those trying to assess our current predicament is in where we place our optimism and our pessimism.  I would say the Crutzen approach and the argument marshalled in favour of it, place their optimism in technology (techno-Laetrile, a miracle cure from the Science Juju or the Energy Fairy);   and the "hardcore green" stance represented by -- picking a name out of the hat -- Gaianne for example, above, is one whose optimism is placed in people, in the ability of human beings to adapt and, when all other options are exhausted, to do the right thing.  Similarly the Gaianne argument places its pessimism in technology -- a disillusioned, disenchanted view based on a long history of huckster claims, frantic bandaiding, malfeasance, arrogance and stupidity in grandiose technomanagerial engineering projects -- and the Crutzen/technoP view places its optimism in technology, a "something will turn up" optimism based on a long history of engineering advances and specifically the series of "miracles" that have been delivered by industrialism over the last 2 centuries.

Obviously from my track record here and elsewhere, I tend to agree with the techno-pessimist and human-optimist assessment.  But arrayed against this stance is the whole weight of the corporate profiteer establishment.  Solutions that involve people living more lightly and spending/consuming less, having more free time, etc. (contraction/convergence) mean less money churning, less skimming off the top, less graft, less extortion, less profit.  Bandaids that involve high-tech fixes, mining, transport, large government or private contracts, etc, mean money churning, embezzlement opportunities, political powermongering, porkbarrel oppos, etc.  What is considered "possible" by politicos who are also CEOs and high-stakes investors in the capitalist casino, has a lot to do with their portfolios and their career ambitions and very little to do with the actual possibilities in the situation.

One of the biggest deposits of sulfur in the world is in the state of Texas.

I think we can expect BushCo to like the Sunscreen Defence Shield idea.

Because I don't believe in Laetrile, faith healing, or "cavalry over the hill" rescues in the final reel (not in real life anyway), I can't get too excited over the time-buying strategy.  Maybe it will not do too much harm (except in lulling the public back to sleep), though given the unintended consequences of every other "let's fix nature" bright idea so far, I'm not sanguine about the predictability of side effects.  But I don't see what is going to miraculously fix all the other symptoms of the underlying autopathy, even if we can buy a few years of stabilised weather.

BTW, The Type I civilisation that has covered the entire planetary surface with human-made structures sounds to me like, literally, Hell.  Suicide looks good by comparison;  but actually the concept itself is suicidal.  If we translate the concept "humans capture all the energy available to the planet's surface," what it really means is "everything else but us has to die."  It is an exterminist idea -- also ludicrous and unattainable since our very lives depend on the biota that capture and transform that energy.  It is as horrifying to me as that other famous exterminist idea, the one that Godwin's Law prohibits me from mentioning.  I shudder to read it.

The difference between theory and practise in practise ...